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The Upper Permian Changhsing Formation is one of the most important gas-bearing formations in the Eastern Sichuan Basin. Nonetheless, the primary factors controlling the reef-shoal gas reservoirs of different regions are diverse. Clarifying the controlling factors for the formation of intra-platform reef-shoal reservoirs will promote the exploration and development process of the Changhsing Formation. By using outcrop data, core observation, thin section identification, scanning electron microscope analysis, physical property study, and logging interpretation, this paper analyse the characteristics and factors controlling the reef-shoal reservoir in Dianjiang area of the Eastern Sichuan Basin. The results show that the reef cap and reef base microfacies fine-to-medium crystalline dolomite reservoir have the best physical properties, and the reservoir spaces are primarily composed of intercrystalline pores, intercrystalline solution pores, karst caves, and fractures. The main lithology of reservoir formation is reef-shoal facies. The results for the carbon and oxygen isotopes show that dolomites of different particle sizes are formed in three environments. Affected by regression in the late Changhsingian Age, the reef-shoal of the Changhsing Formation were subaerially exposed to meteoric water diagenesis environments, resulting in the highly dispersed particles in rocks that formed a honeycomb karst system, which greatly improved the physical properties of the reservoirs. After reflux dolomitization, the honeycomb karst system was well preserved due to strong resistance to pressure and dissolution of dolomite. Therefore, dolomitization in the Changhsing formation contributed to reservoir porosity. Later, tectonic fracturing has adjusted and modified the reef-shoal reservoir. In summary, penecontemporaneous karstification was the key to the formation of the reef-shoal reservoir. [ABSTRACT FROM AUTHOR]

Detrital zircon grains were collected from six samples of sedimentary rock from the Middle Jurassic Zhiluo Formation in the western Ordos Basin of China. U–Pb isotopic geochronology data were measured and used to assess the provenance and directions of sediment transport of these grains. Five age populations were recognised amongst the U–Pb age spectra: 2700–2034 Ma, 1999–1426 Ma, 1296–636 Ma, 556–323 Ma, and 315–207 Ma. Each of these groups corresponds to one or several potential sediment sources around the western Ordos Basin. Based on these data and the spatial distribution of the six collected samples, we interpret that the material of the Middle Jurassic Zhiluo Formation in the western Ordos Basin was mainly derived from the northern margin of the North China Craton and the Qilian Orogenic Belt. In addition, the eastern margin of the Alxa Block contributed as a sediment source for certain regions. Detritus was transported via two major supply directions to the western Ordos Basin, and this basin was connected to the Hexi Corridor during the Middle Jurassic. In conclusion, our results are consistent with the closure of Palaeo-ocean and the formation of the orogenic belt. [ABSTRACT FROM AUTHOR]

The in situ zircon U‐Pb‐Lu‐Hf isotope records from end‐Permian volcanic interlayers in southwest China, integrated with previous studies, restructure the evolutionary history of the Yangtze Craton from Precambrian to Late Paleozoic. This includes early continental crust formation before ∼3.0 Ga and massive juvenile crustal growth at 2.6–2.4 Ga; large‐scale crustal reworking at ∼2.1–1.7 Ga; Neoproterozoic crust addition at ∼1.1 to 0.7 Ga; collision and subduction along the craton margin between ∼700–541 Ma; Early Ordovician to Late Silurian magmatism; and large tectono‐thermal events in the Middle Carboniferous to end‐Permian. Some zircons with TMD2 ages from 4.40 to 4.01 Ga and lower initial 176Hf/177Hf values of 0.280592 to 0.280726 may imply the existence of Hadean crust relics beneath the Yangtze Craton and their provenances could be associated with Hadean crustal remelting. This study further clarifies that the Precambrian‐age zircons between the end‐Permian volcanic interlayers, the complexes in the western margin of the Yangtze Craton, and the sedimentary Kangdian Basin, may share an affinity based on similar U‐Pb age spectra and Hf isotope features. It also shows that the Neoproterozoic tectono‐thermal event may be associated with large‐scale tectono‐rifting activity, which is different from the Grenville‐age continental collision between Yangtze and Cathaysia blocks in South China. The above findings support the inference of a widespread Archean basement extending to the western Yangtze Craton and a provenance in the Kangdian Basin that is derived from the weathering and erosion of Paleoproterozoic continental crust. [ABSTRACT FROM AUTHOR]

Pore-cave systems formed by karstification in the eogenetic stage of carbonate rocks provide abundant potential reservoir space for hydrocarbons. However, whether these dissolution pore-caves can become effective reservoir spaces during the later burial period, serving as the key to the success of hydrocarbon exploration. Therefore, it is important to explore the fluid activities and their alteration effects on eogenetic karst reservoirs during the later burial. Focusing on the Cambrian Longwangmiao Formation in the northwestern Sichuan Basin, this study systematically analyzed the formation of reservoir space in the eogenetic stage and the reworking of the system by fluids in the later stages, based on petrology, geochemistry, burial history, and tectonic evolution data. Results showed that many millimeters to several centimeters scale of pores and caves in the Longwangmiao Formation were produced by eogenetic karstification. These pore-caves underwent by two episodes of dolomite infilling in the shallow burial stage (D1) and in the Caledonian–Hercynian period (D2). Geochemical parameters indicate that D1 and D2 were both affected by meteoric water. In the early shallow burial stage, the dolomitic fluid was enriched in a relatively closed, reducing environment, whereas in the later stage, the fluid was affected by a relatively open oxidizing environment due to Caledonian–Hercynian fractures. Both D1 and D2 took place before the massive hydrocarbon migration from the Cambrian source rocks in the Middle Permian to those of the Middle Triassic. After the formation of the dissolution pore-caves, the precipitation from two episodes of dolomitic fluids led to the degradation of the Longwangmiao Formation carbonate reservoir space in the northwestern Sichuan Basin. In the southern part of the Shatan section-Well MS1, closed to the paleo-uplift of the central Sichuan Basin, where eogenetic karstification was superimposed by Caledonian–Hercynian supergene karstification, may be form effective reservoir and is a significant prospect for exploration. [ABSTRACT FROM AUTHOR]

Several types of dolomite had occurred pervasively in the lower Cambrian Qingxudong Formation in the Sichuan Basin. However, the origin of these dolomites remains particularly controversial until now. Here, we describe the petrographic and geochemical characteristics of dolomites from the lower Cambrian Qingxudong Formation in the southeastern Sichuan Basin, which can potentially provide useful information to constrain the origin and evolution of dolomitization fluid. Three fabric types and five dolomite generations were identified in this study: fabric-retentive dolomites (dolomicrite, Dol-1, and dolomitic microbialite, Dol-2), fabric-destructive dolomites (sucrosic dolomite, Dol-3) and pore-filling dolomites (medium- to coarse-crystalline dolomite, Dol-4, and saddle dolomite, Dol-5). The δ13C and δ18O values, 87Sr/86Sr ratios and rare earth element (REE) pattern of Dol-1, Dol-2 (except the δ13C values) and Dol-3 are approximately consistent with original seawater values, suggesting that the dolomitization fluids were mediated by seawater. The calculated dolomitization temperature (25.66 °C) and Z values (125.2) of Dol-1 with evaporite minerals indicate that the dolomitization fluid was high-salinity concentrated seawater derived from evaporation. The calculated dolomitization temperature (24.20 °C) and Z value (121.8) of Dol-2, and no traces of evaporite minerals in Dol-2 imply that the dolomitization fluid was slightly hypersaline seawater. The more negative δ13C and abundant microbial features of Dol-2 indicate that microbial mats were involved in dolomite formation. The dolomitization temperature (30.55 °C) and Z value (125) of Dol-3 also indicate that the dolomitization fluid was high-salinity concentrated seawater. However, Dol-3 is commonly distributed in reef and shoal facies (high porosity and permeability) throughout the Qingxudong Formation. Moreover, the abundant organic matters in the dolomitization fluid likely overcame the kinetic and thermodynamic barriers in the process of dolomitization. The geochemical data (except the δ18O values) of Dol-4 are approximately consistent with fabric-retentive dolomites, suggesting that the Dol-4 had been inherited from precursor carbonates. The negative δ18O values of Dol-4 revealed a further burial environment. High LREE/HREE ratios, positive europium anomalies and negative δ18O values of Dol-5 suggest that these dolomites were affected by hydrothermal fluids under deep burial conditions. Thus, Dol-1, Dol-2 and Dol-3 are likely attributed to sabkha, organogenic (microbial) and seepage-reflux dolomitization models during early-stage diagenesis, respectively. Dol-4 and Dol-5 are commonly attributed to burial and hydrothermal dolomitization models during deep burial diagenesis. This study provides new insights into the origin of dolomites and is especially useful for the further understanding of dolomite reservoir formation in the Qingxudong Formation. [ABSTRACT FROM AUTHOR]

Two sections of marine carbonate strata, Podu and Lengshuixi, from the Yangtze platform, South China, are investigated in terms of major geological events and carbon-strontium isotope responses. The results show a large-scale regression and a Large Igneous Province (LIP) event occurred in succession in the short interval of the late Guadalupian, inducing a biodiversity crisis. The regression event, marked by an increase in the 87Sr/86Sr ratio, is observed in the Jinogondolella postserrata Zone. It contributed significantly to the reduction in species habitat, leading to notable losses within the shallow-water species assemblage. A subsequent LIP event occurred after the appearance of Jinogondolella altudaensis, releasing CO2 and numerous other deleterious gases that further amplified the crisis in shallow-water to deep-sea environments. The corresponding decline in Sr and C isotopes could have been a response to the basaltic eruption (relative increase from mantle-derived Sr) and release of 12C-enriched CO2. During the process of creation of the LIP, the decline in δ13Ccarb was mainly associated with 12C-enriched CO2 release from organic matter in sedimentary strata, and high-temperature isotope fractionation during the volcanic eruption stage (exceeding 1200 °C). This study could further provide valuable clues regarding the development process of the major geological events and the causes of the biodiversity crisis. [ABSTRACT FROM AUTHOR]

A mature understanding of the sandstone gas reservoir in the Xiaoheba Formation in the southeastern Sichuan Basin remains lacking. To assess the reservoir characteristics and the origin of the high-quality reservoir in the Xiaoheba Formation, this paper uses systematic field investigations, physical property analysis, thin section identification, scanning electron microscopy and electron microprobe methods. The results indicate that the Xiaoheba sandstone is an ultra-tight and ultra-low permeability reservoir, with an average porosity of 2.97% and an average permeability of 0.56×10−3 μm2. This promising reservoir is mainly distributed in the Lengshuixi and Shuangliuba regions and the latter has a relatively high-quality reservoir with an average porosity of 5.28% and average permeability of 0.53×10−3 μm2. The reservoir space comprises secondary intergranular dissolved pores, moldic pores and fractures. Microfacies, feldspar dissolution and fracture connectivity control the quality of this reservoir. The relatively weak compaction and cementation in the interbedded delta front distal bar and interdistributary bay microfacies indirectly protected the primary intergranular pores and enhanced late-stage dissolution. Late-stage potassium feldspar dissolution was controlled by the early-stage organic acid dissolution intensity and the distance from the hydrocarbon generation center. Early-stage fractures acted as pathways for organic acid migration and were therefore important factors in the formation of the reservoir. Based on these observations, the area to the west of the Shuangliuba and Lengshuixi regions has potential for gas exploration. [ABSTRACT FROM AUTHOR]

Understanding the processes and products of carbonate diagenesis is crucial to the exploration and optimum development of palaeokarst hydrocarbon reservoirs in Permian carbonate rocks. Several well-documented outcrops and well drills in the Permian marine carbonate strata around the Sichuan Basin, South China, encompassing well-developed paleokarst landforms in the Middle-Upper Maokou Formation have been investigated. The sections and cores recording paleokarst reservoirs exhibit very similar carbonate diagenetic products and processes from various diagenetic environments by much most cementation and formation of secondary porosity in carbonates. The major diagenetic environments experience the zones of seawater, surface or near-surface atmospheric water, and subsurface mixed water between marine and meteoric. Characteristic diagenetic textures produced in each environment are described and different generations of calcite cements related to the process are responsible for their formation by sedimentary petrology and stable isotope geochemistry. Paleokarst under the constrained condition of surface and near-surface atmospheric water is the most favorable for oil-gas accumulation and presents some karst-related pores-caves-fissures with less or no fillings. Dissolution and corrosion of carbonate by atmospheric freshwater can alter the accumulation spaces and develop better fractures-caves system. These characteristic could exhibit some substantial data for better predicting porosity trends in Permian carbonate rocks and also provide important geological information for further exploring oil-gas resource in marine carbonate strata. [ABSTRACT FROM AUTHOR]

The remains of trilobites and other species were unexpectedly discovered in a volcanic ash layer beneath the Permian–Triassic Boundary (PTB). Based on a biostratigraphic investigation of the Zhongliangshan section in Chongqing, South China, the quantity of the species gradually decreased with subsequent volcanism. This finding provides an opportunity to further understand the disappearance of trilobites and the evolution of the mass extinction event. The temporal coincidence between the volcanic eruption event and the loss of trilobites and other species supports the idea of a cause-and-effect relationship. The species remains in the ash bed appeared before the disappearance ofClarkinayiniand the climax of the negative carbon isotope excursion, which implies that the onset of the mass extinction occurred at the end-Permian. The explosive volcanic events caused massive releases of CO2, toxic gases and volcanic ash and resulted in loss of habitat for certain species in the Tethys domain. This phenomenon may have led to the abrupt death of trilobites and the catastrophic collapse of biodiversity. [ABSTRACT FROM AUTHOR]

Well-documented outcrops around the Emeishan Large Igneous Province (ELIP) in South China, eastern Tethys, encompassing the end-Guadalupian mass extinction have been investigated. Correlatable sections recording the event exhibit very similar lithological characters, positive-then-negative C isotope excursions and massive biotic demise. Detailed analyses of the fossil record and carbon isotopic variations were carried out on the Guadalupian–Wuchiapingian Boundary sections over the inner, middle, outer zones of the ELIP and its margin. Due to a pronounced decrease in marine habitat area and the environmental and ecological change over this part of the Tethys, the biota crisis records show the loss of numerous tropical invertebrate taxa, and exhibit fewer genera and smaller testing sizes and low productivity. The biota crisis was a sustainable and gradual reduction in diversity over the Capitanian. The associated carbon isotopic data reveal unusually high δ13C(carb) values before the late Capitanian, representing higher primary productivity (or buried rate) and more13C-enriched CO2released by hydrothermal carbonate breakdown from the upper crust into the sediments at that time. Subsequently, an accelerated negative excursion across the boundary and the gradual excursion with low carbon isotope amplitude favours an increased influx of light12C sourced by the volcanism around the eastern Tethys. The very similar time–space relation between the biota crisis and the Emeishan volcanism confirms that volcanic eruptions may have triggered the biota crisis event in South China. Intensive volcanism could result in detrimental environmental and ecological stresses, habitat loss, organic material splitting, or the emission of light carbon and thermal fluid (or aerosol), implying that the losses of the shallow-marine invertebrates either occurred geologically instantaneously or in a series of closely spaced crises coinciding with the initial phase of ELIP formation. These findings in South China may reveal the causal relation between mass extinctions and LIPs in a global context. [ABSTRACT FROM AUTHOR]

Recently, a series of high-magnitude earthquakes, including the 2008 Wenchuan earthquake (M 8.0), have occurred in the Longmenshan tectonic region of southwestern China. Many researchers are currently examining the geological aspects of the activity, but little attention has been given to the huge travertine deposits along large fault zones. Carbon isotopes were measured in 106 samples, and the results indicate that the travertine originated from 13 C-enriched CO 2 sourced from deep reservoirs and can be characterized by notable positive δ 13 C values. CO 2 is thought to have originated from isotopic equilibrium fractionation under thermal metamorphism temperature conditions (within 350–850 °C). The re-equilibrium fractionation between the deep CO 2 and the dissolved carbonate composition during travertine formation is due to the declining temperature of thermo-genetic CO 2 , which in turn leads to higher δ 13 C carb values. Based on oil drilling data and geochemical thermometry modeling, the depth of the 13 C-enriched CO 2 source is inferred to be below 4643 m, which coincides with the measured focal depth of the Wenchuan earthquake and subsequent aftershocks. The variation of tectonic activity intensity and the quantity of CO 2 from deep formations are mainly responsible for the δ 13 C fluctuations observed in the travertine. The δ 13 C values in travertine deposits have increased dramatically, which implies the rapid upwelling of the CO 2 -bearing fluid, the enhancement of thermal metamorphism and the continuous increase of the internal pressure in deep reservoirs. These findings may contribute to a better understanding of the deep seismic activity and high-frequency aftershocks in the Longmenshan region. [ABSTRACT FROM AUTHOR]

The Sinian Dengying Formation in China hosts one of the oldest gas-bearing reservoirs in the world. With a long geological history, it has developed into the reservoir rocks, whose quality depends on chemical dissolution and cementation. Despite presence of cavities and pores in the algal dolomites of the Dengying Formation, they were mostly filled with various forms of dolomite cements. Therefore, the investigation of these dolomites and their evolution is significant for analyzing the mechanism of pore space preservation. There are five types of cements in the Dengying Formation, which were well sampled for lab measurements such as thin section, cathodeluminescence, minor elements, isotope analysis of carbon, oxygen and strontium, in attempt to explore the fillers' geochemical characteristics and the diagenetic fluids. They are characterized as follows: firstly, fibrous rim dolomite cement is a typical product formed at sea bottom, and geochemically very similar to the matrix (micrite and microcrystalline dolomite), which could be a reflection of the geochemical characteristics of water at that time; secondly, the diagenetic fluids of the foliated dolomite cements tend to be inheriting, derived from the shallow burial 'imprisoned' seawater; thirdly, fine- to medium-grained dolomite cements with fairly high strontium isotopes and low carbon and oxygen isotopes were precipitated chiefly in freshwater during the period of uplifting caused by Tongwan tectonic movement; fourthly, coarse-grained dolomite cements bear a relatively high percentage of Mn element, but low in Fe and Sr, with carbon and oxygen isotopes remarkably shifted to negative values, which all show that they are related to hydrocarbon maturation and migration, and corresponding Thermochemical Sulfate Reduction; lastly, saddle-shaped dolomite cements are hydrothermally associated, and they were formed in high-temperature brine, which had passed through the areas rich in radioactive strontium isotopes. [ABSTRACT FROM AUTHOR]

In this paper, the genesis of dolomite in Longwangmiao Formation is analysed through field investigation and geochemical methods, and dolomization model is established, which is very important for the next reservoir prediction in Longwangmiao Formation. According to the crystal size, the Longwangmiao Formation dolomite can be divided into two types: microcrystalline dolomite (MID) and macrocrystalline dolomite (MAD). Both types of dolomite have higher Al content than lime mudstone, indicating that during the deposition process of the Longwangmiao Formation, terrigenous materials are continuously mixed into the sediment, which significantly affects the geochemical characteristics of the Longwangmiao Formation. The rare earth element (REE) composition of the two types of dolomite is consistent with that of lime mudstone (LM), with an obvious δCe positive anomaly and no δEu anomaly, which is a typical distribution pattern of marine carbonate sediments. The microcrystalline dolomite has a higher Na element content, lower δ13C value, and higher δ18O value, suggesting that it is formed in a highly saline evaporation environment. Most of the macrocrystalline dolomite has a residual grain texture. The elements, as well as the isotopic geochemical characteristics of the dolomite, are consistent with those of lime mudstone, indicating that macrocrystalline dolomite is formed early and inherits the geochemical characteristics of the precursor limestone. We believe that the dolomitization model of the Longwangmiao Formation is an evaporative concentration + reflux seepage model. In the late sedimentary period of the Longwangmiao Formation, the sea level declined and a large area of lagoon appeared in the restricted and semi-restricted platform. As the evaporation and concentration continued, precursor minerals such as high-magnesium calcite and aragonite mud experienced chemical precipitation. In this process, a large amount of Ca2+ was consumed, resulting in an increase in the Mg2+/Ca2+ ratio. Then, the precursor minerals underwent dolomitization, and microcrystalline dolomite began to form. At the same time, owing to the difference in density between the high-salinity concentrated seawater and the pore water existing in the early limestone, the concentrated seawater flowed back and penetrated downward, causing the dolomitization of the underlying strata, and then the formation of macrocrystalline dolomite. The degree of dolomitization was mainly affected by the development degree of the granule beach and the thickness of the strata. • The highlights of this article are as follows: • This paper provides a case study of terrigenous clastic dolomite. • This paper analyses the genesis of the dolomites of Longwangmiao Formation and establishes a dolomitization model. • We analysed a large number of samples and provided basic geochemical data for Longwangmiao Formation. [ABSTRACT FROM AUTHOR]

• Mid-Permian strata in South China are characterized by high positive δ13C carb values. • Decarbonation during tectono-thermal events is the mechanism for 13C enrichment. • Emplacement causing regression and doming was responsible for δ13C carb fluctuations. During the Permian, Emeishan mantle plume activity created a large igneous province and deeply impacted carbon isotope records in marine carbonate strata in South China. To evaluate the isotopic variations caused by this catastrophic event, 140 carbonate samples from four sections were collected and analyzed. The δ13C carb values in the upper section of the Qixia Formation gradually increased, stabilized at ~4.0‰ in the lower-middle sections of the Maokou Formation, and then abruptly decreased in the upper section of the Maokou Formation. This isotopic evolution trend is likely related to mantle plume geochemical processes occurring during these respective stages. Large amounts of upwelling heat from the emplacement of the mantle plume triggered large-scale decarbonation reactions in the continental crust resulting in CO 2 production enriched in 13C. Substantial amounts of 13C-enriched CO 2 directly affected the isotope compositions of marine carbonates during this time. Temporal and spatial fluctuations in carbon isotopes were also controlled by terrestrial materials as a result of regression and doming caused by plume emplacement. The abrupt decrease in δ13C carb in the upper section of the Maokou Formation reflects the onset of volcanism, resulting in the breakdown of organic materials. These findings emphasize the contribution of a carbon reservoir (with a thermal decarbonation origin) in the upper crust, particularly against the Permian tectono-thermal background. This mechanism, which is similar to biological isotope fractionation in the carbon cycle, should not be overlooked. [ABSTRACT FROM AUTHOR]

Shale has been considered as good gas reservoir due to its abundant interior nanoscale pores. Thus, the study of the pore structure of shale is of great significance for the evaluation and development of shale oil and gas. To date, the most widely used approaches for studying the shale pore structure include image analysis, radiation and fluid invasion methods. The detailed pore structures can be studied intuitively by image analysis and radiation methods, but the results obtained are quite sensitive to sample preparation, equipment performance and experimental operation. In contrast, the fluid invasion method can be used to obtain information on pore size distribution and pore structure, but the relative simple parameters derived cannot be used to evaluate the pore structure of shale comprehensively and quantitatively. To characterize the nanoscale pore structure of shale reservoir more effectively and expand the current research techniques, we proposed a new method based on gas adsorption experimental data and the method of moments to describe the pore structure parameters of shale reservoir. Combined with the geological mixture empirical distribution and the method of moments estimation principle, the new method calculates the characteristic parameters of shale, including the mean pore size (), standard deviation (σ), skewness (Sk) and variation coefficient (c). These values are found by reconstructing the grouping intervals of observation values and optimizing algorithms for eigenvalues. This approach assures a more effective description of the characteristics of nanoscale pore structures. Finally, the new method has been applied to analyze the Yanchang shale in the Ordos Basin (China) and Longmaxi shale from the Sichuan Basin (China). The results obtained well reveal the pore characteristics of shale, indicating the feasibility of this new method in the study of the pore structure of shale reservoir. [ABSTRACT FROM AUTHOR]