Search

Breakthroughs have been made recently in the exploration of hydrocarbon in the middle of Tarim Basin, NW China. The complexity of the Lower-Middle Ordovician carbonate reservoir remains enigmatic and hinders the source, developmental mechanism of the calcite fills, and paleo-environmental conditions of the Yingshan and Yijianfang Formations have been unclear. Here we present the related paleoenvironmental settings and diagenetic history, using petrographic, geochemical, and carbon (13C) and oxygen (18O) isotopic characteristics of calcite fracture, cave, and fissure fillings of a Lower-Middle Ordovician carbonate reservoir. The results indicate that the limestone caves dominantly consist of CaO, FeO, and MgO, and minor quantities of TiO 2 and MnO. Among the trace elements, Ni reported the highest values (with an average of 1162.16 ppm) and other trace elements such as Ba, V, and Ga occur in minor values (with an average of 71.03, 19.18, and 12.92 ppm) respectively. The enrichment of light rare earth elements (LREEs) over the heavy rare earth elements (HREEs), with positive and negative Ce, and negative Eu anomalies indicate the contexts of oxic and anoxic conditions in seawater and sediments interface. The variation in δ18O PDB values ranges from −4.72 to −14.48‰ (with an average of −10.12‰), indicating a setting of paleo-karstification by atmospheric freshwater dissolution. While the variation in δ13C PDB indicates relatively significant values, ranging between 0.82 to −3.80‰ (with an average of −1.07‰), suggests that the paleo-karstification in this region has experienced episodes of supergene and burial diagenesis. The paleo-temperature average value of 59.1 °C, and the paleo-salinity average (Z-value) of 120.07, indicating a relatively warm paleoclimatic environment during the development of calcite-filling deposits. The results suggest several paleo-environmental conditions when paleokarst occurred: a marine deposition environment, an atmospheric freshwater karstification environment, a shallow burial paleo-karstification environment, and a high-temperature deep-burial environment in Yingshan and Yijianfang Formation in the Shunbei area. This study serves as an analog for similar carbonate systems worldwide, providing valuable insights for understanding paleo-karstification processes in different tectonic and climatic settings. Comparative studies between the Shunbei area, Tarim Basin, and other carbonate platforms contribute to our knowledge of karst evolution and its implications for reservoir development and groundwater resources. • Lower-middle Ordovician carbonate reservoir paleo-karstification and depositional environments were investigated. • Paleo-karstification's calcite cementation and diagenetic phases were predicted. • Ascertained the paleo-temperatures that occurred within the paleo-karst development. [ABSTRACT FROM AUTHOR]

The fluid activity in the deep strata of sedimentary basins is commonly related to tectonic activity, and the cements filled in fractures are a good carrier for the tectonic-fluid coupling evolution. Compared to macrofractures, microfractures have characteristics of high frequency and easy identifiable periods. Abundant microfractures infilled by carbonate cements (MCCFs) developed in carbonates of the Ordovician Yingshan and Yijianfang formations in the platform basin area of the Tarim Basin. Based on the study of petrology, U-Pb dating, and geochemical characteristics, this study determined the stages of MCCFs and clarified the tectonic-fluid coupling evolution process recorded by MCCFs in the study area. The formation order of these MCCFs is D1, C1, C2, D2, C3, and C4. The precipitation times of MCCFs have a good correspondence with orogeny around the Tarim Basin and active times of strike-slip faults in the platform basin area. The six stages of MCCFs in the Ordovician Yingshan and Yijianfang formations in the SLU recorded the tectonic-fluid coupling evolution process of concentrated seawater in the late Middle Ordovician, meteoric water at late Ordovician, organic acids during the Silurian, Mg-rich hot brine at the end Devonian-early Carboniferous, and magmatic hydrothermal fluids during the Permian. This not only indicates a close connection between fluid activity and tectonic activity in sedimentary basins, but also confirms that the formation of MCCFs in carbonate formations is closely related to regional tectonic-fluid coupling activities. This study provides a good example for studying macro scale tectonic-fluid coupling activities in basins using microfractures. • Types and intrusion time of fluid recorded by carbonate cements in microfractures were determined. • Microfractures in carbonate formations are closely related to tectonic-fluid coupling activities. • We provide a good example for studying the regional tectonic-fluid activity using microfractures. [ABSTRACT FROM AUTHOR]

Deep sour gases are found in the Ordovician carbonate reservoir of the Tazhong Uplift in the Tarim Basin, northwest China. All sour gases in this field are dominated by gas hydrocarbons (C 1 –C 4 ) with varying amounts of H 2 S. Most of the gases in the eastern Tazhong are characterized by high gas dryness coefficients (>0.95), 13 C-enriched methane (δ 13 C 1 ranging from −44.6‰ to −35.8‰), and small difference between values of the δ 13 C 2 and δ 13 C 1 (δ 13 C 2 - 1 ). In contrast, the gases in the western Tazhong are characterized by relatively low gas dryness coefficients (mostly < 0.90), 13 C-depleted methane (δ 13 C 1 ranging from −54.9‰ to −35.7‰), and high values of δ 13 C 2 - 1 , indicating relatively lower maturity. Gas compositions, carbon isotope ratios, and diamantane data together with thermal pyrolysis experiments using sealed gold tubes suggest that the Ordovician gases in the Tazhong Uplift belong to oil cracking gases mixed with high-maturity 13 C-enriched dry gases, which originated from the deeper paleo-reservoirs cracking. The H 2 S (0.0008%–23.1000%) is attributed to thermochemical sulfate reduction (TSR); in contrast to typical TSR-altered gases, the natural gases in the western Tazhong are characterized by low dryness coefficients, relatively 13 C-depleted methane, and high δ 13 C 2 - 1 , suggesting a low degree of TSR alteration. Observations suggest that the H 2 S contents are generally high in the western Tazhong, with positive correlation to Mg 2+ concentrations and total dissolved solids (TDS) in the Ordovician formation water, indicating that TSR has entered early period of SO 4 2− contact ion pairs (CIPs) oxidation. Compared with the eastern Tazhong, the Mg 2+ concentrations and TDS in the western Tazhong are higher and form more CIPs that accelerate TSR and produce H 2 S. In addition, based on the analysis of the Cambrian formation water and the Cambrian gypsiferous salt, TSR might have also exist in the deeper Cambrian reservoirs, and the TSR-altered gases with high content of H 2 S migrated along faults to Ordovician reservoirs. [ABSTRACT FROM AUTHOR]

Marine red beds (MRBs), typically colored by Fe- and Mn-rich minerals, are often interpreted as indicators of bottom water oxygenation. However, their continuous formation requires long-term input of aqueous derived Fe-Mn oxides and/or stable sources of Fe2+ and Mn2+ ions, challenging the traditional concept of "red equals oxic environment." This study investigates two coeval Upper Ordovician Sandbian MRB carbonate intervals in the Tarim Basin. Previous studies attributed the pigment origin to hematite. Th/U values and total rare earth element and yttrium (REY) contents increased in the MRB interval, indicating a terrestrial source of iron. Hematite was observed within the intercrystal pores of calcite precipitating from porewater, consistent with high Ni carb abundance and MREE-enriched bulge pattern in the red intervals. Both sections exhibit high Fe carb and Mn carb , pointing to substantial reductive dissolution of Fe and Mn oxides. Simulation shows that isotopic discrimination (Δ13C) between carbonate and organic carbon could be biased by increasing benthic Fe-Mn flux, leading to decreased Δ13C values in MRB intervals. Ce/Ce∗ values in the high-Mn carb interval reflect the releasing of Ce by reductive dissolution of Mn oxides. The commonly used carbonate-based redox proxy I/(Ca + Mg) ratio is correlated positively with Mn carb in the shallower section and with TOC in the deeper section, while it is negatively correlated with TOC in the shallower horizon, suggesting that iodine behavior may be influenced by adsorption and releasing of Mn hydro-oxides and organic matter besides oxygen contents. This study links MRB coloring to Fe and Mn mineral cycling in pore water through reductive dissolution and oxidative precipitation, and highlights potential biases of carbonate-based redox proxies that might be susceptible to other electron acceptors/donors such as Fe-Mn oxides and organic matter, in addition to free oxygen in seawater. [Display omitted] • Ordovician red limestone caused by submicron hematite in calcite inter-crystal pores. • Reductive dissolution of detrital provide Fe2+ for reprecipitation of hematite. • Fe-Mn oxides and OM can serve as e−acceptors and donors, influencing redox proxies. • Chemocline depth variation control the iron cycle below SWI. [ABSTRACT FROM AUTHOR]

Strata-bound dolomite occurs in deep-seated Ordovician and Cambrian sediments within the Huron Domain of southern Ontario, Canada. Core samples were analyzed for petrographic, stable and Sr isotopic composition, and fluid inclusion microthermometry to characterize dolomitization and evaluate diagenetic fluid composition. The samples represented a range of host rocks from dolomitized limestones, dolostones, sandy dolostones and sandstones within Ordovician Black River Group and underlying Cambrian formation. The petrographic and geochemical attributes have provided a basis to gain insight on the source fluids that modified these rocks, as well as, the possible timing of formation. Evidence indicates that the formations were subjected to high temperatures (average 115 °C) that cannot be explained by burial history alone. This suggests the occurrence and migration of hydrothermal fluids within the low permeability dolomite horizons, possibly during Paleozoic orogenesis. Dolomite and calcite fracture infill isotopic and fluid inclusion data point to two possibly individual diagenetic fluid systems; i) an earlier Cambrian system that is characterized by a pronounced negative shifts in oxygen and carbon isotopic composition (δ 18 O average −9.0‰ for dolomite and −13.9‰ for calcite, respectively; and δ 13 C −3.06‰ for dolomite and −4.82‰ for calcite, respectively), a more radiogenic (Sr 87 /Sr 86 ratios range from 0.70977 to 0.71100), warm (Th values range from 84 to 156 °C for dolomite; average 113.6 °C and 87–141 °C for calcite; average 101.5 °C) and saline signature (salinity range from 23.2 to 27.2 wt% NaCl eq for dolomite; average 24.3 for dolomite and 23.6 wt% NaCl eq for calcite); and ii) a later Ordovician system that is characterized by less negative shifts in both oxygen and carbon isotopes (δ 18 O average −8.7 ‰for dolomite and −7.5‰ for calcite; and δ 13 C average + 0.37‰ for dolomite and −0.36‰ for calcite, respectively), hypersaline (salinity range from 22.4 to 30.1 wt% NaCl eq.; average 27.0 for dolomite and 27.5 to 29.7 for calcite; average 29.2), comparable homogenization temperature (Th ranges from 85 to 132 °C for dolomite; average 109.6 °C and 66–153 °C for calcite; average 107.2 °C) and a less radiogenic (Sr 87 /Sr 86 ratios range from 0.70818 to 0.71 000) fluid system. The observation of highly discrete, strata-bound dolomites combined with only trace quantities of saddle dolomite and its associated geochemical signature suggest that diagenesis, as a result of hydrothermal fluids, was neither pervasive in volume or extent within the north western Huron Domain. [ABSTRACT FROM AUTHOR]

Thick Upper Cambrian-Lower Ordovician carbonate units in the Tahe Oilfield of northern Tarim Basin were extensively dolomitized during their diagenetic history. Petrographic examination identified three types of matrix dolomites, one type of cement dolomite, and one type of saddle dolomite in these units: micritic to very finely crystalline, nonplanar matrix dolomite (D1); finely crystalline, planar-s(e) matrix dolomite (D2); medium to coarsely crystalline, nonplanar-a matrix dolomite (D3); finely to medium crystalline, planar-e cement dolomite (Cd); and coarsely crystalline, planar-s to nonplanar saddle dolomite (Sd). Isotopic studies (C, O, and Sr), elements measurements, and fluid-inclusion analysis were combined to constrain the origins of dolomitizing fluids. The occurrence of near-micritic dolomite crystals, as well as isotopic compositions which are consistent with the values and ratios of Upper Cambrian-Lower Ordovician marine limestones, coupled with the rare earth elements (REE) patterns with negative Ce anomalies, collectively suggest precipitation of D1 dolomite at low temperatures of near-surface settings from mesosaline to penesaline seawater. D1 dolomite, characterized by high Mg/Ca ratios and MgO values, moderate Sr concentrations, and low Mn and Fe values, was formed in the early diagenetic stage and scarcely experienced recrystallization. The dolomitizing fluid of D2 dolomite was derived from connate seawaters resulting from mechanical compaction and burial dissolution under shallow burial conditions, which is indicated by textures, isotopic compositions, and REE patterns. The relatively low MgO contents and Mg/Ca ratios constrain the formation of the D2 dolomite to a closed diagenetic system with a limited local supply of Mg 2+ ions. The large overlaps of isotopic compositions between D3 dolomite and earlier dolomite, as well as the similar REE patterns and depleted Sr contents, indicate that D3 dolomite was likely the result of intense recrystallization upon previous matrix dolomites in a deeply buried formation-fluid system. The petrographic features and geochemical attributes of Cd dolomite strongly indicate progressive growth or recrystallization in the burial environment. Inclusions with high temperatures developed only in the outer edges of Cd crystals demonstrate diffusionally mediated recrystallization during the later hydrothermal activity. Sd dolomite is interpreted to directly precipitate from hydrothermal fluids, as evidenced by high homogenization temperature measured from fluid inclusions and high Sr contents. High salinity levels suggesting the existence of a very saline fluid (deep-basinal brine). The enriched radiogenic strontium isotope coupled with positive Eu anomalies indicate that hydrothermal fluids are likely to be crustal magmatic. Since Early Hercynian orogeny, long period tectonic activities were responsible for the hydrothermal alteration. High-angle faulting, which appeared as strike-slip faulting initiated along basement-rooted faults, likely acted as the important pathway that channeled the hydrothermal fluids from the depths. Initially, hydrothermal fluids with elevated pressure, high temperature, and low fluid/rock conditions were likely responsible for hydrofracturing and brecciation. When distal fluids of cooler temperatures flowed into permeable formations, the open fluid-dominated system with higher fluid/rock conditions facilitated the occurrence of dissolution vugs and precipitation of Sd dolomite. This study provides a basis for understanding the deep burial carbonate reservoirs in Tarim Basin by clarifying the process of dolomitization. [ABSTRACT FROM AUTHOR]

Abstract In order to meet the demand of shale oil and gas exploration, it is more and more important to study the controlling effect of sedimentary environment on the enrichment of organic matter. In this paper, the Lower Cambrian and Upper Ordovician-Lower Silurian shales in the Yangtze region are studied. Firstly, according to the content of Mo and TOC, the water closure property of the shale depositional period is judged. Then this study selected typical wells, and calculated quantitatively whether there was excess siliceous minerals in shale and the content of it. The origin of excess siliceous minerals is determined by Al, Fe and Mn element. The results show that the Lower Cambrian shale is deposited in a weak to moderate restricted water environment, and the Upper Ordovician and Lower Silurian shales are deposited in a strong restricted water environment in the Yangtze region. Excess siliceous minerals in the Lower Cambrian shale is of hydrothermal origin. On the one hand, hydrothermal activity can enhance the reductivity of the water bottom. On the other hand, it can improve the biological productivity, so that the sedimentary organic matter can be enriched. Excess siliceous minerals in Upper Ordovician and Lower Silurian shale is biogenic. The strong restriction of the water leads to stratification. The oxygen content in the upper layer makes the biological productivity higher and the lower layer more reductive, which is beneficial to the preservation of sedimentary organic matter. Highlights • The shales of two strate are deposited in differently enclosed water environment. • The enrichment of organic matter is controlled by the hydrothermal activity of the Early Cambrian. • The enrichment of organic matter is controlled by the enclosed water of the Late Ordovician-Early Silurian. [ABSTRACT FROM AUTHOR]

Petroleum exploration in many North African intracratonic basins targets Early Paleozoic sandstones as the primary reservoir objective. These sandstones are often characterized by highly variable reservoir quality (0.0001–1000 mD), and the ability to predict and selectively target areas of enhanced porosity and permeability is crucial to unlock the hydrocarbon potential. The objective of this study is to characterize the primary controls on reservoir quality in an Ordovician field in the Illizi Basin of Algeria through detailed core and petrographic analysis, and establish if variations in thermal history across the field have a material impact on reservoir quality. The best reservoir quality is observed in facies where primary intergranular porosity has been preserved in fine to coarse grained quartzarenites with less than 1% fibrous illite. These lithologies are most commonly found within the high-energy, tidally reworked, post-glacial facies sandstones of the uppermost Ordovician succession. Observed differences in quartz cement volume within compositionally and texturally similar samples from the southern and northern parts of the field are interpreted to reflect variations in thermal exposure due to deeper burial. This interpretation is supported by field-wide numerical modelling of sandstone diagenesis. This study indicates that subtle variations in thermal history can have a material impact on the spatial trends in reservoir permeability. Thermal history, therefore, is an important consideration in reservoir quality studies in exhumed basins where variations in present-day burial depth will be a poor guide for evaluating reservoir quality risk across a basin or play. [ABSTRACT FROM AUTHOR]

This paper presents a comprehensive analysis of sedimentary successions and paleoenvironmental indicators in two wells, Rh3 and Rh46, located in NE Jordan, within the context of the Ordovician-Silurian boundary transition. By integrating spectral analysis, cyclostratigraphy and sedimentary architecture, significant shifts and cyclical patterns are identified, shedding light on paleoenvironmental changes and orbital forcing dynamics. The study reveals distinct lithological and geochemical signatures at the Risha/Mudawwara (Hirnantian/Rhuddanian) boundary, indicative of major shifts in depositional environments. Analysis of Milankovitch cycles suggests orbital forcing as a driver of sedimentary cyclicity, with implications for understanding climatic variations during the Ordovician-Silurian transition. Spectral analysis revealed significant cycles with different periodicities. Testing the sediment accumulation rates (SAR) revealed an optimum SAR at 5.9 and 4.6 cm/kyr in Rh3 and Rh46 wells, respectively. A new refined astronomical time scale (ATS) is created based on the 100 kyr eccentricity cycle. The new ATS assigns the Risha Formation to the Hirnantian age and revealed a probable time gap at the base of Risha Formation of about 0.3 myr in Rh46 well; deposition of the Risha Formation commenced at 444.88 myr and 444.58 myr in Rh3 and Rh46 wells, respectively, with sedimentation continuing up to the Ordovician/Silurian boundary at 443.07 myr. The Ordovician/Silurian transition experienced shortening in the obliquity and precession cycles. The tuned data show that the 41, 36 and 31 kyr obliquity cycles, in addition the 21, 19 and 16 kyr precession cycles, were recognised. Furthermore, 740, 425, 250, 147 and 100 kyr eccentricity cycles were also recognised. Notably, the study identifies the glacial/fluvioglacial Risha Formation is bounded by g4-g3 eccentricity nodes coinciding with Hirnantian stage boundaries that correspond to significant geological events, such as the Late Ordovician Mass Extinction and the Hirnantian glaciation events. The g4-g3 eccentricity cycle is estimated to be 1.8 myr. Surprisingly, the lower Silurian OAE coincided with the g4-g3 eccentricity node at the Ordovician/Silurian transition. Additionally, the application of DYNOT modelling has elucidated sea-level fluctuations and their relationship with sedimentary sequences, highlighting the role of tectonic and climatic factors in shaping the sedimentary architecture. Furthermore, the investigation of Th/U and Th/K ratios provides insights into marine and terrestrial depositional settings. • Sedimentary Successions Analysis in NE Jordan's Rh3 and Rh46 Wells. • The Ordovician-Silurian boundary transition characterised by significant shifts and cyclical patterns. • Integration of spectral analysis, cyclostratigraphy, and sedimentary architecture..• Revelation of distinct lithological and geochemical signatures at Risha/Mudawwara boundary. • Milankovitch cycle analysis suggests orbital forcing as sedimentary cyclicity driver. [ABSTRACT FROM AUTHOR]

The Ordovician–Silurian transition represents a critical period marked by the formation of marine shale gas horizons within the South China Block. However, the mechanism for organic matter enrichment of Paleozoic marine shales in the western South China Block remain contentious, primarily due to insufficient understanding of paleogeomorphological evolution. In this paper, we describe the sedimentology of the Ordovician-Silurian succession composed of the Wufeng-Longmaxi Formation in the western South China Block and report new paleontological, and geochemical data for this succession to explore the relationship between basin paleogeomorphology and water mass environment. Nine graptolite zones are identified, spanning from the WF1 (Linxiang Formation) through WF2–WF4 (Wufeng Formation) to LM1–LM5 (Longmaxi Formation) in this well. Based on sedimentological and geochemical analyse results, three distinct stages of redox and paleoproductivity conditions in the water mass have been identified: (1) suboxic environments and low paleoproductivity in WF2–WF4; (2) euxinic and stagnant water conditions in LM1–LM3 accompanying with high paleoproductivity; and (3) anoxic water and low paleoproductivity in LM4–LM5. By integrating marine redox fluctuation, paleoproductivity conditions and regional hiatus (LM1-LM3) observed across multiple successions, we interpret thatthe western margin of the South China Block have been significantly affected by a collision between the South China Block and Yanbian terrane. The results of this study indicate that the accumulation of organic matter in the western margin of the South China Block was predominantly controlled by paleogeomorphology being caused by this collision event. [Display omitted] • Reconstruction of redox and productivity of the South China ocean during the Ordovician–Silurian transition. • Restricted epicratonic sea in the western South China Block is correlated by terrane collision. • Paleogeomorphology controlled organic matter accumulation of Silurian marine black mudstones. [ABSTRACT FROM AUTHOR]

Six types of carbonate platform marginal slope have been identified in the Cambrian-Ordovician of Tarim Basin. These include a progradational carbonate ramp platform, a progradational carbonate rimmed platform, an aggradational rimmed carbonate platform, a retrogradational rimmed carbonate platform, an aggradational margin on an isolated carbonate platform and a retrogradational margin on a submerged carbonate platform. Various platform margin architectures and reef-shoal characteristics occur in different areas of the Tarim basin. A progradational carbonate ramp platform margin began in the Early Cambrian in the Tarim basin. A progradational rimmed platform margin developed in the Middle Cambrian time in the northern Tarim basin, whereas contemporaneously a progradational ramp platform margin developed in the southwestern and central Tarim basin. A retrogradational rimmed carbonate platform developed in the northern Tarim basin, whereas an aggradational rimmed carbonate platform occupied the central Tarim basin in the Early Ordovician time. An aggradational margin on an isolated platform and a retrogradational margin on a submerged platform developed mainly during the Late Ordovician in the central Tarim basin. Platform margin-slopes migrated continually throughout the Cambro-Ordovician. The broad platform area (western Tarim Platform) that occupied most of the northwestern and central Tarim Basin in the Early Cambrian time was split by an east-west oriented shelf seaway that began to develop along the eastern and western margins of the platform in the Early Ordovician time and completely transected the platform by Late Ordovician time. The resultant central Tarim Basin platform area was much reduced, elongate east-west around the Central Tarim paleo-uplift, and terminated eastward towards region of the Late Ordovician basinal deposition across the eastern Tarim Basin. Affected by platform margin structure, Climate and paleodepth, algal reefs and shoals developed mainly in the Cambrian, and intraclastic shoals developed mainly in the Early Ordovician, followed by reef-building of framestone or bafflestone in the Late Ordovician. [ABSTRACT FROM AUTHOR]

The upper Katian (Upper Ordovician) Red River Formation is a carbonate-evaporite supersequence that formed in the epicratonic Williston Basin of western North America. This study focuses on the upper part of the formation in the southern, US part of the basin. The studied succession is made up of three transgressive-regressive sequences (RR1, RR2, and RR3), each ∼0.5–∼1 m.y. in duration and composed of relatively few meter-scale high-frequency cycles (parasequences). Sequence boundaries are characterized by intraclastic breccia-laminated dolomite successions that have been truncated and capped by evaporite deposits. Subaqueous anhydrites within each sequence represent evaporite Lowstand Systems Tracts (LSTs) that formed during relative sea-level lows that lead to the precipitation of gypsum (later recrystallized into anhydrite) in a quiet, salina-type environment. Transgressive (TST) and early Highstand Systems Tracts (HST) are composed of subtidal facies of predominantly skeletal mudstone to wackestone-packstone with abundant burrow mottling. Late HSTs are predominantly composed of peritidal, porous laminated dolomite. The study illustrates how the sequence stratigraphic framework of carbonate-evaporite deposits from the Williston Basin relates to the Late Ordovician glacio-eustasy, transitional climates, and low basin subsidence in an arid setting. [ABSTRACT FROM AUTHOR]

The siliceous rocks are well developed in Northwest Tarim Basin, which plays an important role in indicating its forming environment. In order to determine the origin of siliceous rock and its deposition settings, the paper studies the petrographic and geochemical characteristics of siliceous rock in well KP 1 and outcrops in Northwest Tarim Basin based on core observation, microscopic section identification and analysis of major and trace elements and rare earth elements. The Ordovician siliceous rocks of Northwest Tarim Basin are mainly developed in the middle and upper Penglaiba Formation and the bottom of the Yingshan Formation. Three types of cherts are identified, including cryptocrystalline silica, microcrystalline quartz and fibrous chalcedony depending on the mineral morphology. The terrigenous source is best evidenced by the high Ti (160∼310 ppm) and Al (1300∼3180 ppm), and positive δ30Si (0.9–3.6‰). Whereas, hydrothermal processes can be inferred from relative high content of V, Ni, Sr, Mo and Ba, and no obvious Ce and Eu anomaly. The chert of well KP 1 and Kuluxi profile is deposited in continental-margin settings, which is evidenced by its Al 2 O 3 /(Al 2 O 3 + Fe 2 O 3) (0.58–0.94, average of 0.81) and its La n /Ce n ratio (0.76–1.02, average of 0.93). The continental-margin settings are also evidenced by the close relation between silicification and dolomitization. Silicification is commonly well developed where dolomitization occurs, because the dolomitization mainly occurs in the regression background, where provides more terrigenous source silica to the sea water and results in more silicification. This study highlights the origin of Ordovician siliceous rocks and the outcome from this new contribution may provide useful guidelines for hydrocarbon exploration in the basin and other global analogues. • Based on the characteristics of main and trace elements and rare earth elements, it is concluded that the Ordovician siliceous fluid in well KP 1 is a fluid of mixed hydrothermal and terrigenous origin. • It is concluded that the precipitation form of siliceous fluid depends on the saturation degree and precipitation rate instead of the source of siliceous fluid. • The diagenetic coupling relationship between silicification and dolomitization is also studied. Since dolomitization occurs in the regression background, the decrease of sea level will result in the enhancement of terrigenous clastic weathering. Therefore, the formation with strong dolomitization is also characterized by strong silicification. [ABSTRACT FROM AUTHOR]

Thermal convection arises due to a density gradient between geothermally heated formation water within the underlying strata and the water in the overlying strata. The thermal convection model has long been accepted as a stable and long-lived flow mechanism for dolomitization, which can affect large volumes of rock. This study investigates the dolomitization of the Lower-Middle Ordovician strata in the Tarim Basin using petrological evidence, geochemical data, and the Calculation of Mg2+ flux. The Ordovician dolomites occurred in several types, including 1) individual dolomite crystals; 2) patchy dolomites; 3) layered-massive dolomites; 4) cement dolomites in pores and fractures. The dolomite content decreases vertically with decreasing burial depth. Geochemically, the dolomitizing fluids of the Ordovician carbonates are suggested as deep-formation water slightly influenced by hydrothermal fluids, with an obvious genetic connection with the underlying Cambrian dolomites. In addition, mass balance calculations were conducted to help identify the Mg2+ sources, supporting that the Mg-rich dolomitizing fluids were derived largely from the underlying strata. Large volumes of Mg-rich fluids were released when the underlying contemporaneous Cambrian dolomites were physically and chemically compacted. The released fluids were driven by heat from a high-temperature Early Ordovician magma chamber and migrated upward through fractures and/or permeable strata due to hydrothermal convection. These processes caused thermal convection dolomitization of the overlying Ordovician limestones from bottom to top. This study explains how these widespread dolomites developed in the Ordovician strata of the Tarim Basin, which can help identify the locations of the oil-gas reservoir rocks. In addition, this revised thermal convection model highlights the contribution of Mg-rich fluids derived from the underlying dolomites, and the circulation of fluids may have been driven by the heat from a magmatic source. This model is believed to be of significance to areas affected by abnormal geothermal activities. • Mass balance calculations were conducted to help identify the sources of Mg2+. • The revised thermal convection model highlights the contribution of Mg-rich fluids derived from the underlying dolomites. • The circulation of fluids may have been driven by the heat from a magmatic source. [ABSTRACT FROM AUTHOR]

Ultra-deep (7300 m–8900 m) condensed and volatile oil reservoirs have been discovered in the Ordovician reservoirs of the Shunbei oilfield, in the Tarim Basin. This study describes the petroleum accumulation process in the Shunbei oilfield by combining an analysis of its geochemical characteristics with seismic section observations and an analysis of petroleum filling events. Geochemical techniques are used to determine the sources, maturity, secondary processes, accumulation periods, and timing of petroleum filling. Analysis of fluid inclusion homogenization temperatures and burial-thermal histories indicates distinct petroleum filling events. Specifically, the north sub-section of the No. 5 fault zone experienced two petroleum filling periods during the late Caledonian and Indosinian epochs while the No. 1 fault zones and the mid-south sub-sections of the No. 5 fault zones experienced three distinct filling periods, encompassing the late Caledonian, Indosinian, and Himalayan periods. The CO 2 concentrations and δ13C CO2 values suggest the presence of secondary microbial gases, primarily thermogenic gases. Gas component analysis reveals a transition from kerogen degradation gas to oil cracking gas from north to south. The detection of intact n -alkanes and compounds of the 25-norhopanes series in oil samples suggests a mixture of early biodegradable oil with late fresh oil in the area. Seismic sections reveal diabase intrusions with strong axial reflections. Local hydrothermal activities during the Permian era rapidly cracked saturated hydrocarbon biomarkers, forming polycyclic aromatic hydrocarbons in the oil. Hydrothermal activity in the south sub-section of the No. 5 fault zone depleted the hydrocarbon-generating capacity of the underlying source rocks. Petroleum charged during the Indosinian period likely originated from potential petroleum reservoirs in the Cambrian system. This explains the comparatively low temperature of the SHB57X reservoir (163.32 °C) and the lack of strong hydrothermal activity in the late stage, but no biomarkers of the second-stage charged oil have been detected. Biomarker detection and gas genesis in the mid-south sub-section of the No. 5 fault zone and in the No. 1 fault zone suggest weak or no hydrothermal activity, with Cambrian source rocks maintaining their hydrocarbon generation capacity. Gas filling in the Himalayan period caused evaporative fractionation in the original reservoirs, resulting in a disproportionate (2-MH+2,3-DMP)/(3-MH+2,4-DMP), producing high K1 values in the light hydrocarbons in the present petroleum reservoirs. Differences in gas charging events in the Himalayan period led to the present distribution of petroleum phases. • Coupling of geochemistry with complex accumulation processes in ultra-deep reservoirs. • Validation of late Caledonian oil filling through 25-norhopane series compound detection and reservoir temperature evolution. • Confirmation of Permian hydrothermal disruption using biomarkers and diabase intrusion distribution. • Identification of Cambrian petroleum reservoirs through high K 1 values and the absence of biomarkers. • Control of area phase distribution by late gas filling. [ABSTRACT FROM AUTHOR]

The Tabei Uplift in the Tarim Basin of NW China represents an area with significant hydrocarbon exploration potential. This paper describes the effects of multi-phase faults on structural and hydrocarbon systems in the Tabei Uplift utilizing high-quality 3-D seismic, geological outcrops and drilling and production data. The interpretation and analysis of 10 fault systems from the Cambrian through the Cretaceous, shows that the deep, middle and shallow faults developed in four stages: the Early Caledonian, Mid-Late Caledonian, Hercynian and Himalayan. Specifically, the Early Caledonian fault system is favorable for the hydrocarbon expulsion of Cambrian source rocks, while Mid-Late Caledonian strike-slip faults facilitate hydrocarbon migration and accumulation. Hercynian and Himalayan strike-slip faults were formed as a result of the reactivation of the Mid-Late Caledonian strike-slip faults and have a destructive effect on the older oil reservoirs. Similarly, Permian volcanic activity also caused varying degrees of damage to Ordovician reservoirs. Oil production along fault zones more than 100 km show that the complex fracture-vuggy (FV) structure is more likely to trap oil and gas when it is damaged than the simple FV structure. Fault-controlled, fracture-vuggy carbonate reservoirs (FVCRs) were classified into six grades, which correlate to the productivity of oil wells in the region. Using this classification system, several drilling locations were selected and resulted in 80% success rate. This classification provides a novel approach for assessing the interplay between fault activity and reservoir formation, enhancing the prediction of hydrocarbon potential in varying strike-slip fault zones. This analysis not only refines our understanding of the region's geodynamic history but also aids in optimizing exploration and development strategies. • Geological and geophysical data document four Paleozoic and Mesozoic fault systems. • Deep faults are beneficial to hydrocarbon expulsion and shallow fault destroy cap rock. • A classification of faults that create traps closely aligns with the productivity of oil wells. • Correlation identified between fault activities and the deformation in multi-type fracture-vuggy reservoirs. [ABSTRACT FROM AUTHOR]

The paleokarst-unconformity at the top of the Ordovician Yingshan Formation in the central Tarim basin was exposed to air for at least 10 Ma, providing favorable conditions for the later formation of high-quality reservoirs. During the karstification process, the fault system plays an important role in controlling the development of paleokarst. This study characterized the fault system through the paleokarst features on the northern slope of the Tazhong High and examined in detail the impacts of the fault system on paleokarst distribution. Formation Micro-Imager logs and seismic curvature change rate were used for characterizing the fault system in different scales. The results revealed three sets of faults in this region. Cross-cutting relationships, unconformities, and relation between faults and karstification indicate Mid-to-Late Ordovician thrust faults, Silurian-Devonian strike-slip faults, and Permian tension faults. “Bright spots” in seismic records calibrated by Formation Micro-Imager logs were used for indicating paleokarst features and different depth distributions, respectively. Furthermore, the study employed spectral decomposition technique to characterize the morphology of paleocave complexes in detail. The results show the Mid-Late Ordovician thrust faults heavily impacted the distribution of paleokarst mainly distributed along master and secondary thrust faults in shallow areas, as well as along master basement-involved thrust faults in deep strata, and along the most pronounced area of paleokarst, Tazhong No. 10 fault zone bounded by back thrusts. The data provides new forecasting information for deep hydrocarbon exploration in paleokarst-related reservoirs of the Yingshan Formation. [ABSTRACT FROM AUTHOR]

This study aims to employ an integrated depositional and sequence stratigraphic approach to assess the control of diagenesis on reservoir porosity of a hybrid-energy delta system. The study focuses on examining the outcropped Upper Cambrian-Lower Ordovician Barik Formation representing a highstand system tract of a hybrid-energy delta in the Haushi-Huqf region of Central Oman, which is equivalent to the subsurface gas reservoirs within the Interior Oman Salt Basins. The petrographic, mineralogical, and geochemical assessments revealed that the Barik Formation primarily consists of moderately to very well-sorted coarse-grained siltstones to fine-grained sandstones, with a composition mainly comprisingfeldspathic- and subfeldspathic arenite and the reservoir porosity is controlled by various degrees of diagenetic processes. Eodiagenetic alterations comprise mechanically infiltrated clays and kaolinitization of silicate grains, including detrital feldspars, micas, and mud matrix. The mechanically infiltrated clays (e.g., smectite) were introduced into the sandstones by the percolation of muddy waters under the control of tidal pumps. Although these clays may affect reservoir porosity during eodiagenesis, they positively preserve reservoir porosity during deep burial (mesodiagenesis) by inhibiting cementation by feldspar and quartz overgrowths. Kaolitization of silicate detrital grains was facilitated by the influx of meteoric waters into the sandstones as the delta progrades during the highstand system tract. During progressive burial and lack of extensive eodiagenetic cement, compaction continued and reduced reservoir porosity. Illitization and chloritization of the mechanically infiltrated smectitic clays took place; however, they limited the cementation by quartz and feldspar overgrowths. The quartz and feldspar overgrowths on detrital grains with discontinued smectitic clay coats supported the detrital grains from further compaction and, thus, helped in reservoir porosity preservation. Moreover, extensive feldspar dissolution and kaolinitization has occured during mesodiagenesis and enhanced the reservoir porosity. Subordinate amounts of calcite, dolomite, and gypsum cement have a minor reduction in the reservoir porosity of the Barik Formation. Hence, it can be concluded that, on the one hand, lack of eodiagenetic cementation favored intense reservoir porosity reduction due to compaction. Nevertheless, on the other hand, the mechanically infiltrated clays during the eodiagenetic stage helped preserve the reservoir porosity by hindering the formation of quartz and feldspar overgrowths during the mesodiagenetic stage. This study may serve as an analog model for a similar hybrid-energy delta sandstone reservoirs to better understand the type, and spatial and temporal of diagenetic alterations and their role in controlling the reservoir quality in deeply buried sandstone reservoirs. • Eodiagenetic alterations in a hybrid energy delta system are linked to depositional facies and sequence stratigraphy. • Mesodiagenetic alterations in a hybrid energy delta system is controlled by eodiagenetic alterations. • Porosity deterioration and enhancement can be predicted within depositional facies and sequence stratigraphy. [ABSTRACT FROM AUTHOR]

The nature of the dolomitizing fluids and genesis of the Furongian-Lower Ordovician dolomites and dolomite cements are interpreted on the basis of petrography, geochemistry (trace elements, REEs, O-C-Sr isotopes) and fluid inclusion microthermometry. These carbonate strata were deposited on a shallow marine platform in the Tazhong Uplift in central Tarim Basin and were extensively dolomitized. Based on field investigations and petrographic examination, two types of fabric-retentive dolomitic textures (dolomicrites (DM) and microbial dolomudstone (DMS)) and three types of fabric-obliterated dolomitic textures (very fine- to fine-crystalline planar-e(s) dolomite (RD1), fine- to medium-crystalline planar-e(s) dolomite (RD2) and medium- to coarse-crystalline nonplanar-a, planar-e(s) dolomite (RD3)) are identified. Two types of dolomite cements (medium- to coarse-crystalline dolomite cement (DC1) and coarse- to very coarse-crystalline saddle dolomite cement (DC2) are identified. The REE patterns of the analyzed dolomites and calcite cements are characterized using seawater normalization standard. The DC2 dolomite cement and later-stage calcite cement (LSCAL) display positive Europium anomalies (δEu) and positive Cerium anomalies (δCe). Whereas the DM, DMS, RD1, RD2, RD3 and early-stage calcite cement (ESCAL) samples display positive δCe, light REE (LREE) enrichment and negative δEu. The DM and DMS were formed through seawater flux and mesosaline brine derived from evaporative seawater during early-burial seepage-reflux dolomitization and microbial dolomitization, respectively. The preferential occurrence of RD1 along low-amplitude stylolites suggest that their formation may have linked to pressure dissolution by which Mg-ions were probably released for replacement dolomitization during late early-burial to early shallow-burial dolomitization. Moreover, the tightly packed and curved crystal faces of RD2 and RD3 with irregular overgrowth rims and increasing crystal sizes presumably resulted from the recrystallization and replacement upon the RD1 and/or from the cortical overgrowth and dissolution of remaining precursor limestones by the influx of dolomitizing fluids during late shallow-burial dolomitization. The average range of the homogenization temperatures (Th) of CD2 is 132.4 °C–198.7 °C; and mostly higher than the estimated ambient temperature (95 °C–150 °C) for the Furongian-Lower Ordovician strata, implying that the growth of DC2 in the study area was the result of hydrothermal dolomitization. The comparable isotopic (δ18O, δ13C and 87Sr/86Sr) values between RD2, RD3, DC1, DC2 dolomites and coeval Neoproterozoic to Early Phanerozoic seawater values, in addition to the relatively high Th and salinity levels in these dolomites, indicates that their dolomitizing fluid were derived from radiogenic 87Sr-enrich coeval seawater during late shallow-burial dolomitization and hydrothermal dolomitization. The hydrotectonic driving model of burial dolomitization of the Furongian-Lower Ordovician carbonates in the Tazhong Uplift resulted from compressional tectonics that affected the Tarim Basin during Cambrian to Middle Ordovician and Late Ordovician period. This model is characterized by fluid flow in response to significant thermal heating, pressure-loading and enhanced fluid expulsion by squeezing, and the resultant higher-temperature Mg-rich connate waters and hydrothermal basinal brines are conveyed by convective-advective flows along permeable horizons, fractures and overthrust fault planes, resulting in extensive replacement dolomitization of carbonate materials and recrystallization of early formed dolomites; forming irregular networks of dolomites and calcite cements in the host carbonate rocks. • Dolomitizing fluids were coeval Lower Paleozoic seawater and hydrothermal basinal brines. • Collision tectonics, thermal and hydrotectonic driving convective-advective fluid flow. • Seepage-reflux, burial and hydrothermal dolomitization are proposed. • Intercrystalline pores, vugs and fractures are the main types of porosity in the dolomites. • Carbonate environments were restricted-open platform and platform margins. [ABSTRACT FROM AUTHOR]

The Late Ordovician-Early Silurian transition was a critical interval in Earth's history, marked by mass extinction, the Gondwana glaciation and volcanic events. Here, we present mineralogical and geochemical characteristics across the Ordovician-Silurian boundary (Wufeng, Guanyinqiao and Longmaxi formations) from the Huadi No. 1 well, in the Sichuan Basin, in order to explore the paleoweathering, provenance, tectonic setting and paleoclimatic conditions of the source area. Thin sections, XRD (X-ray diffraction) data and major element results reveal that the rocks were abundant in clay minerals, quartz, minor feldspar, pyrite and carbonate minerals. The samples have similar chondrite-normalized REE patterns to those of PAAS, which are characterized by slight LREE enrichments and accompanied by flat HREE trends with weakly negative Eu anomalies. Paleoweathering indices (CIA and PIA) suggest that the sediments experienced weak to moderate chemical weathering in the source area. CIA values in the lower Wufeng formation range from 67.11 to 72.17, indicating a warm and humid climate, the CIA values in the upper Wufeng formation decrease from 70.35 to 66.15, and the CIA values in the Guanyinqiao and lower Longmaxi formation are persistently low (56.64–62.87), suggesting a cold and dry climate, which is consistent with the widespread presence of glaciation in the Hirnantian period. Further upward in the upper Longmaxi formation, the CIA returns to a high value (65.59), which may reflect that the climate started getting warm and glaciation had finished. Fluctuations in CIA values reflect that the climate shifted from warm to cold and then to warm again during the end Ordovician-early Silurian time period. The A-CN-K triangular diagram and SiO 2 -Al 2 O 3 /TiO 2 , TiO 2 -Zr, and La/Th-Hf bivariate diagrams indicate that the provenance of the rocks from the three formations was primarily felsic igneous rocks. Discriminant-function diagrams and La-Th-Sc, Th-Co-Zr/10 and Th-Sc-Zr/10 triangular diagrams show that the clastic sediments of the Wufeng-Longmaxi formations were derived most likely from an active continental margin. Finally, across the Late Ordovician-Early Silurian, tectonic activity and the end of the Ordovician glaciation dominantly controlled the deposition of black shales and siltstones in the Wufeng-Longmaxi formations of South China. • CIA shows that climate shifted from warm to cold, and to warm during Late Ordovician-Early Silurian. • A-CN-K and elements diagrams indicate that the shales were primarily from felsic igneous rocks. • Tectonic activity and end Ordovician glaciation controlled the deposition of black shales. [ABSTRACT FROM AUTHOR]

Black shales deposited across the Late Ordovician and Early Silurian transition are considered to be the most important source rock and shale gas reservoirs in the Yangtze region of South China. However, the origin of these sediments remains contentious. In this study, we investigated the systematic changes in total organic carbon (TOC), organic carbon isotopes (δ13C org), major elements and trace elements on the Ordovician-Silurian boundary strata from Qiaoting, North Yangtze Sea, to reconstruct the climatic conditions, redox changes, primary productivity, and other factors there, promoting the understanding for the formation mechanism of the organic-rich black shales. Element compositions and their ratios suggest that the black shales studied here were dominantly sourced by the felsic igneous rock, which is similar to granodiorite in composition. Paleoclimatic proxies (CIA, CIW, δ13C org) suggest that the global climate system experienced significant changes from warm-humid to cold-dry and then to warm-humid climates during the Ordovician-Silurian transition. Paleoredox indices (S/C, U/Th, Ni/Co and V/(V + Ni) ratios) also demonstrate fluctuating redox variations from anoxic to oxygenated, and then to anoxic states during this interval in the Yangtze Sea, which describe an oxygenated Guanyinqiao strata sandwiched between the stagnant and anoxic Wufeng and Longmaxi black shales. Paleoproductivity parameters (TOC, Ba bio contents and P/Al ratio) suggest that the organic-rich Wufeng and Longmaxi black shales, were deposited with a high biological productivity, while the organic-lean Guanyinqiao sediments were deposited with a low biological productivity. These data demonstrate large climatic and oceanic fluctuations during the Ordovician-Silurian transition, providing essential controlling factors on the oceanic anoxia, primary productivity, and subsequent organic-rich black shale depositions in the Yangtze region during the Late Ordovician and Early Silurian intervals. • Climatic changes during Ordovician-Silurian transition on the Yangtze Platform. • The organic rich black shales are derived from a provenance region consisting of felsic igneous source rocks. • Redox fluctuations play a significant role in the organic matter accumulation. • Climatic-oceanic systems controlled the deposition of black shales. [ABSTRACT FROM AUTHOR]

Paleokarst systems are major carbonate reservoirs that can form significant and very large oilfields. The Ordovician carbonate reservoirs of the Tahe Oilfield represent a special type of hydrocarbon reservoir consisting primarily of paleokarst caves and fractures. Detailed characterization of paleokarst reservoirs in the Tahe area is challenging because of their heterogeneous distribution and, in particular, because these reservoirs are at depths exceeding 5500 m. This paper describes a detailed approach for integrating data from cores, imaging and conventional well logs, and high-quality seismic datasets to delineate the complex paleokarst systems in the Tahe Oilfield, particularly the paleocaves in the run-off zone. Based on the hydrologic genetic relationships between the karst and paleocaves observed in outcrops, we divided the paleocaves in the run-off zone into chambers, main channels, and branch channels. The chambers, which are generally taller than 15 m, are generally collapsed. The main channels, mostly 5–15 m tall, were the main passages for underground water flow and display good connectivity. The branch channels, typically less than 5 m tall, are relatively dispersed and still have a considerable volume. This method was applied to the area around Well T615, and two layers of paleocaves in the run-off zone were interpreted. Seismic mapping and three-dimensional modeling of the paleocaves were performed, and the lengths, widths, areas and volumes of these three types of paleocaves were statistically analyzed. Although most of the chambers have been penetrated by existing wells, the areas where the main channels and a dense network of branch channels developed are favorable targets for future drilling in the Tahe Oilfield. Our method of detecting and analyzing the paleokarst reservoir structure is applicable to similar paleokarst reservoirs. [ABSTRACT FROM AUTHOR]

The shale lithofacies shows obvious heterogeneity during the Late Ordovician-Early Silurian in the Upper Yangtze area, SW China. To elucidate the Wufeng-Longmaxi shale lithofacies' formation mechanism, we conducted high-resolution cyclostratigraphic analysis and geochemical studies of the W2 well. The findings indicate significant disparities in the sedimentation rates of the Wufeng-Longmaxi Formations, with both units recording the ∼1.2-Myr-long obliquity cycles and the ∼405-kyr-long eccentricity cycles period. Notably, the ∼1.2-Myr-long obliquity cycles predominantly influenced sea-level fluctuations by mediating latitudinal heat distribution between high and low latitudes during the Late Ordovician-Early Silurian. Additionally, leveraging the sedimentary noise model and core log data, we identified five distinct sequences (SQ1∼SQ5) based on reconstructed sea-level fluctuations. During the transgressive system tract (TST1) of SQ1, siliceous shale lithofacies association (S) was prominently developed. In the subsequent highstand systems tract (HST1) of SQ1, shifts in the depositional environment facilitated the enhanced development of calcareous shale lithofacies association (C). During the SQ2 period, the swift elevation of sea-levels facilitated the pronounced proliferation of siliceous shale lithofacies association (S). During the SQ3 and SQ4 period, lower sea-levels coupled with significant terrigenous contributions resulted in the well-developed mixed shale lithofacies association (M). Throughout the SQ5 period, intensified bottom currents and substantial clay material deposition favored the development of the argillaceous shale lithofacies association (CM) developed as the predominant lithofacies. Ultimately, our research reveals that the heterogeneity of shale lithofacies is a multifaceted response to a series of changes in the paleoredox environment, paleoproductivity levels, and terrigenous influx, all driven by sea-level fluctuations forced by astronomical orbits. Paleoredox conditions dictate biosilica's degradation and preservation, influencing sediment composition. Paleontogenic types and productivity levels set the sediment's mineral composition. Meanwhile, terrigenous input crucially impacts the accumulation of clay and clastic materials in the sediment. • Division of marine shale sequences using sedimentary noise model. • The ∼1.2-Myr-long obliquity cycles drive the glacial eustasy changes in the Upper Yangtze area. • Eustatic changes driven by Milankovitch cycles is the key for formation of shale lithofacies. • Sea-level changes control shale lithofacies development patterns. [ABSTRACT FROM AUTHOR]

Abstract A large, typical, marine, organic-rich shale is developed in the Ordovician Wufeng Formation-Silurian Longmaxi Formation in the Sichuan Basin, China, which has rich shale gas resources. This paper uses this type of shale as an example to analyze the influence of the depositional environment on the enrichment mechanisms and sedimentary processes of its organic matter (OM). Based on thin sections and field emission scanning electron microscope observations from well core samples, along with X-ray diffraction and geochemical analysis, the mineralogy, organic and inorganic geochemistry characteristics, as well as the lithofacies, are analyzed. The results show that the shale is mainly composed of quartz and clay minerals but also contains calcite, feldspar, pyrite, etc. The OM includes solid bitumen, algae, graptolite, spongy spicule, radiolarians, etc. The total organic carbon (TOC) content ranges between 0.5% and 6.17%, with an average value of 2.5%. Based on the mineral compositions, the TOC content and sedimentary structure, five kinds of lithofacies are identified. According to the primary productivity, redox conditions, paleosalinity and sedimentation rate characteristics, the formation's sedimentary environment is divided into five intervals. During the OM enrichment processes, the primary productivity, sedimentation rate and redox conditions affect the generation, accumulation and preservation of OM. There is a good positive correlation between primary productivity and TOC content, as well as the redox condition. Sedimentation rate is inversely proportional to TOC content. The change and evolution of the sedimentary environment over these five intervals form different OM occurrence characteristics and associations with lithofacies. Based on the characteristics of OM, lithofacies and the sedimentary environment, as well as the geological background of the study area, the mode of OM occurrence and enrichment was proposed. Highlights • There is a significant difference of OM occurrence and TOC in different lithofacies. • Five intervals of sedimentary environment have been divided. • Primary productivity, sedimentation rate and redox conditions affect the enrichment of OM. • Sedimentary environment affects the OM occurrence and lithofacies associations. [ABSTRACT FROM AUTHOR]

The Ordovician crude oils in the Fuman Oilfield usually show varying physical properties and phase states along a single strike-slip fault. This study collected nineteen crude oil and gas condensate samples from the fault zones (F I 17, F I 19, and F I 20) located in the eastern zone of the Fuman oilfield. Organic geochemistry methods were used to investigate the oil-source correlation, thermal maturity and secondary alternation processes of the samples, to reveal petroleum charge and accumulation models in ultra-deep reservoirs in the Fuman oilfield. The results showed that biomarkers of most crude oils have been modified by thermal stress, and the primitive geochemical information were distorted. Similar light hydrocarbon chromatogram characteristics and parameter ratios indicated that crude oils in the study area were generated from the same source rock. The carbon isotope of the whole oils (−31 ‰ to −33 ‰) and n -alkanes (−31 ‰ to −37 ‰) was comparable to the kerogen of the Lower Cambrian Yurtus Formation source rock (−30 ‰ to −37 ‰) considering both the carbon isotope fractionation between oils and kerogen and the effect of thermal maturity. The calculated equivalent vitrinite reflectance (%VRE) of oil and gas condensate based on adamantane, aromatic hydrocarbon and light hydrocarbon parameters ranged from 0.77 % to 1.44 %, suggesting a mature to highly mature hydrocarbon-generating stage. The lower dibenzothiophene content, 2-methylhexane/3-methylhexane ratios and relatively stable K 1 values from light hydrocarbons revealed that the oil and gas condensates fluids were not subject to significant modification by thermochemical sulfate reduction (TSR). The calculated n -alkanes loss based on the whole oil gas chromatography ranging from 8 % to 26 %, suggested that the oil and gas condensate in the study area underwent relatively weak gas washing. Additionally, the concentrations of 3-methyldiamantane (3-MD) and 4-methyldiamantane (4-MD) in the samples ranged from 40.42 μg/g to 121.43 μg/g, showing realtively low oil cracking degree (<40 %) except for the gas condensates from the F I 20 fault zone, whose oil cracking degree can reach 61 %. The comprehensive analyses in this study indicate that the differences in oil and gas physical property and phase state in the F I 17 fault zone were primarily controlled by the vertical connectivity of the fault and the differences of crude oil charging periods. The hydrocarbon generation occurred during the early Caledonian period, followed by migration and accumulation in the Hercynian period, and subsequent adjustment and transformation in the Himalayan period. This study could provide significant exploration guidance for further ultra-deep oil and gas exploration in the Fuman oilfield. • Crude oil and gas condensate in the F I 17 fault zone generated from the Cambrian Yurtus Formation. • The maturity of crude oils ranged from 0.77 %VRE to 1.44 %VRE, with a cracking degree of less than 40 %. • The crude oils did not undergo significant gas washing and TSR. • Vertical connectivity of faults and differences of oil charging periods affect the physical properties of crude oil. [ABSTRACT FROM AUTHOR]

Microstructural and textural measurements from two Ordovician shale units (Goldwyer and Bongabinni formations) within the Palaeozoic–Mesozoic Canning Basin indicate that the former unit was affected by mechanical compaction and clay mineral transformation whereas the latter preserves an early fabric due to syn-depositional precipitation of authigenic dolomite and anhydrite. Conventional petrographic analysis coupled with quantitative mineralogy, electron micro probe analyses, X-ray Texture goniometry (XTG) and cathodoluminescence spectroscopy of core samples were used to decipher the post-depositional evolution of marine and supratidal facies in the Goldwyer and Bongabinni formations, respectively. Differences in diagenesis are strongly reflected in the orientation of clay minerals as quantified by XTG: in both cases the c-axes of illite diffract strongest normal to the bedding plane but the measurements clearly illustrate that shale in the Goldwyer Formation has a stronger preferred orientation relative to the Bongabinni Formation, with multiple of random distributions (m.r.d.) values of 5.77 and 2.54, respectively. Laboratory measurements conducted at 10 MPa effective stress also indicate distinct rock physics signatures: the Bongabinni Formation shows very low anisotropy, whereas the Goldwyer Formation displays a higher degree of elastic anisotropy in terms of both P- and S- waves. The crystallographic preferred orientation of illite, highlighted by the XTG, is likely to contribute to the significant difference in elastic anisotropy observed in the two units. Therefore, the Bongabinni Formation is mechanically stronger and stiffer than the Goldwyer Formation, likely due to the early dolomite and anhydrite cementation of the former providing a rigid microstructure framework. [ABSTRACT FROM AUTHOR]

Abstract: The Catoche Formation of the St. George Group in eastern Laurentia (western Newfoundland) consists of early Ordovician (Arenigian) shallow marine platform carbonates (∼160 m thick), which were extensively dolomitized during the course of their diagenetic history. The dolomites occur as both replacement and pore-filling cements and are a major control on porosity distribution in the formation. The origin and diagenetic history of the Catoche dolomites at Daniel''s Harbour (western Newfoundland) were analysed in comparison to equivalent successions at Port au Choix (PaC) and Port au Port Peninsula (PaP) to assay the reservoir potential of these dolomites in eastern Laurentia. Petrographic examination identified at least three generations of dolomites in the Catoche Formation, which are: (1) an early replacement sub- to eu-hedral micritic dolomite (<4 μm–30 μm, D1), (2) eu- to sub-hedral dolomite (70 μm–1 mm) often with cloudy cores and clear rims (D2), and (3) subhedral to anhedral saddle dolomite cement (200 μm–3 mm, D3). The micritic dolomite (D1) exhibits a dull cathodoluminescence (CL) under cathodoluminoscope, whereas dolomite D2 exhibits consistent concentric CL zonation. Some subhedral crystals of D3 appear zoned both in plane polarized light and cathodoluminoscope, otherwise D3 exhibits a dull CL. Stoichiometric dolomite occurs in all three generations with D2 as the dominant dolomite by abundance. The low strontium (47 ± 25 ppm) content coupled with depleted δ18O value of dolomitizing fluids (−10 to −11.2‰ VSMOW) and near-micritic grain size, suggests an early precipitation of dolomite D1 at low temperatures of near-surface conditions from solutions likely formed by mixing of early Ordovician sea and meteoric waters. In contrast, microthermometric measurements of primary two-phase fluid inclusions in dolomite D2 (homogenization temperatures of 102–168 °C with a salinity range of 19.8–25 eq wt% NaCl) and dolomite D3 (homogenization temperatures of 158–190 °C with a salinity range of 20.2–22.2 eq wt% NaCl), suggest that both dolomite generations were generated in mid to deep burial settings from high salinity, low temperature (<200 °C) hydrothermal fluids likely under suboxic conditions. This is consistent with the low Sr concentrations for D2 (36.4 ± 8 ppm) and D3 (38.7 ± 9 ppm), δ18O values of dolomitizing fluids for D2 (+2.1 to +8.1‰, VSMOW) and D3 (+6 to +8.1‰ VSMOW), coupled with Fe contents of D2 (1684 ± 1096 ppm) and D3 (1783.7 ± 618 ppm) as well as the respective Mn (D2 = 131.2 ± 50 and D3 = 197.5 ± 55 ppm) concentrations. ΣREE and shale normalized (REESN) values of Catoche carbonates indicate enrichment in rare earth element (REE) composition of the earliest calcite (C1) relative to those of Arenig seawater, whereas the REESN profiles of the dolomite generations mimic that of calcite C1. The Ce (Ce/Ce*)SN and La (La/La* = Pr/Pr*)SN anomalies of the Catoche dolomites are consistent with precipitation in equilibrium with source fluids in slightly oxic to suboxic conditions whereas Eu (Eu/Eu*)CN anomalies suggest similar source fluids for D2 and D3. Results of fluid inclusion gas analysis are consistent with petrographic features and geochemical compositions and support the exclusion of magmatic fluids during dolomitization. Visual estimates of porosity (ϕ) from thin sections indicate four porous (ϕ = 4–12) horizons. Vugs and intercrystalline pores are two types of porosity associated with the dolomites with the latter being the dominant type and associated mainly with dolomite D2. [Copyright &y& Elsevier]

Knowledge of the paleoenvironment is crucial to understand organic matter enrichment and shale gas development. Like organic matter, siliceous microfossils such as radiolarians and sponge spicules are widely distributed in Late Ordovician to early Silurian shales. However, there are few characterizations of the depositional conditions of siliceous microfossil-enriched layers. Here siliceous microfossil-rich shales from the Ordovician–Silurian transition in South China were selected for this study. We used major/trace element geochemistry to determine the sedimentary environment of siliceous microfossil-rich shale and to strengthen the understanding of the effect of siliceous microorganisms on organic matter enrichment. Our results show that most siliceous microfossils were deposited in anoxic reducing conditions with the presence of free hydrogen sulfide and accompanied by a moderately to weakly restricted seawater circulation. In addition, the nutrient-rich water bodies could upwell from the deep sea into the Yangtze Sea, and siliceous microfossil-rich shale was deposited during a period of high primary productivity. The shale layers of siliceous microfossils generally correspond to high gas-producing intervals and high organic matter abundance. The accumulation and preservation of organic matter in siliceous microfossil-rich shales is attributed to anoxic benthic conditions, high levels of primary productivity and flourishing of siliceous microorganisms. This study addresses the knowledge gap of the depositional conditions in siliceous microfossil-rich shale, and can help better understand the history of organic matter accumulation. • Siliceous microfossils were mostly deposited in an anoxic environment with H 2 S. • Siliceous microfossil-rich shale was deposited under high levels of productivity. • Organic-enrichment is controlled by depositional conditions and Si-microorganisms. [ABSTRACT FROM AUTHOR]

Source rock studies are one of the key issues of petroleum exploration activities. In the supercontinent of Gondwana, ice ages related to the Upper Ordovician (Hirnantian) and rising sea levels caused by glacial melting at the end of the Ordovician and Early Silurian (Llandoverian) created excellent source rocks along the margin of Gondwana. Investigations conducted in the Arabian Peninsula have been indicated indicating that the lower Qalibah Formation (the so-called Qusaiba Member or Hot Shale) is a good source rock for the Paleozoic petroleum system in this area. Likewise, the Sarchahan Formation was recently introduced as a source rock in the Zagros Basin of Iran, which is probably equivalent to the Qalibah Formation in the Arabian Peninsula. In this study, samples were prepared from surface and subsurface Paleozoic rock units in Iran's Zagros Basin. The emphasis of the paper was on the Sarchahan Formation in Kuh-e Faraghan, ranging in age from the Late Ordovician (Hirnantian) to Lower Silurian (Llandoverian) to determine whether the high richness of organic matter in the Sarchahan Formation is related to the Late Ordovician or Lower Silurian. The basal part of the Sarchahan Formation belongs to the Late Ordovician (Hirnantian) because of the presence of the persculptus graptolite biozone, while the remainder belongs to the Lower Silurian. The Ordovician and early Llandoverian parts of the Sarchahan Formation contain type II and III kerogen with TOC ranging from 2.94 to 7.19, but the rest of the Sarchahan Formation (late Llandoverian) has TOC ranging from 0.1 to 0.58. Therefore, the Hot Shale in Iran falls within the Hirnantian and early Llandoverian (Rhuddanian), and not the latest Llandoverian (Aeronian and Telychian). Utilizing organic petrography, kerogen type was found II/III. The carbon stable isotope studies revealed that the source rock of hydrocarbons in Dalan and Kangan reservoirs has been the Sarchahan Formation. Based on analytical data, the kerogenous shales in the lower part of the Sarchahan Formation are at end of gas window, and the gamma ray amount is approximately 180 API. This research indicates the differences between the source rocks in the southern and northern Persian Gulf and suggesting, the Hot Shale should be considered in different views and used in modeling studies of sedimentary basins for future exploration targets. [ABSTRACT FROM AUTHOR]

The organic-rich Upper Ordovician sediments (Wufeng and Guanyinqiao Formations) on the Yangtze platform are considered to be one of the main source rocks. Here we present geochemical proxies, including redox indicator (S/C ratios and sulfur isotopes) and productivity indices (TOC, Mo and Ba contents), from Nanbazi section in North Guizhou province, South China, in order to investigate the mechanism of organic matter accumulation. The geochemical data suggest a stagnant and anoxic environment predominated the Yangtze Sea during the Wufeng period, whereas ventilated and oxygenated marine conditions pervaded the Yangtze Sea during the Guanyinqiao period. Variations in the concentration of TOC, Mo and Ba indicate that higher organic carbon export in the Wufeng intervals than those in the Guanyinqiao intervals. These variations in redox and productivity during the late Ordovician were associated with different mechanisms and forcing processes. The abrupt change from anoxic to oxygenated condition at the beginning of the Guanyinqiao was concomitant with the global glacial period, likely resulted from the glacio-eustatic sea-level fall and subsequent circulation of cold, dense oxygenated waters upon the shelf seabed. The productivity variations were related to the change of nutrient supply, which is consistent with volcanic activities and runoff to the Yangtze Sea. Redox changes, together with primary productivity fluctuations could have played a significant role on the variation of organic matter accumulation during the late Ordovician in South China. [ABSTRACT FROM AUTHOR]

Abstract The Middle Ordovician Majiagou Formation in the eastern Ordos Basin (central China) represents a typical case of karst in gypsum-bearing carbonate strata. It is unique in a long karst history of ca. 120 Ma since the deposition to the Late Carboniferous. The lithology is complex with the development of both fine-grained and granular dolomites. Thus, the karstification and its impact on reservoir quality is challenging. To improve the understanding of this critical issue and further provide data for exploration, we collected core samples within the representative first and second sub-members of the fifth Member of the Majiagou Formation (herein referred to as Ma 5 1+2) and conducted comprehensive analyses of their physical and geochemical properties. Results show that Ma 5 1+2 is generally composed of gypsum-bearing micritic dolomites and grain dolomites. Karst breccias were observed and two periods of karstification were recognized. The first period is syngenetic and shows typical features such as gypsum molds and intragranular pores. It is predominantly developed within gypsum-bearing micritic dolomites due to selective fabric dissolution. Isotopic data indicate the influence of syngenetic meteoric water and seawater, as indicated by slightly depleted δ13C values (average −0.03‰), relatively low δ18O values (average −8.35‰), and slightly elevated 87Sr/86Sr values (average 0.70999). The second period is interpreted as inland eogenetic karstification and shows typical features such as non-selective fabric dissolution, including pores and vugs, breccias, and karst fillings in grain rocks. The rocks were subjected to differential dedolomitization. Grain dolomites were strongly altered by meteoric water and record a terrestrial input, as suggested by negative carbon and oxygen isotope excursions (average δ13C and δ18O values are respectively −3.07‰ and −8.97‰) as well as high 87Sr/86Sr values (average 0.71093). The two periods of karstification had a profound and heterogeneous influence on the reservoir quality. Gypsum molds formed due to the alteration of gypsum-nodule-bearing micritic dolomite by early syngenetic karstification, and represent the best reservoir space in Ma 5 1+2. This reflects the key role of syngenetic karstification in the formation of high-quality reservoirs. In contrast, the late inland eogenetic karstification is mainly manifested as strong alteration of grain dolomites, with little effect on micritic dolomites due to its relatively poor physical properties. The collapse resulting from strong dissolution destroys the continuity of reservoirs. In addition, relatively high porosity and permeability promote fluid movement, resulting in the infilling of the karst cave system by mud and carbonate sand. Thus, a so-called inversion of physical property occurs, namely the filling of original large pores in the grain dolomites leads to poor reservoir at last, and the preservation of initially small pores in the micritic dolomites leads to relatively good reservoir at last. This indicates that the evolution of reservoir is complex under the influence of karst. Highlights • Syngenetic and eogenetic karsts were identified in the Middle Ordovician of the Ordos Basin. • Syngenetic karst was featured by selective fabric dissolution. • Eogenetic karst was featured by facies-controlled non-selective fabric dissolution. • The two periods of karstification resulted in the inversion of physical properties. [ABSTRACT FROM AUTHOR]

The Lower-Middle Ordovician shallow-marine carbonates in western Tarim Basin were dolomitized to different degrees. The resultant dolomites are mainly concentrated in the successions of platform interior to shallow subtidal shoal, and display an upward decrease in abundance. Based on petrographic textures, three types of matrix dolomites were identified: (1) very finely to finely crystalline, nonplanar-a to planar-s dolomite (Md1), (2) finely to medium crystalline, planar-e(s) dolomite (Md2), and (3) finely to coarsely crystalline, nonplanar-a dolomite (Md3). Origins of matrix dolomites are interpreted on the basis of field investigation, petrography and isotopic (O, C and Sr) geochemistry. Md1 dolomite was formed from slightly evaporated (i.e., mesosaline to penesaline) seawater in near-surface to very shallow burial settings, possibly facilitated by microbial mediation (i.e., bacterial sulphate reduction; BSR). Md2 dolomite was precipitated from connate seawater stored in coeval limestone in shallow burial realms. In contrast, Md3 dolomite was likely formed as a consequence of intense recrystallization of early-formed dolomites (e.g., Md1) at depths. The formation of early matrix dolomites (i.e., Md1) was likely determined by depositional facies and sea-level variations of variable orders. This study provides a useful analogue to understand the dolomitization processes related to depositional facies and variable-order sea-level fluctuations in the Tarim Basin and elsewhere. • Dolomitizing fluids were derived from slightly evaporated seawater and connate coeval brines. • Early dolomite was formed by the reflux of mesosaline to penesaline seawater. • Late dolomite likely resulted from intense recrystallization of early-formed dolomites. • Early dolomite development was likely determined by depositional facies and sea-level variations of variable orders. [ABSTRACT FROM AUTHOR]

Rock-Eval pyrolysis and molecular organic geochemical analyses as well as organic petrographic observations were carried out on a rare black shale xenolith core drilled from a Late Jurassic-Early Cretaceous kimberlite on the Hall Peninsula in southern Baffin Island, Canada. The results are compared with those of outcrop shale samples from the Upper Ordovician Amadjuak Formation, the only organic-rich shale deposit discovered so far in the region. The shale xenolith is found to be organic-rich and thermally mature at the stage of peak oil generation, and contain type II organic matter similar to that of the immature Amadjuak shale. Discounting the differing thermal maturation effect, similar depositional environments and original organic inputs are suggested for both the shale xenolith and the Upper Ordovician shale based on their bulk, molecular and isotopic signatures as well as organic maceral assemblages. The present study cannot exclude the possibility of the shale xenolith being originated from an organic-rich Lower Silurian deposit, as suggested by previous Re Os isotope dating study. It is clear, however, that depending on their areal distribution and thickness, the lithostratigraphic unit from which the organic-rich black shale xenolith is derived would make an excellent petroleum source rock in the study area. • Black shale xenolith in a Jurassic-Cretaceous kimberlite on Baffin Island is very organic-rich. • It likely originated in a depositional environment similar to that of the organic-rich Ordovician shale from the region. • Organic matter in the shale xenolith is originally Type II kerogen. • The shale xenolith is thermally mature at peak oil generation. [ABSTRACT FROM AUTHOR]

Black shales in the Holy Cross Mountains area of Poland provide a record of environmental change across the Ordovician–Silurian boundary. The changing depositional conditions have generated a variation in organic matter contents above and below the boundary. Investigating the organic constitution of these black shales has the potential to reveal how their organic matter contents were generated and how suitable these rocks and their lateral equivalents may be for exploitation as shale gas reservoirs. One outcrop at the Holy Cross Mountains with a continuous section across the Ordovician–Silurian boundary occurs near the village of Bardo Stawy in the Kielce region, and contains sandy-silty mudstones, as well as grey and black shales. Organic geochemical analyses of samples at Bardo Stawy reveals low Total Organic Carbon (TOC) contents for Ordovician samples and higher TOC values for Silurian samples. Organic biomarkers indicate that the Ordovician rocks were deposited in a shallow-marine shelf setting, while the Silurian rocks were deposited in a deeper marine environment. The progressive increase in TOC from the uppermost Ordovician to the lowermost Silurian rocks reflects increasingly oxygen-poor depositional conditions during the post-glacial transgression. Following the deposition and preservation of organic matter in the Ordovician and Silurian rocks, these rocks were buried and subjected to thermal maturation. Rock Eval and biomarker thermal maturity parameters all indicate that the organic matter is mature and lies within the oil window. The Ordovician and Silurian shales have direct relevance to recent attempts to discover and exploit shale gas reservoirs in Poland. Our data and interpretations suggest that the relatively low TOC values (<2%) and low maturities for gas generation render these rocks unsuitable for commercial shale gas production. The progressive improvement in conditions for preserving organic matter across the Ordovician–Silurian boundary does, however, leave the possibility that more suitable deposits occur in Early Silurian rocks in the other parts of the basin. [ABSTRACT FROM AUTHOR]

Silica diagenesis has been proved to significantly affect rock composition and fabric. However, it still needs to be clarified how silica diagenesis affects the mechanical properties of shales. To further investigate this impact, we collected shale samples from the Upper Ordovician Wufeng and the Lower Silurian Longmaxi formations, southeast Sichuan Basin. The total organic carbon (TOC) content, mineralogical composition, and major oxide concentrations were measured to characterize the shale composition. Thin-section observations and high-resolution imaging techniques were used to characterize the rock fabric and identify quartz types. The Young's modulus, Poisson's ratio, and the measured hardness were used to characterize the shale mechanical properties. Four major lithofacies were identified, namely, siliceous shale, siliceous-argillaceous mixed shale, silty mudstone, and argillaceous shale. Three types of authigenic quartz in the Wufeng-Longmaxi formations were observed, namely, quartz overgrowth, matrix-dispersed microquartz, and microquartz aggregates. Matrix-dispersed microquartz is typically mixed with clay matrix, while microquartz aggregates typically coexist with migrated organic matter. The combined analysis of Fe–Al–Mn ternary diagram, cross-plots of Si versus Al, Rb versus K 2 O, and TOC content versus excess silica demonstrate that the dominant silica source of quartz cementation is biogenic. The cross-plot of excess silica contents versus Young's modulus, Poisson's ratio, and hardness suggest that silica diagenesis significantly affects the mechanical properties of shale rocks. Microquartz aggregates greatly contribute to shale brittleness, while detrital quartz minimally affects shale brittleness if the detrital quartz was enclosed by ductile clay minerals. • Four quartz types were identified in the Wufeng-Longmaxi shales. • Silica cementation could significantly enhance shale rock mechanical properties. • The organic-rich siliceous shale typically manifests the best brittleness behavior. • The evolution patterns of silica diagenesis in different lithofacies were discussed. [ABSTRACT FROM AUTHOR]

The characteristics of Ordovician paleokarst reservoirs in the Tahe Oilfield are complex. Many prediction studies on the distribution of karst fractures and caves have been carried out, but the filling types and distribution characteristics of caves have been less studied and are thus less understood. First, the development characteristics of the early paleohydrological landforms in the early Hercynian are considered, the paleokarst landforms are restored, the characteristics of the paleohydrological network are clarified, and the lateral distribution of the surface and groundwater systems is illustrated. Additionally, based on a qualitative analysis of core thin section photographs, conventional logging curves, full-bore formation microresistivity scanning, imaging, logging and other characteristics, the existing filling types are divided into four classes: vertical flow filling, river transport filling, cave collapse and hydrothermal transformation. Moreover, reservoir facies division standards for the different filling types are established. Different filling types are constrained in the context of paleogeomorphology and paleohydrology to further study the filling effects and the corresponding controlling factors. As research progresses, five common joint control modes are proposed: joint control of sinkholes and underground rivers, joint control of ancient landforms and underground rivers, joint control of multilayer caves and ancient landforms, control due to collapse and fracture, and joint control of early collapse and late hydrothermal transformation. Based on the model, the relevant mechanisms and the concentrated areas of development are analyzed in depth. The distribution map of the plane filling type is obtained by combining the development model with the identification of the filling type. The relationships between oil and gas production and the reservoir filling degree and between oil and gas migration channels are analyzed by the classification and statistics of different filling types laterally, and the relationships between oil and gas production and the reservoir filling degree and between oil and gas migration channels are analyzed. Hydrothermal action has the strongest ability to transform reservoirs, mainly due to hydrothermal dissolution and fault zones transporting oil and gas; the yield of caves filled with sinkholes is relatively low, which is related to the good reservoir space retained by oil and gas filling and the migration channels provided by fault zones. The yield of the underground river filling type is lower than that of the water-falling cave filling type, mainly because the underground river filling type is located in the karst slope and lower part of the basin. The water energy in the karst slope is relatively strong, and the interior of the cave is filled with medium-fine sandstone. In the lower part of the karst zone, the water energy becomes weak; both of these conditions lead to further filling of the cave reservoir space and a lack of migration channels. The collapse filling type has the strongest impact on the reservoir because this discussion is mainly about late collapse, which causes the cave to collapse, and the internal filling is severe; even if a fault zone acts as a migration channel, oil and gas production is relatively low. • The filling type in the cave is redefined according to the five effects of landform height, vertical dissolution, river transport, collapse and hydrothermal fluid. • Through different filling types, five reservoir development models were studied. • The relationship between different filling patterns and yield was discussed under the background of landform and hydrology. • The conclusion that the hydrothermal effect has the strongest ability to transform the reservoir is confirmed. [ABSTRACT FROM AUTHOR]

Late Ordovician pre-glacial and syn-glacial deposits form economically important hydrocarbon reservoirs throughout North Africa and the Arabian Peninsula. Internal subdivision of the unit is difficult due to the short time span of deposition resulting in coarse biostratigraphic control. The arenaceous character of the sediments has also contributed to poor biostratigraphic recovery. The present study focuses on the chemostratigraphic correlation of this interval encountered in ten wells across a complex, multi-phased pro-glacial outwash fan dominated by hyperpycnal (jet-efflux) turbidites fed by glacial outburst floods. The high resolution reservoir layering scheme presented here utilizes detailed sedimentological core descriptions, borehole image-derived palaeoflow indicators, 3D seismic attributes and chemostratigraphy. The Inductively Coupled Plasma – Optical Emission Spectrometry (ICP-OES) and Inductively Coupled Plasma – Mass Spectrometry (ICP-MS) techniques were used to acquire data for 50 elements, with the chemostratigraphic scheme based on changes in the following ‘key’ element ratios: Zr/Y, Zr/Lu, Th/Nb, Th/P, Ta/P, Cr/P, P/Lu, Th/Ti, Cr/Yb and Th/Ta. G may give the impression that there are two principal aims are defined by distinct values of key element ratios, with variations in these mainly reflecting changes in source/provenance. In order to avoid complications relating to changes in grain size/lithology, separate schemes were proposed for sandstone and mudrock lithologies, with the former comprising a hierarchical order of two zones, three subzones and seven divisions. Owing to the lower number of analyzed mudrocks, the scheme produced for this dataset is less detailed but, nevertheless, consists of three principal zones and five subzones. At the formation level, chemical stratigraphy allowed for consistent identification and correlation of the pre-glacial Qasim, syn-glacial Sarah and post-glacial Qalibah formations. The earliest post-glacial unit referred to as the Uqlah Formation has an inherited signature from the syn-glacial chemozone (i.e. uppermost Sarah Formation) below, and represents initial reworking of the post-glacial landscape following isostatic rebound. Evidence for glacial isostasy is recorded in a regional deformed zone at the top of the Sarah Formation and immediately beneath an isostatic rebound unconformity at the base of the Uqlah Formation. Chemical zonation of this unit clearly illustrates significant local truncation in wells located close to NW-SE trending lineaments. It is proposed that chemozones are missing across reactivated fault block highs as a result of increased erosion on the shallow marine post glacial shelf. High resolution reservoir layering of the hydrocarbon-bearing syn-glacial Sarah Formation highlights a complex depositional system characterized by an initial phase of deeply entrenched pro-glacial turbidite channels and younger sheet-like depositional lobes. Cores and image logs from both elements of the outwash fan system record extensive climbing dune cross-bedding, an unusual sedimentary structure described from many Upper Ordovician glacial successions in North Africa, and considered indicative of quasi-steady sediment-charged flows initiated by glacial outburst floods. There is no shared chemical link between the lowest channels and overlying lobes, suggesting a large fan system may be found downdip of this deeper channel system. Integration of the zonation scheme with 3D seismic attribute analysis clearly illustrates a second and even third phase of small channels cross cutting the lower larger vintage that fed the sheet-like younger lobe system. This is substantiated by a shared chemical signature and provenance between these small channels and their attached lobes. [ABSTRACT FROM AUTHOR]

Thick Upper Cambrian-Lower Ordovician carbonates were deposited on a shallow marine platform in the northern Tarim Basin, which were extensively dolomitized, particularly for the Upper Cambrian carbonates. The resulting dolomite rocks are predominantly composed of matrix dolomites with minor cement dolomites. Based on petrographic textures, matrix dolomites consist of very finely to finely crystalline, nonplanar-a to planar-s dolomite (Md1), finely to medium crystalline, planar-e(s) dolomite (Md2), and finely to coarsely crystalline, nonplanar-a dolomite (Md3). Minor cement dolomites include finely to medium crystalline, planar-s(e) dolomite (Cd1) and coarsely crystalline, nonplanar saddle dolomite (Cd2), which partially or completely fill dissolution vugs and fractures; these cements postdate matrix dolomites but predate later quartz and calcite infills. Origins of matrix and cement dolomites and other diagenetic minerals are interpreted on the basis of petrography, isotopic geochemistry (O, C and Sr), and fluid inclusion microthermometry. Md1 dolomite was initially mediated by microbes and subsequently precipitated from slightly modified brines (e.g., evaporated seawater) in near-surface to very shallow burial settings, whereas Md2 dolomite was formed from connate seawater in association with burial dissolution and localized Mg concentration (or cannibalization) in shallow burial conditions. Md3 dolomite, however, was likely the result of intense recrystallization (or neomorphism) upon previously-formed dolomites (e.g., Md1 or Md2 dolomite) as the host carbonates were deeply buried, and influenced by later hydrothermal fluids. Subsequent cement dolomite and quartz crystals precipitated from higher-temperature, hydrothermal fluids, which were contributed more or less by the extensive Permian large igneous province (LIP) activity in Tarim Basin as evidenced by less radiogenic Sr in the cement and parts of matrix dolomites. This extensive abnormal hydrothermal activity could also have resulted in recrystallization (or neomorphism) on the previous matrix dolomites. Faults/fractures likely acted as important conduit networks which could have channeled the hydrothermal fluids from depths. However, the basin uplift triggered by the Late Hercynian Orogeny from the Late Permian would have facilitated downward infiltration of meteoric water and dilution of hydrothermal fluids, resulting in precipitation of later calcites in which lighter C and more radiogenic Sr components demonstrate such a switch of fluid properties. This study provides a useful analogue to understand the complicated dolomitizing processes and later hydrothermal alteration intimately related to the Permian LIP activity within Tarim Basin and elsewhere. [ABSTRACT FROM AUTHOR]

In this paper, we analyse the Ordovician condensate in the Tazhong area of the Tarim Basin, NW China, using a new high-resolution comprehensive two-dimensional gas chromatography with time-of-flight mass spectrometry (GC × GC-TOFMS). Results show that 4778 compounds at a signal-to-noise ratio of more than 100 were detected, reflecting the advantage of this method in the analysis of condensate. Abundant adamantane series compounds and some diamantane series compounds, as well as a series of sulphur-containing compounds (including the benzothiophene series, and the dibenzothiophene series) were detected for the first time in the study area. Combined with the results of a study on oil and gas sources and migration/accumulation, we address the origin of these compounds. It is implied that the diamondoids were not formed in-situ due to oil cracking (in reservoir approximately 5500 m deep at temperature around 139 °C), whereas the cracking of oil to gas may have occurred in the deeper-buried Cambrian dolomite reservoir at a reservoir temperature of 180–220 °C (8500–11,000 m depth). The oil-cracked gas (secondary cracking product) might have migrated to and accumulated in the shallower-buried Ordovician reservoir, accompanied by abundant diamondoids. The formation of sulphur-containing compounds may be related to thermochemical sulphate reduction (TSR), which, similar to the oil cracking, took place in the deeper-buried Cambrian reservoir. Under high-temperature TSR, SO 4 2− in the formation water is reduced to H 2 S and organic sulphur compounds, leading to the enrichment of sulphur in crude oils (e.g., benzothiophene, and dibenzothiophene series). As such, it is implied that the Tazhong area could potentially be explored for deep-buried oil-cracked gas. [ABSTRACT FROM AUTHOR]

This research reveals the depositional environment can affect the organic matter enrichment. The major elements and trace elements are important indexes to reconstruct the depositional environment of Ordovician subsalt marine formations in the central-east Ordos Basin, combining organic parameters with inorganic parameters. Based on the microfacies, the study area is divided into gypsum-bearing dolomite flat, dolomite flat, gypsum-salt depression and limy dolomite flat. The higher Ti/Al ratios and the characteristics of trace elements show that there were detrital inputs in the sedimentation process. When the amounts of detrital inputs are large enough, they can obviously adsorb the elements and dilute the organic matter. Therefore, gypsum-bearing dolomite flat and dolomite flat have lower organic matter accumulation with higher detrital input. Lower P/Ti and Ba excess indicate the deficiency of paleoproductivity. Sr/Ba is higher than 0.6, indicating marine-continental transitional and marine environment. Even the paleosalinity is higher, previous researches have demonstrated the descending sea levels and strong hydrodynamic conditions are unfavorable for organic matter preservation. Lower paleoclimate index "C" and higher Sr/Cu illustrate arid and hot conditions. The value of V/(V + Ni) suggests that the limy dolomite flat is under the oxidizing conditions, and the three other environments are reductive to a certain extent. The Mo-U covariation shows that the water body is under strongly restricted conditions. These conditions are not conducive to biological growth and the accumulation of organic matter, causing low ancient productivity. The partial correlation analysis shows: the organic matter enrichment is affected by multiple factors; paleoproductivity, paleosalinity and paleoclimate have greater influence on the results of organic matter enrichment, and detrital input and oxidation conditions can decrease the paleoproductivity, indicating multiple factors are interrelated and interact with each other. • Major and trace elements are used to reconstructs the depositional environment of four types microfacies. • Paleoproductivity, paleosalinity and paleoclimate have greater influence on the organic matter enrichment. • Depositional environment has direction relationship with low paleoproductivity, causing low organic matter enrichment. [ABSTRACT FROM AUTHOR]

There is no doubt that organic matter plays an important role in shale gas accumulation and storage. Thus, successful evaluation and production strategies of organic-rich shale deposits require an understanding of the range of factors that contribute to the accumulation and preservation of organic matter in these deposits. This paper reports results of a multi-faceted study of two Ordovician–Silurian transition sections of the Upper Yangtze Platform of South China. Organic and inorganic geochemical data are used to reconstruct paleoenvironmental changes across this critical interval of Earth history and to identify those factors that were most important to organic matter accumulation. Deposition of organic-rich shale of the Katian lower Wufeng Formation was favored by rising sea level and diminished clastic sediment content. Black shale of the upper Wufeng preceded Hirnantian glaciation whereas the overlying lower Longmaxi Formation accumulated in association with a post-glacial rise of sea level as well as the peak of Caledonian tectonism. The carbonaceous deposits appear to reflect the combined effects of elevated primary productivity as suggested by Si/Al and Ti/Al ratios, especially in the upper Wufeng, and consequent depletion of water column oxygen (preservation). Consideration of the relationships of redox-sensitive trace element concentrations and total organic carbon content suggest that bottom water conditions fluctuated between anoxic and euxinic during this period, perhaps related to rapid oscillations of sea level. Moreover, the basin appears to have remained connected with the global ocean in spite of Caledonian uplift of bordering areas. Thus, black shale sedimentation on the Upper Yangtze Platform at the Ordovician-Silurian boundary reflects the interplay of climate change and tectonism and their influences on sea level, clastic sediment content, and paleo-productivity. • Geochemical data is used to reconstruct the Ordovician-Silurian transition. • Paleoredox conditions fluctuated between anoxic and euxinic. • The basin was connected with the global ocean at the Ordovician-Silurian boundary. • Sedimentation reflects the interplay of sea level, clastic content, and productivity. [ABSTRACT FROM AUTHOR]

Abstract The Ordovician large-scale gas reservoir in the Tazhong III region (burial depth > 6000 m) is a typical deep strata gas reservoir in the Tarim Basin of NW China. The origin, migration and accumulation of the Ordovician natural gas in the Tazhong III region were investigated by integrating geological, geophysical, and geochemical parameters. The parameters that were successfully analyzed included the gas compositions and carbon isotope ratios, production data, burial and geothermal histories, oil compositions and properties, together with the diamantane index. The results indicated that the Ordovician natural gas in the Tazhong III region originated from Cambrian-Ordovician paleo-oil cracking, accompanied by a large amount of H 2 S generated from thermochemical sulfate reduction (TSR). When the oil cracking gas migrated vertically into the Ordovician reservoir through the Tazhong I fault and strike-slip faults, it was preferentially charged in the pinnate fault zones of strike-slip faults with high porosity and permeability, leading to the enrichment of natural gas and an abnormal distribution of the compositions and properties of hydrocarbon. Then, oil cracking gas intruded into and altered the previous oil reservoir from northeast to southwest through the strike-slip faults, unconformities and pore-fracture reservoir belt, leading to the differential accumulation of hydrocarbon in districts of the Tazhong III region. The Ordovician natural gas migration-accumulation model in the Tazhong III region indicates a great exploration potential in the deep or super-deep strata of the Tarim Basin. Further exploration activities in the Ordovician reservoir of the Tarim Basin should be conducted in the regions where Cambrian-Ordovician source rocks are well developed, and the stress fracture zones-related deep fault should be emphasized. Highlights • Oil accumulated in the southwestern Tazhong III region, while gas predominated in the northeastern Tazhong III region. • Ordovician gas of the Tazhong III region is oil-cracking gas from deep Cambrian-Ordovician reservoirs. • Oil cracking gas migrated vertically through faults, and migrated laterally from northeast to southwest. • Cambrian-Ordovician source rock distribution regions should be conducted in further exploration activities. [ABSTRACT FROM AUTHOR]

Abstract The Central Uplift region of the Tarim Basin contains a series of thick Ordovician carbonates that form fractured-vuggy hydrocarbon-rich reservoirs along an unconformity. These fractured-vuggy reservoirs are difficult to characterize using traditional well logging and seismic interpretation methods because these carbonates feature complex and heterogeneous lithologies and variations in scale, geometry and spatial distribution. In this paper, a series of deeply buried fractured-vuggy carbonate reservoirs are characterized on micro-, macro- and mega-scales. These characterizations are based on previous studies detailing the carbonate sedimentation, diagenesis, and tectonic processes in the study area and abundant geological and geophysical data, including core samples, thin sections, and well logging and seismic datasets, which have been obtained and upscaled using the "choosing-classification-evaluation- generalizing-upscaling" (CCEGU) method. In this area, different facies can be identified at different scales: (1) Detailed core and thin section observations performed based on the Dunham classification demonstrate that 7 lithofacies and 19 rock types are present at the micro-scale (0.01 m); (2) logging data and assorted reservoir characteristics indicate that these 19 different rock types consist of 9 different electro facies at the macro-scale (0.1 m–1 m), and the presence of electro facies in individual wells can be recognized; and (3) these 9 different electro facies comprise 6 distinct seismic facies at the mega-scale (10 m), and can be used to delineate the spatial outlines of potential reservoirs and evaluate their properties. The targets of particular interest in the study area are a series of partially dissolved dolomites located near large faults and a series of vuggy soluble limestones located near NE-trending strike-slip faults. This study combines geological and geophysical response data from fractured-vuggy carbonate reservoirs to achieve seamless connections among micro-scale, macro-scale and mega-scale observations to effectively characterize a series of strongly heterogeneous fractured-vuggy reservoirs. This method can also be applied to characterize other deeply buried paleokarst reservoirs in the Tarim Basin, as well as other strongly heterogeneous reservoirs. Highlights • Integrated Geological-Geophysical method connects reservoir characterizations at microscopic, macroscopic and mega scales. • Seamlessly connects data between each scale to effectively characterize fine-scale fractured-vuggy reservoirs' heterogeneity. • Results provide new insight into strongly heterogeneous, fine-scale, and deeply buried fractured-vuggy reservoirs. [ABSTRACT FROM AUTHOR]

The strike-slip fault-controlled ultra-deep fractured-vuggy carbonate reservoir is an increasingly important exploration target. However, the high heterogeneity of fault-controlled reservoirs and the ineffective methods available for pressure prediction make it difficult to assess the risk of potential overpressure in ultra-deep carbonate reservoirs. Abnormal overpressure is common in the ultra-deep strike-slip fault-controlled carbonate reservoirs in the Shunbei Oilfield of the Tarim Basin. According to measured pressures, petroleum accumulation history, and fault framework, this study investigates the generation mechanisms of overpressure in fault-controlled carbonates and reconstructs the pressure evolution of reservoirs. Finally, the effect of strike-slip faults on current overpressure distribution is discussed. Measured pore pressure data from 90 wells showed that all reservoirs present in the major fault zones are normally pressured, and the reservoirs present far away from the major faults commonly develop overpressures. The distribution patterns of overpressure in the Middle-Lower Ordovician reservoirs coincide with areas with fault reactivation during the Himalayan Period. The results of the comprehensive analysis of petroleum accumulation history, fault activity, and well production suggested that dry gas generation is a primary cause of overpressure in the Middle-Lower Ordovician reservoirs. The relatively low temperature in the reservoir, in addition to the highly compartmentalized nature of overpressure, indicated that these overpressured gas migrate vertically into the Ordovician formation from deep strata along strike-slip faults. The pressure evolution restored by basin modelling and fluid inclusion showed that distinct differences in overpressure magnitude in the different reservoirs were formed at the stage of gas charging but not at the early stages of oil accumulation. The difference in physical properties of the two types of fault-controlled reservoirs resulted in different pressure distributions and oil-bearing levels in the ultra-deep carbonate reservoirs at present. The strike-slip fault zones are the primary pathways by which overpressured fluids migrate from the deeper Cambrian formation into the Ordovician reservoirs and act as barriers to separate the pressure systems in the ultra-deep carbonate reservoirs. These findings are expected to help mitigate the drilling risks with overpressure in deep strike-slip fault-controlled carbonate reservoirs. • Faults act as barrier to separate the carbonate into many isolated pressure systems. • Overpressured gas from deep strata vertically migrate into reservoir via active fault. • The two types of fault-controlled reservoirs have different pressure states. [ABSTRACT FROM AUTHOR]

Molecular and compound specific isotope analysis of volatile organic sulfur compounds (VOSC), alkanes and CO 2 in natural gases from the Ordovician Majiagou Formation of the Daniudi field (Ordos basin, China) was performed. The analysis of VOSC demonstrated abundant thiols, sulfides and thiophenes in several samples while others had low concentrations of sulfides only. Molecular analysis of main alkanes and non-hydrocarbons showed the samples vary by the nitrogen and H 2 S concentrations. Thiols in the gas were at isotopic equilibrium with H 2 S bearing isotopic signature similar to that of Ordovician anhydrites within the reservoirs, hence the source of H 2 S was likely thermochemical sulfate reduction (TSR) of these anhydrites. In contrast, gas samples that lacked thiols also lacked H 2 S. Since all gases came from the same formation and similar thermal regime, the lack of thiols and H 2 S indicates that the occurrence of TSR in the field was limited, likely by contact between hydrocarbons to the anhydrites. The isotopic signature (δ34S) of thiophenes in the gas samples suggests the source of the gas is unlikely to be Carboniferous or Permian coals which were previously suggested as the source rocks for the gas. The δ13C and δ2H of alkanes further supports this suggestion as it demonstrates Carboniferous and Permian gases are unlikely to undergo TSR and generate the gas present in the Ordovician reservoirs. Instead, the δ13C and δ2H of alkanes hint that the gas source is shale rather than coal. This study demonstrates the applicability of compound specific sulfur isotope analysis of VOSC in identification of H 2 S sources, performing gas-source rock correlation and acting as a complementary proxy in case where routine analysis of hydrocarbons (δ13C and δ2H) is insufficient for identification of the gas source. • Volatile organic sulfur compounds were measured in natural gas from the Ordovician Majiagou Formation in the Daniudi field. • Molecular and sulfur isotopic distribution of thiols indicate that the origin of H 2 S is TSR from Ordovician anhydrites. • The natural gas of the Majiagou Formation was likely generated in-situ from an Ordovician source rock. • Distribution of VOSC isotopes indicates TSR in Ordovician gas reservoirs was likely localized due to limited sulfate presence. [ABSTRACT FROM AUTHOR]

Abundant oil and gas are yielded from Ordovician reservoirs in the Fuman oilfield of Tarim basin and behave in multi-phase states at comparable burial depths, including light oil, volatile oil, and condensate gas. Oil and gas phase are distributed in the west and east, respectively, with heavier oil property in the north than that in the south. However the genetic mechanism of oil and gas phase states in the study area remains obscure. This has been examined based on fluid inclusions and geochemical parameters of light hydrocarbons and diamondoids in this paper. The results show that most Ordovician reservoirs were charged by oils in the late Caledonian and the late Hercynian, and the oil maturity charged in the same period decreased from south to north. In the Himalayan, dry gas and condensate oil-wet gas were respectively filled into the Yuke area in the east and local areas in the west. The multi-period differential charging of oil and gas with different properties is the main cause of the current oil and gas phase states. Two and three periods of oil charging are responsible for the formation of light and volatile oil, respectively, in western regions. Condensate gas reservoirs in Yuke area were formed by three periods of oil and gas charging, with the dominance of dry gas in the Himalayan. The phase states were transformed into volatile oils with the decrease in the charging intensity of dry gas to the west. The oil property becomes lighter from north to south is mainly related to the oil maturity charged during the same period, but also affected by the gradually decreasing cracking degree of oils from south to north. The current oil and gas phase states were ultimately produced by the genetic model of "multi-period differential charging and spatial differential cracking". • Ordovician reservoirs in Fuman oilfield of Tarim basin behave in multi-phase states at comparable burial depths. • The genetic mechanism of multiple oil and gas phase states in Fuman oilfield was revealed based on fluid inclusions and oil geochemistry. • Multi-period differential charging of oil and gas with different properties is the main cause of the current oil and gas phase states. • Oil property becomes lighter from north to south is mainly related to the oil maturity charged in same period, but also affected by spatial differential cracking. [ABSTRACT FROM AUTHOR]

Gases, oils, solid bitumen samples and fluid inclusions were analyzed for chemical composition and δ 13 C and δ 34 S values, and production data were collated, to elucidate the origin and fates of the H 2 S in the Tazhong area. The H 2 S in the Ordovician was measured to have δ 34 S values from 12.1‰ to 23.4‰, which are significantly lighter than the counterpart in the Cambrian (33.0‰), indicating that the H 2 S did not migrate from the Cambrian, but may have been derived from thermochemical sulfate reduction in situ , as partially supported by 12 C-rich CO 2 in fluid inclusions and TSR calcites with δ 13 C values from −3.6‰ to −17.7‰ in the Ordovician. The H 2 S rich gas subsequently mixed with H 2 S-poor, 13 C rich methane-dominated gas as indicated by the roughly negative correlations of H 2 S content to dryness and gas/oil ratio, and CH 4 from fluid inclusions having δ 13 C values 1.2–3.6‰ lighter than the associated free gas. The H 2 S has been dissolved in formation water, precipitated as pyrite and incorporated into oils and solid bitumens, resulting in the generation of alkylthiolanes, alkylthiols and alkyl 2-thiaadamantanes, and increase in δ 34 S values and S/C ratios of the solid bitumen. The content of H 2 S produced from the water intervals or petroleum–water transition zones is significantly contributed from exsolved gas and thus higher than that of the gas-cap gas in the reservoirs. Thus, H 2 S contents cannot be used to indicate TSR extents in the Ordovician. However, in the Cambrian, whole oil and saturates show positive shifts in δ 13 C values with increasing H 2 S contents, indicating that the H 2 S in the Cambrian was derived from TSR by liquid hydrocarbons. [ABSTRACT FROM AUTHOR]

A long, continuous core (1155 ft; 352 m) from the Ellenburger Group in Upton County, Texas allowed for a detailed reservoir description of depositional setting, lithofacies, pore networks, and impact of subsequent karst processes on the reservoir geology. This reservoir description furthers the understanding of the origin and characteristics of pore types relevant for waste-water disposal, carbon dioxide storage, and hydrocarbon reservoir quality in the Ellenburger Group. The Lower Ordovician Ellenburger Group in West Texas is a thick dolomite sequence that was deposited on a broad shelf over a 16 m.y. Time period in the Tobosa Basin. The shallow-water restricted depositional setting produced carbonate sediments that are dominantly composed of middle-shelf microbial thrombolite boundstones and inner-shelf laminated (stromatolitic) mudstones. Recognizable skeletal grains are rare. The lower two thirds of the cored section in the area of investigation in Upton County, Texas is dominated by microbial thrombolite complexes with associated stromatactis features. Interbeds of oolites and evaporites are associated with the complexes and may represent small-scale fluctuations in relative sea level. Up-section, lithofacies transition into a peritidal complex composed of stromatolites and laminated mudstones, some displaying evaporite nodules. The lithofacies stacking pattern is similar to the regional established stacking pattern where the most open-marine lithofacies are at the base of the stratigraphic section and the most restricted lithofacies are at the top of the stratigraphic section. This consistent stacking pattern suggests a stable structural setting for millions of years where constant environmental conditions existed. Following deposition and subsequent lithification and dolomitization of the Ellenburger Group, the section underwent intense meteoric karsting (epigenic) during Late Middle Ordovician Sauk-Tippecanoe megasequence long-term exposure. The meteoric karsting affected the upper 700 ft (215 m) of the Ellenburger Group. The pore network in the studied core is composed of vuggy (small cavities) pores in the thrombolite lithofacies and crackle and mosaic breccia fracture pores and chaotic breccias interbreccia pores in karst-affected intervals. These pore systems support hydrocarbon production, waste-water disposal, and carbon-dioxide storage. • Reservoir-characterization description of thick Ellenburger section. • Defines lithofacies stacking pattern on broad restricted carbonate shelf. • Recognizes a thick (hundreds of feet) section of stacked thrombolite complexes. • Dual pore network of matrix pores and karst-related fracture pores. • Epigenic karsting affected the upper 700 ft (215 m) of section. [ABSTRACT FROM AUTHOR]

There are many oil and gas fields around the world that are sealed by tight carbonate caprocks. Compared with gypsum rocks and shales, tight carbonate caprocks are characterized by brittleness and ease of structural fracturing and dissolving in acidic groundwater. The breakthrough pressure (BP) is a significant indicator that indicates the sealing capacity of the caprocks. Therefore, we conducted a series of experiments, including effective porosity, permeability, casting thin-sections, BP, mineral composition, microstructural characteristics and clay minerals contents on carbonate samples from Ordovician system in the Kalpin area, Tarim Basin, China to research factors that affect the BP of carbonate caprocks. According to the experimental data analysis, the BP deceases with macropore proportions and increases with proportions of micropore and mesopore volume. Due to the large macropore proportion and low clay content (<10%), expansion of clay minerals with water hardly reduces effective porosity and permeability. Pore throat and cement influence effective porosity and permeability, and thus influence the BP. Generally, the BP decreases with the increase of effective porosity and permeability, and linear correlations exist between the reciprocal of effective porosity (A), −0.48 power of permeability (B) and the BP. Due to the interdependency between A and B, a principal component regression procedure (PCRP) is implemented with the statistical analysis system (SAS) software to eliminate the interdependencies and obtain a fitting formula of BP. The calculated BP with depth by this method matches gas logging results. This study provides a research idea and a mathematical calculation model of the BP of carbonate caprocks to further evaluate and predict carbonate caprocks behaviour. • The influence factors of BP (breakthrough pressure) of carbonate rocks have been investigated for the first time. • PCRP is used by SAS software to deduce a computational formula of BP values of carbonate rocks. • The application effect of the evaluation method of BP values in the Kalpin area shows the feasibility of this method. [ABSTRACT FROM AUTHOR]

Abstract Evaluating the heterogeneity and sealing capacity of carbonate cap rocks is of great significance for hydrocarbon production. However, the main controls on pore size distribution in carbonate cap rocks are not well constrained. In this contribution we analyze drill core samples of carbonate cap rocks from the Ordovician Yingshan Formation (Tarim Basin) to study their pore properties and investigate the controls of pore-throat structure and fractal dimension on the sealing capacity of the cap rock. Results from petrography and a combination of mercury injection capillary pressure and nitrogen gas adsorption tests reveal that there are four lithological cap rock types, with micrite being the most prevalent one, followed by calcarenite. Six types of pore-throat structures and fractal dimensions (from D 1 to D 6) are classified based on the pore size distributions, including macropore-I, macropore-II, mesopore-I, mesopore-II, transitional pore-I and transitional pore-II. The fractal dimensions show that there is a decreasing trend from D 1 to D 6 with decreasing pore-size diameter, suggesting that the macropore and mesopore types present more complex pore-throat structures than the transitional type. The total fractal dimension (D total) indicates that the pore throat structures are complex and heterogeneous. The sealing heights of the gas column have a positive correlation with cement content in calcarenite and a negative correlation with microfracture density. Mesopore-II and transitional pore types make a significant contribution to enhancing the breakthrough pressure. The fractal variations mainly result from macropores and mesopores, while transitional pores and micropores weaken the impact of the whole pore system on the D total value. The heterogeneity and anisotropy of macropores is the main control on the sealing capacity of the cap rock. The increasing of D 1 considerably contributes to the pore shape heterogeneity and enhances the capillary resistance force to hydrocarbon migration. Therefore, the fractal dimension is an effective approach for investigating the pore throat structure and sealing capacity of carbonate cap rocks. Highlights • Pore size distributions of carbonate cap rock were quantitatively investigated using MICP and N 2 GA. • Controls of microfractures and cementation on sealing capacity were analyzed. • Fractal dimensions were adopted to characterize the heterogeneity of cap rock. • Correlation between fractal dimension and sealing capacity was discussed. [ABSTRACT FROM AUTHOR]