Search

The Eastern Tianshan provides significant insights in relation to unraveling the history of Early Paleozoic magmatism, ocean-continent transition, and tectonic evolution of the southern Central Asian Orogenic Belt. This investigation represents a detailed geochronological and geochemical study of the Early Paleozoic volcanic rocks from the Harlik and Dananhu terranes in the Eastern Tianshan. U Pb zircon ages indicate periods of volcanism in Harlik Early Silurian (441–434 Ma) and Dananhu Late Ordovician (448–445 Ma). The Harlik Early Silurian volcanic rocks unconformably overlie Ordovician sediments, and consist of alkaline (434 Ma) and tholeiitic (440 Ma) basalts. The alkaline basalts originated from low degree partial melting of previously subduction-modified lithosphere and subsequent interaction with upwelling asthenosphere. The tholeiitic basaltic magmas were derived from partial melting of a spinel facies lithospheric mantle source metasomatized by early slab-derived fluids, which then underwent crystal fractionation of clinopyroxene and olivine. In contrast, the Dananhu Late Ordovician high-Mg andesites were generated by partial melting of a hydrous, depleted mantle wedge and these melts experienced a low degree of crystal fractionation. Combined with results from previous geological studies, it is proposed that the Harlik and Dananhu terranes had different volcanism and tectonic histories. The Harlik tholeiitic basalts were generated in a post-collision setting, which may be associated with the closure of the Armantai Ocean. Contrarily, the alkaline basalts were formed in an intraplate setting during the initial stages of intracontinental extension that gave rise to the Karamaili Ocean. The Dananhu Late Ordovician andesites represent the initial phase of arc-related volcanism due to the subduction of the North Tianshan Ocean, where the Dananhu arc may have temporally evolved from an Andean- to oceanic island-type arc in Early Paleozoic.

To improve the understanding of the relationship between silica source and organic matter accumulation, the origin of silica and its implications for organic matter enrichment in the Upper Ordovician-Lower Silurian (O3w-S1l) black shale in western Hubei Province in the middle Yangtze area, were investigated through geochemical and petrological analyses. The results show that the O3w-S1l black shale is mainly composed of five lithofacies with varying graptolite abundance, total organic carbon (TOC), and silica contents. Biogenic silica and terrigenous siliciclastic input constitute the main silica sources in O3w-S1l black shale and they exhibit an upward inverse variation trend interpreted to be related to sea-level changes. Moreover, with the increase in biogenic silica content or decrease in terrigenous siliciclastic input, TOC values in black shale initially rise and then fall, which is different from originally expected simple linear relationship. We infer that organic matter enrichment and the distribution of silica from different origins are controlled by sea-level changes and variations in terrigenous input in a continental shelf with little hydrothermal influence. An environment with appropriate sea level and terrigenous input should be most favorable for organic matter accumulation, rather than one with excessive high sea level and less terrigenous input.

Strike-slip faults, proven to be closely linked to hydrocarbon migration and accumulation, are widespread in the Tarim Basin Craton. Deformation patterns in the eastern part of the Central Uplift Belt of the Tarim Basin are analyzed in detail to understand the deformation model and genetic mechanism of strike-slip faults, how their patterns impact reservoir formation and influence oil and gas potential. Regional tectonic events are taken into consideration to identify the primary factors that controlled the development of strike-slip faults in the region. The Tazhong Uplift in the eastern part of Central Uplift Belt is believed to be a complex anticlinal uplift formed by compression-and-torsion acting on the Cambrian – Middle Ordovician platform during the middle–late Caledonian period. Multi-stage development of this north-trending fault–fold belt and the NE-trending strike-slip faults make it structurally complex. The NE-trending Paleozoic strike-slip faults were formed jointly by the differential thrusting of the Tazhong Uplift along Fault No. I and the NE-trending shear sliding of the basal fault blocks during the middle–late Caledonian – early Hercynian. Based on the distribution of effective source rocks, the tectonic setting during the period critical to hydrocarbon accumulation, and the distribution of conductive faults, the northern slope of the Tazhong Uplift— especially its west-central part where the NE-trending faults are developed is not only located close to the northern hydrocarbon source rock area but also possesses conditions suitable for the development of carbonate reservoirs. In addition, the NE-trending faults provide passage for initial development of the reservoir and the subsequent migration of oil and gas from the Aman area to the uplifted zone, making the Tazhong Uplift an extremely favorable area for oil and gas accumulation with substantial potential for exploration.

Long-lived subduction and accretion of the Paleo-Asian Ocean (PAO) led to the formation of the Central Asian Orogenic Belt (CAOB). The central part of the southern CAOB is crucial to understand the tectonic evolution of the entire CAOB. However, the Paleozoic evolution and the final closure of the central PAO are controversial, and the nature of the Early Paleozoic basins is confusing. The relatively complete Paleozoic strata are well-preserved in Zhusileng area in the central part of the southern CAOB and provide an ideal object to constrain the Paleozoic tectonic setting of the PAO. In this study, detrital zircon U-Pb dating and sedimentological and paleocurrents analyses are conducted on the Cambrian-Devonian strata and Lower Permian strata. Most detrital zircons from the Cambrian-Devonian strata have Precambrian ages, while the detrital zircons from the Lower Permian strata are mainly of Paleozoic age. The North China Craton, Alxa Block and Tarim Craton are not the provenances of the Cambrian-Devonian strata. The central part of the southern CAOB was located in a passive continental margin setting during the Cambrian to Ordovician and received the detritus from the Beishan Orogenic Belt. During the Silurian to Middle Devonian, a nearly east-west-trending foreland basin that resulted from the collision between the Zhusileng-Hangwula terrane and southern Mongolian microcontinent developed. From the Late Devonian to Early Permian, bidirectional subduction of the PAO occurred between the Alxa Block and Zhusileng-Hangwula terrane, resulting in numerous arc-related magmatic rocks. Detritus in the Lower Permian strata was mainly from the Paleozoic magmatic rocks and Cambrian-Devonian strata in Zhusileng and surrounding areas. The Late Paleozoic shortening deformation and the switch of the tectonic setting from subduction/collision to post-collision occurred in the central part of the southern CAOB imply that the central PAO closed between the Early and Late Permian.

Ordovician limestone and Cambrian dolomite are the main oil and gas producing layers in eastern Tazhong field, Tarim basin. Industrial oil flow has been obtained from drilling in this zone, proving that the Lower Paleozoic strata is important for oil and gas exploration and development. However, the main controlling factors and distribution of high-quality reservoirs remain unclear. Based on the logging constrained seismic inversion , FMI (Fullbore Formation MicroImager) log interpretation and core analysis of six wells, the main controlling factors of Paleozoic carbonate reservoirs in the study area were investigated. The results reveal that the two buried cuestas control the distribution of the Cambrian and Ordovician strata, the ratio of mudstone in north buried cuesta (NBC) is more than that in south buried cuesta (SBC), and the average porosity of 3xq dolomite formation is more than that of O 1 − 2 y limestone formation. FMI logging interpretation results show that fracture density of well TZ-1 is significantly higher than that of well TZ-2, combined with core observation and microscopic analysis we can believe that reconstructed dolomite reservoir has developed dissolution pores and vugs. By the means of seismic impedance reconstruction, the most favorable production areas, so-called sweet-spots, are karst residual mounds that formed independent small anticlinal high points and have been depicted. Finally, total 28 traps in the Cambrian–Ordovician​ carbonate buried hill are characterized, covering an area of 66.4 km2. 17 anticline traps with an area of 26.9 km2 in dolomite reservoirs are optimized to be the next exploration targets.

Dolomitized fault–fracture structures in the Trenton and Black River formations (TBR) are the type example for “hydrothermal” petroleum reservoirs world-wide. However, fluid histories of these structures are only partially understood. Trenton and Black River reservoirs in the southern Michigan Basin are composed of fault-associated, vertical dolomite bodies that are highly fractured and brecciated. Open spaces are partially to completely filled by saddle dolomite and less frequently by calcite cement. Cathodoluminescence microstratigraphies of void-filling carbonate cements are not correlatable between oil fields. Fluid inclusion homogenization temperatures (Th) measured in carbonate cements indicate two fluid endmembers: a warm fluid (∼ 80° to 180° C) and a hot fluid (180° to ∼ 260° C). Increasing Th proximal to the underlying Proterozoic Mid-Michigan Rift (MMR) suggest that the hot fluids emanated from the rift area. Included fluids are saline (16.1–49.4 wt. % NaCl equivalent), and salinity likely is sourced from overlying Silurian Salina Group evaporites. First melting temperatures (Tfm), interpreted as eutectic temperatures (Te), of fluids range from –112° C to –50° C, indicating a complex Na–Ca–KCl brine; the expected composition of dissolved Salina salts. Lower Te proximal to the MMR suggest the rift as a source of additional complexing ions. C and O isotope values for carbonate cements are depleted with respect to δ18O (–6.59 to –12.46‰ VPDB) relative to Ordovician seawaters, and somewhat depleted with respect to δ13C (–1.22 to +1.18‰ VPDB). Equilibrium calculations from δ18O and Th values indicate that cement precipitating waters were highly evolved (+1.3 to +14.4‰ δ18O‰ VSMOW) compared to Ordovician and Silurian seawaters (–5.5‰ δ18O‰ VSMOW). Strontium isotope values indicate two fluid sources: Proterozoic basement and Late Silurian evaporites. Values of 87Sr/86Sr for cements in the Freedom, Napoleon, Reading, and Scipio fields (0.7086–0.7088) are influenced by warm water sourced from Silurian strata, and values for cements in the Albion, Branch County, and Northville fields (0.7091–0.7110) record continental basement signatures. Cement precipitating fluids in TBR oil fields likely have similar sources and timing. However, water–rock interactions along fault pathways modified source waters, giving each oil field a unique petrographic and geochemical signature. Fluid movement in TBR oil fields likely were initiated by reactivation of basement faulting during Silurian–Devonian tectonism.

A diverse mid-Darriwilian trilobite fauna from the Table Cove Formation, western Newfoundland, has long been known on the basis of calcareous specimens from the west coast of the Great Northern Peninsula. Discovery of silicified faunas in localities on the east coast provides additional morphological information for previously known species, and also reveals the presence of multiple new genera and species. Many of these species are important, as they represent some of the earliest Laurentian members of the diversifying Whiterock Fauna, and seem phylogenetically near to the base of their respective clades. The concept of Sphaerexochinae is restricted to the genus Sphaerexochus itself, with the possible inclusion of Newfoundlandops n. gen. (type species N. karimae n. sp.), which shares with Sphaerexochus potential synapomorphies including the structure of the hypostome and the presence of fine granular sculpture on the librigenal border and field. Most of the Early and Middle Ordovician taxa with three pygidial segments previously classified as Sphaerexochinae by many authors are reassigned to Acanthoparyphinae on the basis of multiple putative synapomorphies. Other new cheirurid genera from the Table Cove Formation are the pilekiine Harebayaspis n. gen. (type species H. plurima n. sp.), and the deiphonine Mainbrookia n. gen. (type species M. becki n. sp.). Other species revised or described include the cheirurine Laneites polydorus (Billings, 1865), and the acanthoparyphines Cydonocephalus tiffanyae n. sp., Kawina stougei n. sp., and Kawina? sp.

The Harihada–Chegendalai ophiolitic mélange, which is located between the Bainaimiao arc and the North China Craton, holds significant clues regarding the tectonic setting of the southern margin of the Central Asian Orogenic Belt. The ophiolitic mélange is mainly composed of gabbroic and serpentinized ultramafic rocks. Here, zircon U–Pb dating, in situ zircon Hf isotopic, whole-rock geochemical and in situ mineral chemical data from the ophiolitic mélange are reported. The zircons in the gabbroic rocks yielded concordia U–Pb ages of 450–448 Ma and exhibited slightly positive ϵHf(t) values (0.87–4.34). The geochemical characteristics of the gabbroic rocks indicate that they were generated from a mantle wedge metasomatized by subduction-derived melts from sediments with continental crust contamination, in a fore-arc tectonic setting. These rocks also experienced the accumulation of plagioclase. The geochemical characteristics of the ultramafic rocks and their Cr-spinels indicate that they may constitute part of residual mantle that has experienced a high degree of partial melting and has interacted with fluids/melts released from the subducted slab in the same fore-arc tectonic setting. The ophiolitic mélange may therefore have formed in this fore-arc tectonic setting, resulting from the northward subduction of the South Bainaimiao Ocean beneath the Bainaimiao arc during Late Ordovician time, prior to the collision between the Bainaimiao arc and the North China Craton during the Silurian to Carboniferous periods.

Based on field outcrop data, the effects of cyclic change of astronomical orbit and volcanic activity on organic carbon accumulation during the Late Ordovician – Early Silurian in the Upper Yangtze area were studied using cyclostratigraphic and geochemical methods. δ13C and chemical index of alteration (CIA) were used to filter the astronomical orbit parameters recorded in sediments. It is found that the climate change driven by orbital cycle controls the fluctuations of sea level at different scales, obliquity forcing climate changes drive thermohaline circulation (THC) of the ocean, and THC-induced bottom currents transport nutrient-laden water from high latitude regions to the surface water of low-latitude area. Hence, THC is the main dynamic mechanism of organic-carbon supply. The marine productivity indexes of Ba/Al and Ni/Al indicate that volcanic activities had limited effect on marine productivity but had great influences on organic carbon preservation efficiency in late Hirnantian (E4). Paleo-ocean redox environmental indicators Th/U, V/Cr and V/(V+Ni) show that there is a significant correlation between volcanism and oxygen content in Paleo-ocean, so it is inferred that volcanisms controlled the organic carbon preservation efficiency by regulating oxygen content in Paleo-ocean, and the difference in volcanism intensity in different areas is an important factor for the differential preservation efficiency of organic carbon. The organic carbon input driven by orbital cycle and the preservation efficiency affected by volcanisms worked together to control the enrichment of organic carbon in the Middle–Upper Yangtze region.

Quartz crystallinity index (QCI) was used to reflect the crystallisation of silica in the Late Ordovician Wufeng (WF) and Early Silurian Longmaxi (LM) Formation shale, as well as the airborne volcanic ash-derived silica in the Lucaogou Formation tuffaceous shale, to distinguish the two types of silica. The silica in different graptolite biozones exhibited different crystallisation. The WF2–3, LM1–4 graptolite biozones showed obviously lower QCI values than the LM5–9 graptolite biozones and the Lucaogou Formation samples. The graptolite organisms played the role of adsorption, fixation, and precipitation in silicon accumulation and enrichment in stratum. The biogenic origin caused the poorest quartz crystallisation in WF2–3 and LM1–4 graptolite biozones samples. The airborne volcanic ash-derived silica in the Lucaogou Formation tuffaceous shale exhibited relatively poor quartz crystallisation because of weaker diagenesis intensity. Generally, although the WF2–3 and LM1–4 graptolite biozones underwent strong diagenesis and contained a small amount of detrital quartz, the silica still exhibited lower QCI values than the airborne volcanic ash-derived silica in the Lucaogou Formation tuffaceous shale. The biogenic silica crystallisation was much poorer than that of the airborne volcanic ash-derived silica. QCI is an effective quantitative index to demonstrate the biogenic silica in the organic-rich and silica-rich shale.

Based on a large number of geological and geophysical data, the formation, fracture-caves types and hydrocarbon distribution of hoodoo-upland on the Ordovician karst slope in the Tahe area, Tarim Basin, are discussed by analyzing faults and strata thickness. The hoodoo-upland was made of high peaks and narrow valleys in the Ordovician karst slope during the Early Hercynian karst period, which were distributed along the NNE positive flower structure and had inherited evolution. The fault-fractures and fracture-vugs complex were extremely developed, with a thickness of 100 m. The cumulative oil production of 60% oil wells was more than 20×104 t per well in the hoodoo-upland, where the residual thickness of the Ordovician Yingshan Formation was greater than karst depressions. Caves formed by the shelter of collapsed breccias were developed in the valleys. They were 1.6 to 13.5 m high, with a filling rate of 51.6%. The positive flower structure under the settings of strike-slip compression controlled the early formation of the hoodoo-upland on the karst slope, resulting in the differences of drainage distribution and karstification. Compared with the water-rich karst valley, the hoodoo-upland with lean water suffered weaker karstification, had thicker residual stratum, and was higher in terrain. In rainy season, the meteoric water flew and corrode along the cracks, forming a complex network of fractures and caves. Combined with inherited uplift and the effective match of the NNE deep faults, oil and gas continuously charged into the reservoir space in the upland, forming the hoodoo fracture-cave reservoir with vertically quasi continuous distribution, high hydrocarbon abundance and high production.

Based on outcrop, core, logging, seismic and production data, and the formation of fault-controlled karst reservoirs, the types and characterization of Ordovician fault-controlled karst reservoir architectures in the Tuoputai area of the Tahe oilfield are studied. According to the concept of genetic geologic body, the fault-controlled karst reservoir is divided into architecture elements of four levels, the strike-slip fault impact zone is the level-1 architecture element, the fault-controlled karst reservoir the level-2 architecture element, the fracture-cave zone (which can be further subdivided into dissolution cave, dissolution pore and vug, and fracture zones) inside the fault-controlled karst reservoir the level-3 architecture element, and fillings inside caves is the level-4 architecture element (which can be further divided based on the filling degree and lithologic types of the fillings). Specific characterization techniques have been optimized according to the characteristics of various architecture elements. The zone impacted by strike-slip fault is characterized by seismic coherence and artificial interpretation. Under the constraint of zone impacted by strike-slip fault, fault likelihood (FL) property is used to characterize the outline of fault-controlled karst reservoir. Under the constraint of fault-controlled karst reservoir outline, the internal structures are divided based on seismic texture attribute. Finally, the cavern filling pattern is interpreted based on drilling and logging data. The fault-controlled karst reservoirs can be interpreted in 3-dimensional space by architecture element levels, and the characterization technology combining log and seismic data for fault-controlled karst reservoir has been worked out, which has complemented the development theory and technologies for this kind of reservoirs in the Tahe oilfield.

In the Eastern Cordillera of Peru, observations related to the accretion of the Arequipa Terrane to the Amazon craton are scarce and reactivation of sutures in a backarc basin may make interpretation more difficult. Cambro-Ordovician backarc successions located in proximity to a proposed suture in the Eastern Cordillera of southern Peru were investigated at Umachiri and Ollantaytambo in order to disentangle the early Paleozoic tectonic history of the Arequipa Terrane. At Umachiri, newly identified fossil assemblages in the upper Cambrian Llallahue Formation (Fm) and in the Lower to Middle Ordovician Umachiri Fm constrain the age of these successions and allow correlation of these strata throughout the Central Andes. Using U–Pb geochronology on zircon, we establish that the Ollantaytambo Fm is an upper Carboniferous succession and that the Umachiri Fm instead correlates with the Lower to Middle Ordovician Veronica and San Jose formations, which form the base of a backarc succession in the Eastern Cordillera of southern Peru that extended into northwestern Argentina. Detrital zircons from the Umachiri Fm display age signatures characteristic of Arequipa basement and contain a prominent 540–510 Ma peak. Precise dating and trace element analysis yielded 539 Ma arc-derived and 522–510 Ma mantle-derived zircons. We infer that a Cambrian backarc system related to deposition of the lower Furongian (~494 Ma) Llallahue Fm developed following ~ 540 Ma Pampean arc magmatism. Closure of the backarc in late Cambrian to Early Ordovician time resulted in regional deformation, uplift, and erosion followed by renewed backarc subsidence in the Ordovician Period. In summary, our results support a new tectonic model of late Neoproterozoic to early Cambrian Pampean collision of an exotic Arequipa Terrane with Amazonia, opening and closing of a Cambrian backarc basin, the presence of parallel Ordovician continental arcs associated with Famatinian subduction, and the opening of an Ordovician–Devonian backarc basin.

New, distinctive, well-preserved and previously undescribed constituents of a Middle Ordovician (middle Darriwilian, Dw2) radiolarian assemblage from the Table Cove Formation in Newfoundland are described. Three-dimensional X-ray micro-computed tomography (μ-CT) facilitates detailed examination of key specimens revealing hitherto unknown details of the internal morphologies of key lower Paleozoic taxonomic groups, among which a lack of knowledge has previously impeded resolution of higher taxonomic rankings.Twenty-seven archaeospiculid and entactinarian taxa are described and illustrated including six new species: Westernbrookia polygonata n. sp., Neopalaeospiculum piccadilliensis n. sp., Ramuspiculum laxum n. sp. Spongentactinia nazarovi n. sp., Aspiculum irregulare n. sp., and Nyfrieslandia ramosissima n. sp. The investigation extends the known ranges of the species: Pararcheoentactinia reedae Won and Iams, 2002; Sphaeroentactinia robusta Won and Iams, 2015; Varispiculum ectospiculatum Won and Iams, 2015; and Svalbardospiculum multifurcatum (Won, Iams, and Reed, 2005), together with the genus Echidnina to the mid-Darriwilian.UUID: http://zoobank.org/74826b7b-bb86-45d5-ad23-e6e65e0706df.

Seven conodont biozones are recognized in the carbonate-dominated shelf-marine Middle Ordovician developed in the intracratonic sedimentary basins (Canning, Amadeus and Georgina) of central and north-western Australia, in the Lachlan and New England orogens of New South Wales, and in the Takaka Terrane of New Zealand. A separate scheme identifying seven conodont biozones spanning the Middle Ordovician has also been developed for siliciclastic sequences deposited in slope-basinal environments in the Lachlan Orogen in New South Wales and Victoria. This biozonal classification consisting of two parallel biostratigraphic schemes for the shelf-marine and deep-marine successions respectively has significantly increased precision in regional and global biostratigraphic correlation and laid a solid foundation for the Middle Ordovician chronostratigraphy of Australia and New Zealand. Recognition of short-ranging pandemic species as the eponymous species of the biozones also supports direct correlation with the classical conodont successions established in Baltoscandia and the North American Midcontinent, and with those of the major Chinese terranes (South and North China and Tarim). The Lachlan Orogen appears to be globally unique in enabling correlation of contemporaneous conodont faunas over a considerable spectrum of water depths and biofacies ranging from carbonate shelves, slopes to deep-water basins.

The Salair fold-nappe terrane (a.k.a. Salair orogen, Salair) is the northwestern part of the Altai-Sayan folded area of the Central Asian Orogenic Belt. It is composed of Cambrian – Early Ordovician volcanic rocks and island-arc sedimentary deposits. In plan, Salair is a horseshoe-shaped structure with the northeast-facing convex side, which is formed by the outcrops of the Early Paleozoic folded basement. Its inner part is the Khmelev basin composed of Upper Devonian – Lower Carboniferous sandstones and siltstones. The Early Paleozoic volcanic rocks and sediments of Salair are overthrusted onto the Devonian-Permian sediments of the Kuznetsk basin. The Paleozoic thrusts, that were reactivated at the neotectonic stage, are observed in the modern relief as tectonic steps. Our study of the Salair deep structure was based on the data from two profiles of magnetotelluric sounding. These 175-km and 125-km long profiles go across the strike of the Salair structure and the western part of the Kuznetsk basin. Profile 1 detects a subhorizontal zone of increased conductivity (100–500 Ohm·m) at the depths of 8–15 km. At the eastern part of Profile 1, this zone gently continues upward, towards a shallow conducting zone that corresponds to the sediments of the Kuznetsk basin. Two high-resistance bodies (1000–7000 Ohm⋅m) are detected at the depths of 0–6 km in the middle of the section. They are separated by a subvertical conducting zone corresponding to the Kinterep thrust. The main features are the subhorizontal positions and the flattened forms of crustal conductivity anomalies. At the central part of Profile 2, there is a high-resistance block (above 150000 Ohm⋅m) over the entire depth range of the section, from the surface to the depths of about 20 km. In the eastern part of Profile 2, a shallow zone of increased conductivity corresponds to the sediments of the Kuznetsk basin. The subhorizontal mid-crust layer of increased conductivity, which is detected in the Salair crust, is typical of intracontinental orogens. The distribution pattern of electrical conductivity anomalies confirms the Salair thrust onto the Kuznetsk basin. The northern part of the Khmelev basin is characterized by high resistivity, which can be explained by abundant covered Late Permian granite massifs in that part of the Khmelev basin. The Kinterep thrust located in the northeastern part of the Khmelev basin is manifested in the deep geoelectric crust structure as a conducting zone, which can be considered as an evidence of the activity of this fault.

As the remnant of the South Qilian Ocean, the South Qilian suture zone recorded abundant information on the Cambrian–Ordovician subduction history of the southern branch of the Proto-Tethyan Ocean. However, the closure timing of the South Qilian Ocean and subsequent collision are poorly constrained. In this study, we report early Silurian (433–435 Ma) U–Pb ages of felsic subvolcanic rocks from Lianhuashan, Ayishan and Shihuiyao of the Lajishan district within the South Qilian suture zone. They intruded the Late Ordovician – Silurian sedimentary or Late Ordovician volcanic rocks and have high SiO2 (61.43–73.06 wt%), Sr/Y ratios with significant different rare earth elements (REEs) and trace-element spider diagrams, and Sr–Nd isotopic compositions, probably implying that they were formed through distinctly different generation mechanisms. Geochemistry of the Lianhuashan dacites reveals compositions typical of adakitic rocks derived from partial melting of lower crust in a thickened setting. The Ayishan dacites were derived from partial melting of crustal materials with the involvement of minor peridotite mantle, and the Shihuiyao rhyolites were derived from partial melting of felsic crust. The similar geochemical characteristics of coeval post-collisional igneous rocks in the Central Qilian and South Qilian blocks indicates that the lower Silurian subvolcanic rocks were generated in a thickened crust of post-collisional setting. Considering their intrusive contacts with Late Ordovician – Silurian retro-foreland basin and Late Ordovician collisional volcanic rocks, we propose that the South Qilian suture zone was at a transitional stage from collisional to post-collisional during the early Silurian Period.

To discuss the hydrochemical evolution characteristics of the mining process of Peigou Coal Mine, based on the test results of 43 water samples collected at different times from three main discharge aquifers, namely, Carboniferous Taiyuan Formation limestone water (L7–8 + L5–6 water), Ordovician limestone water (including Taiyuan Formation L1–4), and Permian main mining coal seam roof and floor sandstone water (roof and floor water), a hydrochemical evolution model of the mining disturbances since 2003 has been established. The carbonate and sulphate dissolution and pyrite oxidation in Ordovician limestone water significantly decreased and then increased in 2006, and silicate weathering was weak. The carbonate and sulphate dissolution, silicate weathering and pyrite oxidation of roof and floor sandstone water increased. At the same time, a water source identification model suitable for the Peigou Coal Mine was developed by comparing the Fisher discriminant and the BP (back propagation) neural network discriminant. The accuracy rates of Fisher discriminant and BP neural network discriminant are 81.40% and 83.72% respectively, which indicates that BP neural network is more accurate. Finally, the evolution of hydraulic connection between aquifers is analysed. We speculate that there is a fracture development channel between Ordovician limestone water and roof and floor water aquifers that is affected in 2005 by the mining disturbance. This study has significance for examining the hydrochemical evolution of groundwater in mines and acting as a guideline to prevent and control water inrushes.

Groundwater, an important source of drinking water globally, is susceptible to contamination by naturally occurring metals and radionuclides. Regional trends in groundwater quality are useful in pr...

A tectonic melange containing blueschists and eclogites, Middle Ordovician mid-ocean ridge basalt, cherts, and clastic sediments occurs at Port Macquarie in the southern New England Orogen,...

With the reduction of oil and gas reserves and the increase of mining difficulty in Northern China, the carbonate rocks in Southern North China Basin are becoming a significant exploration target for carbonate reservoirs. However, the development characteristics, formation stages, formation environments and mechanisms of the carbonate reservoirs in Southern North China Basin are still unclear, which caused the failures of many oil and gas exploration wells. This study focused on addressing this unsolved issue from the Ordovician carbonate paleokarst in the Huai-Fu Basin, which is located in the southeast of Southern North China Basin and one of the key areas for oil and gas exploration. Based on petrology, mineralogy and geochemical data, pore types, distribution characteristics, and formation stages of the Ordovician paleokarst were analyzed. Then, in attempt to define the origins of porosity development, the formation environments and mechanisms were illustrated. The results of this study showed that pore types of the Ordovician carbonates in the Huai-Fu Basin are mainly composed of intragranular pores, intercrystalline (intergranular) pores, dissolution pores (vugs), fractures, channels, and caves, which are usually in fault and fold zones and paleoweathering crust. Furthermore, five stages and five formation environments of the Ordovician paleokarst were identified. Syngenetic karst, eogenetic karst, and paleoweathering crust karst were all developed in a relatively open near-surface environment, and their formations are mainly related to meteoric water dissolution. Mesogenetic karst was developed in a closed buried environment, and its formation is mainly related to the diagenesis of organic matters and thermochemical sulfate reduction in the Permian-Carboniferous strata. Hydrothermal (water) karst was developed in a deep-buried and high-temperature environment, where hydrothermal fluids (waters) migrated upward through structures such as faults and fractures to dissolve carbonate rocks and simultaneously deposited hydrothermal minerals and calcites. Lastly, a paleokarst evolution model, combined with the related porosity evolution processes, nicely revealed the Ordovician carbonate reservoir development. This study provides insights and guidance for further oil and gas exploration in the Southern North China Basin, and also advances our understanding of the genesis of carbonate paleokarst around the world.

Lower Paleozoic dark shale is developed in the western Middle Yangtze Block, which lays a material foundation for the enrichment and accumulation of marine shale gas. In order to ascertain the control action of geological structures on the differential preservation of shale gas and reveal the key factors in shale gas preservation, this paper firstly analyzed the structure characteristics of this area, carried out structure pattern recognition and structural belt division, and studied structural deformation mode and intensity. Based on this, the relationships between different structure styles and shale gas preservation conditions were analyzed. Finally, combined with the structural deformation and the lithofacies paleogeographic characteristics of marine shale, the favorable exploration zones of shale gas were proposed. And the following research results were obtained. First, the western Middle Yangtze Block can be divided into four structural deformation belts, and three types of piggyback structural patterns have been identified, including restricted type, weakly reformed type and strongly reformed type. Second, the restricted type is located in the northwestern part of Hunan and Hubei Provinces. In this pattern, piggyback structure is incomplete and thrust belt and Compressive fold belt are developed. Third, the weakly and strongly reformed types are located in the western parts of Hunan and Hubei, and Wulingshan area, respectively. They both have complete piggyback structures, but the former has lower deformation intensity and has never undergone the late superimposed reformation. Fourth, there are three structural transfer belts in the western Middle Yangtze Block, i.e. the structural transfer belt between the East Sichuan fault–fold belt and West Hunan–Hubei fault–fold belt, the structural transfer belt between West Hunan–Hubei fault–fold belt and Wulingshan fault–fold belt, and the structural transfer belt between the outcrop and the hinterland of Middle Yangtze Block. The first one is structurally transformed at the Qiyueshan fault. The East Sichuan fault–fold belt on the west is an ejective fold with low fault density and formation denudation intensity, where shale gas is enriched in anticlines and slopes; while the West Hunan–Hubei fault–fold belt on the east is a trough-like fold with strong faulting and high formation denudation intensity, where shale gas is enriched in residual synclines. In conclusion, shale gas preservation conditions of Upper Ordovician Wufeng Formation–Lower Silurian Longmaxi Formation in this area are the best in Zigui syncline, thrust–detachment zone and western margin of Qiyueshan fault. The favorable exploration areas of shale gas of Lower Cambrian Niutitang Formation are distributed in the western flank of Yichang Slope, Kaixian thrust zone, compressive fold zone and thrust–detachment zone.

To further ascertain the origin of the Ordovician Majiagou Formation reservoirs in the Ordos Basin, the M54-M51 sub-members of the Ordovician Majiagou Formation in the eastern Sulige gasfield of Ordos Basin were taken as examples to analyze the vertical development characteristics of eogenetic karst and to discover the dissolution mechanism and its control on reservoirs through observation of a large number of cores and thin sections. According to detailed analysis of petrologic characteristics, the reservoir rock types include micritic dolomite, grain dolomite and microbialite which have mainly moldic pore, intergranular (dissolved) pore, and (dissolved) residual framework pore as main reservoir space respectively. The study area developed upward-shallowing sequences, with an exposure surface at the top of a single upward-shallowing sequence. The karst systems under the exposure surface had typical exposure characteristics of early dissolution and filling, indicating these reservoirs were related to the facies-controlled eogenetic karstification. With the increase of karstification intensity, the reservoirs became worse in physical properties.

Ophiolite pulses, which are periods of enhanced ophiolite generation and emplacement, are thought to have a relevance to highly active superplumes (superplume model). However, the Cambrian-Ordovician pulse has two critical geological features that cannot be explained by such a superplume model: predominance of subduction-related ophiolites and scarcity of plume-related magma activities. We addressed this issue by estimating the mechanism and condition of magma generation, including mantle potential temperature (MPT), from a ~500 Ma subduction-related ophiolite, the Hayachine-Miyamori ophiolite. We developed a novel method to overcome difficulties in global MPT estimation from an arc environment by using porphyritic ultramafic dikes showing flow differentiation, which have records of the chemical composition of the primitive magma, including its water content, because of their high pressure (~0.6 GPa) intrusion and rapid solidification. The solidus conditions for the primary magmas are estimated to be ~1450 °C, ~5.3 GPa. Geochemical data of the dikes show passive upwelling of a depleted mantle source in the garnet stability field without a strong influence of slab-derived fluids. These results, combined with the extensive fluxed melting of the mantle wedge prior to the dike formation, indicate sudden changes of the melting environment, its mechanism, and the mantle source from extensive fluxed melting of the mantle wedge to decompressional melting of the sub-slab mantle, which has been most plausibly triggered by a slab breakoff. The estimated MPT of the sub-slab mantle is ~1350 °C, which is very close to that of the current upper mantle and may reflect the global value of the upper mantle at ~500 Ma if small-scale convection maintained the shallow sub-slab mantle at a steady thermal state. We, therefore, conclude that the Cambrian-Ordovician ophiolite pulse is not attributable to the high temperature of the upper mantle. Frequent occurrence of slab breakoff, which is suggested by our geochemical compilation of Cambrian-Ordovician ophiolites, and subduction termination, which is probably related to the assembly of the Gondwana supercontinent, may be responsible for the ophiolite pulse.

The Siratur gold deposit occurs in the northern part of the South Uralian segment of the Main Uralian Fault. The ore zone of the deposit is confined to an area of a tectonic junction of serpentinites of the Nurali massif and a longitudinal band of carbonaceous schists of the Ordovician Polyakovka Formation, which also hosts abundant basalts with petrogeochemical features of mid-oceanic ridge mafic volcanic rocks. The gold mineralization of the Siratur deposit mostly includes stringer-disseminated gold-sulfide type (1) in black shales and stringer-disseminated gold-quartz-low sulfide type (2) in listvenite–beresite complex. The microprobe analysis of gold grains showed that the fineness of type 1 and 2 gold is 670–820 and 940–970, respectively. The deposit can be ascribed to the polygenic and polychronous type.

Fault-controlled karst carbonate reservoirs are one of the most important reservoir types in the Tahe oilfield of the Tarim Basin. These reservoirs have a large oil reserve and belong to a strongly reconstructed reservoir type with a highly heterogeneous distribution of pores and fractures. This study characterizes a fault-controlled karst reservoir by using integrated methods, including outcrops, well logging, structure interpretation, seismic inversion, and statistical geomodeling. We have established a fault-/fracture-controlling karstic geologic model and classified the internal architectural elements so that we adopted an origin-controlled hierarchical geomodeling strategy based on the fault-controlling characteristics. The results determined that large strike-slip faults provide an important tectonic framework and that its derived fractures act as important channels and spaces for dissolution. Flower structure fault zones and the associated fractures are the main range of karst development, within which a high stress is concentrated during the strike-slip shear process with a high-density fracture development. This is the highly developed karst reservoir, which mainly is concentrated along large faults. The coexistence of fractures and karst dissolution has resulted in a complicated reservoir architecture (karst architecture), which can be classified into four types: (1) large caverns, (2) small caverns and vugs, (3) fractured zones, and (4) matrix (tight limestone). Controlled by the degree of dissolution, the karst architecture is quite different from the sedimentary facies. Large caverns are formed under the strongest degree of dissolution and are the most favorable reservoir type. Small caves and vugs are created under relatively strong dissolution; they are distributed outside large caves and also can act as favorable reservoirs. The fractured zones are not necessarily affected by strong dissolution but have high conductivity and act as important channels for fluid movement. The carbonate matrix is less reconstructed. The architecture development model of the fault-controlled karst carbonate reservoir presented a tree system, within which the karst reservoir caves are connected by the fractures and faults similar to fruits and trunks. The new geomodeling method revealed the constraining characteristics of faults, seismic attributes, and hierarchical architectural elements. Furthermore, we also have built a 3D model of the Tuoputai unit in the Tahe oilfield to show the robustness of this workflow. This research enables us to better understand the structure of fault-controlled karst reservoirs, and it could provide a specified characterization approach that is considered to be theoretically and practically useful.

It is difficult to determine flow pathways of paleofluids in sedimentary basins. Petrography, fluid inclusion, and isotopic compositions from different phases of calcite veins in the Ordovician in the Tazhong area were studied. Our results reveal a complex diagenesis process with two importantly crossformational fluid movements, coupled with three phases of calcite veins (C1, C2, and C3) precipitation and thermochemical sulfate reduction by hydrocarbons. Fluids that precipitated C2 and C3 calcites were saline and enriched in 18O and thus may have originated from underlying evaporates, whereas fluid that precipitated C3 had an additional contribution from 87Sr-rich, Ba-rich, and Zn-rich Precambrian basinal clastic units. Fluid inclusion homogenization temperatures, salinities, and δ18Owater values of C2 veins show decreasing trends with increasing distances from strike-slip faults. This may indicate that C2 fluid might migrate into Ordovician carbonate reservoirs through strike-slip faults within 6 km during late Hercynian orogeny (late Permian). The similarly decreasing trends are observed in C3 veins, indicating that the eastern part of No. 1 fault seemed to be the main fluid-flow pathway for C3 calcite precipitation during Himalayan orogeny. Hydrocarbon may have migrated along the similar pathways as supported by (1) abundant primary hydrocarbon-bearing fluid inclusions in the C2 and C3 calcites and (2) well-documented hydrocarbon migration and mixing. This is an ideal case showing how spatial variation in geochemical data from fracture fillings can be used to trace paleowater and likely hydrocarbon flows in sedimentary basins; it thus may be useful in predicting fluid flow and distribution of hydrocarbons in the Tarim Basin and other basins.

The Siratur gold deposit is located in the northern part of the South Ural segment of the Main Ural Fault. Its ore zone is confined to the area of the tectonic junction of the serpentinite of the Nurali massif and the submeridional strip of carbonaceous schists of the Ordovician Polyakovka Formation. Its composition is widely represented basalts with chemical characteristics of the main effusive rocks of the mid-ocean ridges. In the black shale deposits of the Siratur deposit, gold ore mineralization is mainly represented by the gold-sulfide vein-disseminated type, and in the listwanite-beresite complex, gold-quartz-low-sulfide vein-streaked type. Microprobe studies of zolotins showed that gold in ores of the first type has a low-qality of 670–820, and in the second — 940–970. The deposit can be attributed to the polygenic and polychronous type.

Calcified cyanobacteria are of considerable research value for reconstructing the ecology of Paleozoic and Mesozoic benthic communities on carbonate platforms due to their ability to produce oxygen and fix nitrogen and CO2. The diversity and abundance of calcified cyanobacteria was initially suggested to have declined in the Middle and Late Ordovician, although more recent work suggests that complex and diverse assemblages persisted throughout the Ordovician. Here, calcified cyanobacteria and associated microfossil flora from the Middle and Late Ordovician of the Ordos Basin, North China Block, are systematically described for the first time based on 1330 thin sections from seven outcrop profiles and four drill cores. In total, there are 18 species belonging to 16 genera, including a new species,Proaulopora ordosian. sp.Girvanella,Subtifloria,Acuasiphonoria,Xianella, and Oscillatoriaceae gen. indet. are assigned to Osillatoriales of cyanobacteria;Ortonella,Hedstroemia,Cayeuxia,Zonotrichites,Proaulopora, andPhacelophytonare assigned to Nostocales of cyanobacteria; andGarwoodia,Renalcis,Izhella,Rothpletzella, andWetheredellaare assigned to calcified Microproblematica. A literature survey of Ordovician microfloral assemblages shows that cyanobacteria and associated microfossils occur in reef, open platform, lagoon, and tidal facies. Most genera occur on at least two independent blocks, and many have a cosmopolitan distribution in similar sedimentary facies. Our research suggests that calcified cyanobacteria and associated microfossils formed complex ecosystems and played greater ecological roles on carbonate platforms during the late Middle and Late Ordovician than was previously thought.UUID:http://zoobank.org/1812ccf8-136c-4cff-92ba-faeaf06523ef

Granitoid rocks are universal in continental crust and are of special significance in understanding tectonic settings. This paper presents detailed zircon U-Pb dating, Hf isotope, whole-rock geochemistry, and Sr-Nd-Pb isotope analyses, and mineralogy of two Ordovician granitoid intrusions and one quartz diorite intrusion in Western Kunlun, NW Tibetan Plateau. The Yutian Complex is composed of diverse rock suites, including monzogabbros, quartz monzodiorites, monzogranites, and monzodioritic enclaves. These suites have similar rock formation ages (447–440 Ma) and minerals, e.g., amphibole grains from different suites belonging to pargasite. Moreover, they exhibit geochemical similarities, such as broadly parallel trace-element patterns characterized by enrichments in light rare earth elements and large ion lithophile elements, and depletions in high field strength elements, which are typical features of arc rocks. Furthermore, the studied samples display homogeneous zircon Hf values, e.g., εHf(t) = −1 to −3, and whole-rock isotopic compositions, e.g., εNd(t) = −4 to −6. Thus, they were most likely derived from a mantle wedge enriched by subducted sediments and fluids, which then evolved into different suites through fractional crystallization of hornblende and plagioclase. The ca. 440 Ma North Yutian quartz diorite intrusion, with an average of εHf(t) value of −6, was a product of the partial melting of mafic lower crust through slightly fractional crystallization of hornblende. In contrast, the ca. 470 Ma Aqiang granodiorite intrusion has εHf(t) values varying from −5 and −2, but it has heterogeneous petrological and geochemical features. It is considered to be a product of the partial melting of the overriding mantle wedge modified by fluids derived from the subducted Proto-Tethys slab and some mixed crustal materials. The Aqiang samples belong to the slightly fractionated I-type series, but they have variable alumina saturation index (ASI = molar Al2O3/[CaO – 3.33 × P2O5 + Na2O + K2O]) values (0.74–1.03) due to variable peraluminous biotite contents. The different suites in the Yutian Complex display low ASI values (

M54−1a is a new natural gas exploration target of the Ordovician in the Ordos Basin and is expected to have large-scale reserves and a production increasing area after M51 and M52 weathering crust gas reservoir. Based on the data of the drilling core, thin section, cathodoluminescence, physical properties, and geochemical characteristics, a study has been conducted about the genesis and distribution of the M54−1a reservoir in the central and eastern parts of the Ordos Basin. The study results showed the following: (1) The M54−1a reservoir is composed of fine-silt-crystalline dolomite containing anhydrite nodules in lithology. The reservoir storage space is mainly the moldic pores of anhydrite nodules with a small amount of anhydrite crystal moldic pores and micro-fractures. The average porosity is 5.54%, and the average permeability is 1.53 × 10−3μm2. (2) The reservoir pores have experienced anhydrite precipitation, atmospheric freshwater dissolution, mesocrystalline calcite filling, quartz filling, medium-coarse crystal dolomite filling, megacryst calcite filling, and cataclasis. (3) Differential dissolution and differential filling influenced by paleogeomorphology controlled the planar distribution of the reservoir. The results of the comprehensive analysis indicate that the superimposed area between the high part of sedimentary paleogeomorphology and the karst slope at the west of Yulin-Jingbian-Yanan as well as the Hongdunjie-Qingyangcha area are high-quality reservoir distribution areas with strong dissolution and low filling degree. The study results provide the theoretical basis for the next exploration and well deployment.

A new bryozoan species Dianulites altaicus sp. nov. (order Esthonioporata) is described from the Sandbian Stage of the Upper Ordovician of the environs of Lake Teletskoe in Gorny Altai. A collection of bryozoans of the genus Dianulites Eichwald, 1829 from the Lower and Middle Ordovician of the Novaya Zemlya archipelago and Vaygach Island in the Russian Arctic has been revised. Bryozoans from the Lower and Middle Ordovician of the Novaya Zemlya archipelago previously classified as D. janischevskyi Modzalevskaya, 1953 were shown to belong to D. altaicus sp. nov. Two groups of species of the genus Dianulites have been identified in the Ordovician of the Russian Arctic: the representatives of the first group had mesozooecia, whereas the representatives of the second group, which includes the new species, did not have mesozooecia, but some of them had minutopores.

The Ordovician carbonate reservoir is the most productive deep-earth reservoir in Tahe Oilfield and other oilfields in Tarim Basin. Exploration and production successes in recent years reveal a new reservoir type, namely faulted karst reservoirs, which is closely related to regional strike-slip faults and very different from the well-recognized paleo-karst reservoir. The paleo-karst reservoirs distribute mainly in weathering crust regions in the northern Tahe Oilfield. Their primary reservoir spaces are meter-scale caves and the fluid conduits are predominantly the unconformable surfaces. In production, paleo-karst reservoirs always have sufficient energy, therefore high productivity. The faulted karst reservoirs mainly develop in southern Tahe Oilfield, controlled by the different ordered strike slip faults and related dissolutions. Their reservoir space is smaller than which of paleo-karst reservoirs. The predominant fluid conduits in these reservoirs are the faults. In production, reservoirs along major strike-slip faults have sufficient energy, high productivity and slow watercut increase like paleo-karst reservoirs. While in areas with less strong energy, faulted karst reservoir exhibits weak productivityand rapid watercut increase, implying a rule of “big fault big reservoir, small fault small reservoir, no fault no reservoir. A comprehensive understanding of the geophysical features, distribution characteristics, reservoir property, and production behaviors of the two reservoir types will assist further exploration and production in Tahe Oilfield and other basins containing such reservoirs. Cited as : Lu, X., Wang, Y., Yang, D., Wang, X. Characterization of paleo-karst reservoir and faulted karst reservoir in Tahe Oilfield, Tarim Basin, China. Advances in Geo-Energy Research, 2020, 4(3): 339-348, doi: 10.46690/ager.2020.03.11

The Palaeozoic basin of Jordan and Iraq occupies the NW part of the Arabian Plate. The Lower Palaeozoic succession is the most under-explored succession in Iraq, particularly in western Iraq where it is considered the most promising target for future exploration. The western Iraqi subsurface and Jordan geology are characterized by similar major chrono-stratigraphic rock units. In western Iraq, the Akkas-1 well is the only gas and condensate discovery in sandstone reservoirs of Early Silurian and Late Ordovician age. Lower Silurian sandstones of the Akkas Formation are an oil reservoir characterized by a mean porosity of 6.5% and permeability of 0.2mD. Underlying Upper Ordovician sandstones of the upper part of the Khabour Formation are also reservoir, but for sweet gas and condensate. The latter are comparable to both the Risha and Wadi Sirhan discoveries in Jordan. Several deeper potential sandstone reservoirs (of older Ordovician and Cambrian ages) are also present across the region. Equivalent Ordovician age Jordanian reservoirs are primarily developed in homogeneous, highly mature quartz arenites of storm-dominated clastic-shelf stratigraphies. Within the Risha Field and the Wadi Sirhan area, are also end-Ordovician (Hirnantian) glaciogenic and periglacial sandstones with more variable reservoir quality characteristics compared to the underlying quartz arenites. These often comprise highly immature arkoses (as diamictites) that provide a mineralogically much less stable assemblage in which there is a possibility of developing secondary porosity. Sedimentological and XRD mineralogical studies suggest that diagenesis is an important factor in developing of reservoir quality. Nevertheless, reservoir quality is a major exploration risk where burial was significant that led to quartz overgrowth and clay cementation. It is notable that some sandstone reservoirs have chlorite-clay coatings, which prevented formation of the quartz overgrowth and preserved good reservoir quality. The widespread occurrence of associated source and seal rocks together with the proven and potential reservoirs, suggests that Lower Palaeozoic prospects below the deserts of western Iraq are promising targets for future hydrocarbon exploration.

Research objects. Paleozoic (Ordovician–Middle Devonian) volcanic-sedimentary complexes of the contiguous Sakmara and Mugodzharskaya structural zones of the Southern Urals, which have a paleo-oceanic and island-arc nature. Мaterials and methods.The work was carried out on the basis of many years of personal research of the author with the involvement of literature data on other folded zones of the Southern Urals. Main results. In the early Devonian, the first intense clustering of different facies complexes of rocks took place, which determined the tectonic face of the modern western part of the fold belt. The uniformity and set of Paleozoic formations in all structures of the Urals are maintained along its entire strike and are not repeated in neighboring zones.The drilling data showed the primary character of bedding of the rocks of the Upper Devonian Zilair Formation on the dislocated volcanogenic-sedimentary deposits of the Lower-Middle Paleozoic. Thrust contacts are observed only in the marginal parts of the zone. The almost textbook views on the cover bedding of the Kraka and Kempirsai hyperbasite massifs are in contradiction with the data on the presence of “roots” up to 4–8 km deep in them. By the beginning of the Devonian, the Kempirsai massif was located within the Sakmara zone and was “cross-linked” with the surrounding effusive rocks of the Middle Ordovician by gabbro-diabase dykes. According to the results of seismometric studies, features of similarity of the deep structure of the base of the Sakmara zone with the structure of the Magnitogorsk-Mugodzharskaya zone, which also revealed a melanocratic basement, have been established. Conclusions. The formed structural zones of the Southern Urals are distinguished by a set of rocks of the same age, their stratigraphic range, and the autonomy of feeding areas. These differences have survived to this day. All subsequent tectonic episodes, including the sharpening epochs, only complicated the appearance and structure of the Southern Urals, without changing the relative position of structures that arose in the Early Devonian. There was no transfer, swarming of rocks from one zone through another in any of the subsequent stages of compression. The Early Devonian stage was the first, but at the same time the most intense, which determined the tectonic face of the modern western part of the Urals.

The Ordovician (485–444 Ma) saw a global shift from microbial- to skeletal-dominated reefs, and the rise of corals and bryozoans as important reef-builders. Hypothetically, increasingly morphologically diverse and abundant reef-building metazoans increased spatial habitat heterogeneity in reef environments, an important component of reefs' capacity to support diverse communities. Quantifying the spatial scale and extent of this heterogeneity requires three-dimensional exposures of well-preserved reefs whose composition and spatial arrangement can be measured. The Darriwilian (c. 467–458 Ma) carbonate sequence of the Mingan Archipelago, Quebec, presents such exposures, and also provides an opportunity to establish how the distribution of skeletal-dominated metazoan reefs contributed to, and was influenced by, seafloor relief. This study includes two transects through a 200–300 m wide paleo-reef belt, which developed along a rocky paleo-coast line. The reefs are typically micrite-rich, meter-scale mounds, locally forming larger complexes. Here, we present quantitative evaluations of the composition of these reefs, and detailed mapping of reef distributions. There is high compositional heterogeneity between reefs at spatial scales ranging from meters to kilometers, contributed by differences in the volumetric contribution of skeletal material to the reef core, and in the identity of the dominant reef-builders. We suggest that the abundance and morphological diversity of Middle Ordovician reef building metazoans made them important contributors to environmental and substrate heterogeneity, likely enhancing the diversity of reef-dwelling communities.

Widely distributed in North China, Ordovician karst is characterized by having high thickness, nonuniform aquosity, and significant water pressure-bearing properties. Deep mining in North China is threatened by associated water hazards; hence, research on the hydrogeological characteristics of deep Ordovician karst is needed. In this study, the Weibei coalfield in Shaanxi Province, China, was selected as the study area, especially mines in the Hancheng and Chenghe mining areas. In situ experiments, including water pumping, water drainage, water injecting and water pressure, and laboratory experiments, were conducted to study the hydrogeological characteristics of the Ordovician karst top in the study area. A comprehensive analysis was conducted on controlling factors for the development of the Ordovician karst top in the study area, and a method for evaluating the water inrush risk in coal mining areas based on karst hydrogeological characteristics was proposed. The research results indicated that the Ordovician karst top in the study area was characterized by heterogeneity, vertical zonation, and partially filled properties, which were mainly controlled by two factors: sedimentation and tectonism. The hydrogeological conditions of the Ordovician karst could be divided into three types: nonfilled and nonsignificant tectonism, filled and nonsignificant tectonism, and significant tectonism. Among them, the filled and nonsignificant tectonism type Ordovician karst top type had a filling thickness of 20 m. Based on karst hydrogeological characteristics, the methods were proposed to evaluate the water inrush risk in the coal mining floor. The practical tests verified the methods.

Subject. The Upper Ordovician reef Bol’shaya Kos’yu, located on the western slope of the Northern Urals in the basin of the Ilych River is a unique object among coeval reefs due to the weak dolomitization of rocks and the ability to trace the phases of its development. Materials and methods. The article presents the results of paleontological, paleoecological and microfacial analyses of reef limestones selected by the author from 12 natural outcrops with a total capacity of about 150 m located along both banks of the Bol’shaya Kos’yu River. Results. The structure of the reef, morphology and taxonomic composition of the frame-forming biota were established, and the important role of sphinctozoal sponges was first identified. Among the reef limestones in the main genetic characteristics that reflect specific depositional environments, was identified three types of microfacies. Type 1 – reef microfacies – framestones and buflestones. Type 2 – microfacies of interbiohermic space and inter-carcass cavities exposed to wave and tidal action – bioclastic and peloid pakstones, biolithoclastic grainstones and lithoclastic rudstones. Type 3 – microfacies of the hidden microcosmic cavities, protected from the active wave impact – wackstone. Сonclusion. The composition of the microfacies and frame-forming biota characterize difficult differentiated, both on a section, and structure the reef, which was formed in the conditions of the carbonate platform-shelf margin with active hydrodynamics. Its growth occurred during transgression, which ended by the end of the middle katian a major regression that stopped its development. The revealed paleontological, paleoecological and microfacial data indicate that starting from the middle katian the first identified sphinctozoal sponges could settle in sublittoral environments with active hydrodynamics at depths up to 20 m.

This work analyses the development of reefs, which represent thick morphologically expressed carbonate massifs created by the activity of frame-forming organisms. Major stages in the development of reefs include the periods of Early Cambrian, Ordovician, Silurian-Devonian, Late Visean-Serpukhovian and Permian, the upper boundaries of which are generally determined by mass extinction events. The main reef builders during the Early Cambrian were archaeocyatha and epiphytaceans, while Ordovician reefs were created by sponges, stromatoporoids, tabulata and bryozoans. Stromatoporoid, tabulata, rugose and, to a lesser extent, bryozoans were the reef builders during the longest Silurian-Devonian period. Bryozoans and corals served as the frame-forming organisms for Upper Visean-Serpukhovian reefs. However, during the Permian, reefs were built primarily by sponges and, to a lesser extent, by bryozoans and corals. It is shown that there is no strict correspondence between the termination of reef formation and the beginning of extinction events. During extinction periods, potential reef builders did exist, although beyond the reef biocoenosis structure. Similarly, the beginning of reef formation occurs somewhat later than the appearance of the corresponding frame-forming organisms. Apparently, it takes time for organisms to develop such a biocoenosis that could trigger the formation of reef ecosystems. Reef ecosystems are stable under relatively constant external conditions, changes in which cause first the degradation of the biocoenosis and then the extinction of its constituent organisms. Therefore, the process of reef formation is terminated before the actual extinction of organisms.

The Strona-Ceneri Zone is located south of the Insubric line, where Alpine overprint is mainly brittle and of low grade or even absent. Apart from the unmetamorphic Permo-Carboniferous sediments and intrusiva the Strona-Ceneri Zone is an Ordovician gneiss complex composed of paragneisses, migmatites, peraluminous orthogneisses and banded amphibolites associated with meta-gabbros and meta-ultramafics. Despite of the individual characters of the other Ordovician gneiss terranes north of the Insubric line, most prominently the Aar Massif, the Gotthard, Silvretta and Ötztal nappes, they show analogous lithological formations and pre-Mesozoic structures indicating similar genetic processes. A revision of geological maps and new field observations in these gneiss terranes indicate characteristic spatial relationships of banded amphibolite formations with migmatites and orthogneisses. The contrasting chemistries of the peraluminous rocks (para- and orthogneisses) and the basaltic amphibolites with a lack of intermediate lithologies in between can be explained by the setting of peraluminous arc magmatism within a subduction–accretion complex. In this model the amphibolites represent primary basalts which ponded at the base of the subduction–accretion complex and delivered the heat for the production of peraluminous melts. Volume estimations indicate a “zone of intermingling” composed of immiscible basalts and peraluminous melts with a total thickness of several kilometers. In this “zone of intermingling” the protoliths of the banded amphibolite formations were generated. Steep strike-slip faults, which are important structures for the syn-magmatic cratonization of subduction–accretion complexes, provide pathways for the emplacement of magmas, migmatites and intermingled materials. This results in the formation of steeply oriented sheets of orthogneisses, deformed migmatites and banded amphibolite formations, respectively. Finally, the paper lists many other peri-Gondwanan regions with an early Paleozoic peraluminous arc magmatism, indicating a similar setting on a global scale.

The results of U–Pb geochronological studies of Early Paleozoic granitoids from the northern part of the Bureya continental massif are presented. It has been established that porphyroblastic hornblende–biotite granites of the Kivili massif referring to the complex of the same name are of 502+6 Ma in age, while weakly gneissic biotite granites of the Kivity massif, Sularin complex, are of 492+6 Ma in age. Thus, granitoids of both complexes are of Cambrian rather than Ordovician age, as was previously assumed. In combination with materials already obtained, these data indicate that intensive granitoid magmatism manifested itself in the Middle Cambrian–Early Ordovician within the Bureya continental massif. It is noteworthy that, in the structure of the Jiamusi–Khanka continental massif, which is adjacent to the Bureya one, Early Paleozoic magmatism manifested itself in a broader range, which is from the Early Cambrian to the Silurian. This may indicate both differences in the formation history of these continental massifs and insufficient knowledge of the Bureya massif.

Industrial and municipal wastewater and oilfield brines have been disposed of into the Cambrian-Ordovician Arbuckle Group for decades in Kansas and nearby states in the midcontinent United States. The industrial and municipal wastewater disposal wells (designated Class I disposal wells) are regulated by the Kansas Department of Health and Environment. The oilfield brines are disposed of in Class II disposal wells, which are regulated by the Kansas Corporation Commission. Annual testing of formation pressure and static fluid levels in Class I wells compose a body of data that is useful in monitoring movement of water and fill-up of Arbuckle disposal zones. In western Kansas, the depth to water in wells penetrating the Arbuckle can be several hundred to more than a thousand feet (305 m) below ground surface, but in parts of southern and southeastern Kansas, the depth to water locally can be less than 100 ft (31 m). Furthermore, most Class I wells indicate Arbuckle fluid levels in central and south-central Kansas are rising ~10 ft (~3 m) annually, suggesting that at current disposal rates, the Arbuckle may lose its capacity to accept wastewater under gravity flow in parts of the state in the next few decades, principally south-central and southeastern Kansas along the Oklahoma state line. At present in parts of six Kansas counties along the Oklahoma state line, low-density (~1.0 g/cc or slightly greater density) wastewater in a wellbore does not have a sufficient hydrostatic head by gravity alone to force its way into the more dense resident Arbuckle formation water. In general, Arbuckle formation water flows west to east in Kansas. Arbuckle disposal wells in Kansas collectively dispose of ~800,000,000 barrels (~127,000,000 m3) of wastewater per year, although some of this is recycled from Arbuckle oil production. Declines in oil price since mid-2014 have resulted in less oilfield disposal in the Arbuckle since 2015. The number of Class I wells recording annual fluid rises have also declined since 2015, as has the median of their annual change in static fluid level, but overall, more Class I wells are still recording fluid rises. There is a poor correlation between changes in fluid levels in Class I wells and the volume of fluid disposed in them annually, thereby indicating that more regional characteristics may control water movement in the Arbuckle. More monitoring wells are needed to better understand the movement of water in the deep subsurface and to anticipate any potential problems that may occur with reduced disposal capacity and possible migration of fluids through unplugged or improperly plugged older wells.

The southern margin of the Ordos Block (S-OB) is located in the western part of the southern region of the North China Craton (S-NCC), and has been involved in multiple orogenic episodes that uplifted the North Qinling Orogenic Belt (NQB). Determining whether the S-OB deposits and sources were controlled by late Precambrian or Ordovician tectonic sedimentation is extremely important for understanding the paleo-tectonic evolution of the S-NCC and its relationship with the North Qinling Terrane (NQT). In this study, the depositional age and trace of the source area of the Tangwangling Group (TWL) were constrained using detrital zircon U-Pb isotopic analyses. Six sandstone samples collected from the TWL Group yielded similar detrital zircon age patterns, with four distinct clusters at 2588-2305 Ma, 2100-1600 Ma, 1251-1005 Ma 995-744Ma which correspond to peak ages of 2.45 Ga, 1.80 Ga, 1.10 Ga, and 0.82 Ga, respectively. The youngest zircon ages that peak at 0.82 Ga suggest that the depositional age of the TWL Group is the closest to the late Neoproterozoic, based on comparisons with geochronological data from late Precambrian and Ordovician successions that outcrop in the OB. The detrital zircon age ranges of the TWL Group, in comparison with those of the NCC and NQT, also indicate that clastic grains deposited in the TWL Group were mainly sourced from NCC basement rocks and less so from the late Precambrian NQT complex, which was coincident with a Grenvillian-aged tectonomagmatic event. Thus, we conclude that the NQT had accreted onto the S-NCC before the TWL Group were deposited during the late Neoproterozoic, implying that the S-NCC or the S-OB may have experienced or was at least influenced by the assembly and breakup of Rodinia.

At >7 km depths in the Tarim Basin, hydrocarbon reservoirs in Ordovician rocks of the Yijianfang Formation contain large cavities (c. 10 m or more), vugs, fractures and porous fault rocks. Although some Yijianfang Formation outcrops contain shallow (formed near surface) palaeokarst features, cores from the Halahatang oilfield lack penetrative palaeokarst evidence. Outcrop palaeokarst cavities and opening-mode fractures are mostly mineral filled but some show evidence of secondary dissolution and fault rocks are locally highly (c. 30%) porous. Cores contain textural evidence of repeated formation of dissolution cavities and subsequent filling by cement. Calcite isotopic analyses indicate depths between c. 220 and 2000 m. Correlation of core and image logs shows abundant cement-filled vugs associated with decametre-scale fractured zones with open cavities that host hydrocarbons. A Sm–Nd isochron age of 400 ± 37 Ma for fracture-filling fluorite indicates that cavities in core formed and were partially cemented prior to the Carboniferous, predating Permian oil emplacement. Repeated creation and filling of vugs, timing constraints and the association of vugs with large cavities suggest dissolution related to fractures and faults. In the current high-strain-rate regime, corroborated by velocity gradient tensor analysis of global positioning system (GPS) data, rapid horizontal extension could promote connection of porous and/or solution-enlarged fault rock, fractures and cavities. Supplementary material: Stable isotopic analyses and the velocity gradient tensor and principal direction and magnitude calculation are available at https://doi.org/10.6084/m9.figshare.c.4946046 Thematic collection: This article is part of the The Geology of Fractured Reservoirs collection available at: https://www.lyellcollection.org/cc/the-geology-of-fractured-reservoirs

Preservation bias may significantly impact the application of detrital zircon geochronology in reconstructing Earth surface processes. Here we compare detrital zircons from the actively eroding Murchison River channel in Western Australia with Ordovician fluvial sediments that have drained similar source rocks along the western margin of the West Australian Craton. In addition to standard analysis of detrital zircon age spectra we apply multivariate statistics to test the relation between 3-D grain shape, U-content and U–Pb ages, with the objective to quantify differences between both sample groups and track preservation along the transport pathway of the Murchison River. Our results show that zircon grains in modern river sands display an upstream trend toward larger surface areas, volume equivalent diameters and grain widths, as well as toward higher U-contents and lower apparent grain densities. 3-D grain shape, size and age spectra of Murchison River zircons evolve consistently downstream, but even at the river outlet remain distinct from the Ordovician samples, as a less mature representation of source. We interpret Ordovician river zircons to represent a significantly depleted subset from which up to 22% of the zircon population may have been lost compared to the actively transported detrital load. This discrepancy between the characteristics of detrital zircons in modern active rivers and ancient fluvial Ordovician sandstones demonstrates a bias that could be relevant for other source-sink detrital transport systems throughout Earth history. Keywords: X-ray tomography, Detrital, Zircon, Grain shape, Preservation, Fluvial

Karsted carbonates of the Ordovician Yingshan Formation represent significant hydrocarbon reservoirs in the Tarim Basin, China. Due to the geologic complexity of the formation, realistically predicting and modeling karst zones and rock properties is challenging. This drives the need to apply diverse techniques for building a suitable geologic model. We have developed a static model approach that uses fully automated seismic facies classification processes for predicting and modeling patterns associated with karst elements. Our method uses a seismic attribute and well logs as input data. We initially processed a seismic facies volume using the hierarchical clustering technique. This is based on seismic attribute values that take into account an optimal number of classes. The outcome reveals various patterns illustrated with low amplitudes highlighting the geomorphology of paleokarst elements. Simultaneously, a seismic traces map of the karsted interval was processed using the hybrid clustering technique conducted on seismic trace shape. In this case, the karst facies was extracted from the output and used as secondary input data in trend analysis of the model. Both outputs obtained from clustering techniques are processed in a volume of the most probable facies, which delineate the karst patterns. The results of the modeling process are visualized in various time slices and cross sections, appropriately recognizing the relationship of estimated patterns with karst zones. We have evaluated the karstification thickness and porosity map obtained from the 3D model that detail a reasonable connectivity between karst elements. This is based on the paleogeographic location and type of filling, as well as the dissolution development along the main striking faults. Finally, our method outputs a logical model of karst zones located within the host rock, which reduces the uncertainty and identify nonperforated segments.

The study on a unique set of outstandingly preserved sedimentary surface textures (SSTs) found in the late Tremadocian Áspero Formation of northwestern Argentina, coupled with the sedimentological and ichnological analysis, indicate that they were formed in the intertidal to supratidal setting of a mixed-energy estuary recording storm and tide sedimentation. We recognize seven types of SSTs: probably biotic microbial mat-related SSTs (Kinneyia, elephant skin, exfoliating sandy laminae), abiotic SSTs (elliptical scours and convex parallel ridges type I “wrinkle marks” sensuAllen 1985), and problematic (convex parallel ridges type II and dot matrix texture). Elliptical scours and convex parallel ridges type I show features which indicate reworking of a cohesive sandy substrate in an intertidal or supratidal setting. Abundance of biotic SSTs with specific associated trace fossils reflect matground development and mat-grazing ichnofaunas, indicating the suppression of intense, penetrative bioturbation due to intense physicochemical stress. The “dot matrix” texture, described here for the first time, consists of a regular horizontal network of millimeter-scale pits; it appears associated with exfoliating sandy laminae, probably reflecting a mat-related origin. Three facies associations are defined through paleoenvironmental analysis. Facies association 1 is dominated by high-energy sandy and bioclastic storm deposits with tidal flat facies, and corresponds to the outer bay of a mixed-energy estuary; highly impoverished Cruziana assemblages and distal expressions of the Skolithos Ichnofacies reflect high energy and sedimentation rate. Facies association 2 shows tidal-channel and tidal-flat facies with subordinated storm deposits, representing the middle bay; impoverished Cruziana assemblages dominated by simple facies-crossing structures, with high-density monogeneric opportunistic suites, evidence physicochemical stress associated with subaerial exposure, frequent episodic deposition, high water turbidity, and/or brackish water conditions in these relatively sheltered tidal flats. Facies association 3 is formed by interdistributary-bay deposits with intercalation of channel-fill deposits in the upper part, and represents the river-dominated bay-head delta; low degrees of bioturbation in fine-grained facies indicate brackish- to fresh-water conditions. SSTs are found in tidal flat facies of facies association 2; they indicate an intertidal to supratidal environment subject to localized conditions of intense physicochemical stress. The paleoenvironmental interpretation of SSTs converges with the one performed through sedimentological and ichnological analysis, producing a robust and more detailed paleoenvironmental model for the Áspero Formation. Our study highlights the use of SSTs as a tool for supporting and refining paleoenvironmental analysis.