The Mississippi Valley-type (MVT) Pb-Zn deposits and hydrocarbon reservoirs coexist around the margin of the eastern Sichuan basin. This study examined the fluid inclusions, Sr isotope and systematic Rb-Sr, Sm-Nd geochronology for the distinct ore and gangue minerals of four orebodies from two MVT Pb-Zn deposits in the margin of the eastern Sichuan basin, combined with the existing research foundation of oil and gas accumulation and evolution, which was designed to understand the internal relationship between oil and gas accumulation and the involvement of organic matter in metal mineralization. High-density methane, moderate temperature, and salinity inclusions were discovered in the studied MVT Pb-Zn deposits, combined with relatively higher ⁸⁷Sr/⁸⁶Sr ratios (0.71088~0.714749), indicating that the ore-forming fluids were derived largely from the hydrocarbon associated basinal brines. Rb-Sr isochron of paragenetic sphalerites and pyrites and Sm-Nd isochron of paragenetic fluorites and calcite from the studied MVT Pb-Zn deposits define isochron ages of 144.5 Ma~138.5 Ma, and 147.6 Ma, respectively, indicating that the Late Jurassic to Early Cretaceous Yanshanian orogeny was an important metallogenic event in the margin of the eastern Sichuan basin. The close temporal and spatial relationship between the MVT mineralization and hydrocarbon accumulation and destruction in the strong structural deformation area of the margin of the eastern Sichuan basin allows us to propose a possible model in which the Yanshanian compressional tectonics drove a large-scale flow of metal- and sulfate- bearing basinal fluids passed under or through methane-bearing carbonate reservoirs, resulting in the MVT mineralization and hydrocarbon accumulation and destruction.

Diagenesis has been demonstrated to significantly affect porosity development in shale reservoirs, however, the effect of diagenetic modifications on shale pore structures is still unclear. For clarifying this issue, this paper focuses on the Upper Ordovician to Lower Silurian Wufeng and Longmaxi shales, which are the only commercially gas-produced shale plays in China. This study aims to reveal the influence of diagenetic alterations on the WF-LMX shale reservoir quality by integrating total organic carbon (TOC) content, X-ray diffraction (XRD), low-temperature gas (N₂) and carbon dioxide (CO₂) adsorption experiments, field-emission scanning electron microscopy (FE-SEM), energy dispersive X-ray (EDS), and cathodoluminescence (CL) analyses. Three major shale lithofacies were identified, mainly including siliceous, siliceous–argillaceous mixed, and argillaceous shale; the siliceous shale has a relatively high TOC content. The organic pores, intergranular pores, intragranular pores, and fractures are generally developed in the WF-LMX shales. The pore volume (PV) and specific surface area (SSA) of micropores, mesopores, and macropores of siliceous shales are higher than those of mixed shales and argillaceous shales. The TOC content has a strongly positive correlation with PV and SSA for micropores and mesopores. After combustion, the PV and SSA of micropores and mesopores were decreased, whereas the PV and SSA of macropore were significantly increased. In the siliceous shale, organic pore is the dominant pore type due to the fact that a large amount of authigenic microcrystalline quartz aggregates can protect organic pores from compaction. The argillaceous shale has high clay and low TOC content, and the dominant pore type is pores between clay flakes. The siliceous shale has a relatively high TOC content, large PV and SSA, and so are the dessert lithofacies for shale gas exploration.

The deeply buried Upper Ediacaran Qigebrak Formation dolostones in the Tarim Basin are promising future hydrocarbon exploration targets in China. However, the origin of these pervasive matrix dolomites is not well understood, which hampers further hydrocarbon exploration. In this study, petrographic, isotopic (C, O, and Sr), rare earth element (REE), and clumped isotope analyses were performed to unravel the mechanisms of early dolomitization. Petrographic investigations indicate that the Qigebrak Formation carbonates were completely replaced by three distinct types of dolomites: (1) dolomicrite (MD-1), (2) fabric-preserving dolomite (MD-2), and (3) fabric-destructive dolomite (MD-3). Despite different crystal textures, these three dolomite types have a narrow range of δ¹³C and ⁸⁷Sr/⁸⁶Sr values similar to those of coeval seawater. Furthermore, their seawater-normalized REE compositions display a seawater-like REE pattern with positive Ce anomalies. These findings suggest that the dolomitization fluids were seawater derived. From the clumped isotope temperature (T_Δ47 ≈ 60 °C) and the δ¹⁸O water values of the dolomitization fluids, it can be inferred that the main mechanism for the formation of matrix dolomites was seepage-reflux dolomitization by mesosaline to penesaline seawater in the evaporative environment. MD-1 and MD-2 precipitated from mesosaline to penesaline seawater in slightly evaporated settings. MD-3 was likely formed via recrystallization of MD-1 and/or MD-2 at a greater depth. This study provides an insight into early dolomitization processes related to mesosaline to penesaline seawater, which may make the origins of dolomite reservoirs with similar geological backgrounds better understood.

The hydrothermal dolomitization, facilitated by basement fault activities, had an important impact on the Permian Maokou Formation dolomite in the Sichuan Basin, which experienced complex diagenesis and presented strong reservoir heterogeneity. The source and age of diagenetic fluids in this succession remain controversial. In this study, various analyses were implemented on samples collected from outcrops and wells near the No. 15 fault in the eastern Sichuan Basin to reconstruct the multi-stage fluid activity and analyze the impact on reservoir development, including petrology, micro-domain isotopes, rare earth elements, homogenization temperature of fluid inclusions, and Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) U-Pb dating. The homogenization temperature of primary brine inclusions in fine-grained matrix dolomite and saddle dolomite is concentrated between 100 and 150 °C, which indicates that the impacts of abnormally high temperatures of other geological bodies. The δ¹³C and δ¹⁸O value and low ⁸⁷Sr/⁸⁶Sr value indicate that the diagenetic fluid of fine-grained matrix dolomite is mainly Permian seawater. The U-Pb ages of fine-grained matrix dolomite are ~260 Ma, which coincides with the age of the main magmatism of Emeishan Large Igneous Province (ELIP), and hydrothermal fluid provided a favorable high-temperature environment in the penecontemporaneous stage. While highly radiogenic ⁸⁷Sr/⁸⁶Sr compositions suggests those of saddle dolomite, the high-temperature Sr-rich formation fluid. The U-Pb ages of saddle dolomite are 245–250 Ma, which coincides with the age of the 255~251 Ma magmatism of ELIP. This indicates that those should be the diagenetic products of the ELIP hydrothermal fluid in the shallow burial stage. The U-Pb age of coarse-grained calcite is 190–220 Ma, and it should be the diagenetic product of the deep burial stage. Brine inclusions associated with primary methane inclusions were developed in coarse-grained calcite, with a homogenization temperature range of 140.8–199.8 °C, which indicates that the formation fluid activities were related to hydrocarbon charging. The Permian Maokou Formation dolomite was firstly formed in the penecontemporaneous shallow burial stage, and then it was subjected to further hydrothermal dolomitization due to the basement faulting and the abnormally high heat flow during the active period of ELIP. Hydrothermal dolomitization contributed to the formation and maintenance of intercrystalline and dissolution pores, whereas it also formed saddle dolomite to fill the pores, and reduce the pore space. The influence of deep fluid activities on reservoir evolution is further distinguished.

Risk assessment of deep shale reservoirs is very important for subsurface energy development. However, due to complex geological environments and physicochemical interactions, shale reservoir fabric parameters exhibit variability. Moreover, the actual investigation and testing information is very limited, which is a typical small-sample problem. In this paper, the heterogeneity and statistical characteristics of deep shale reservoirs are clarified by the measured mechanical parameters. A deep learning method for deep shale reservoirs with limited survey data information is proposed. The variability of deep shale reservoirs is characterized by random field theory. A variable stiffness method and stochastic analysis method are developed to evaluate the risk of deep shale reservoirs. The detailed workflow of the stochastic risk assessment framework is presented. The frequency distribution and failure risk of deep shale reservoirs are calculated and analyzed. The risk assessment of deep shale reservoirs under different model parameters is discussed. The results show that a stochastic risk assessment framework of deep shale reservoirs, using a deep learning method and random field theory, is scientifically reasonable. The scatter plots of the elasticity modulus (EM), cohesive force (CF), and Poisson ratio (PR) distribute along the 45-degree line. The different distributed variables of EM, CF, and PR have a positive correlation. The statistical properties of the measurement data and deep learning data are approximately the same. The principal stress of deep shale follows the normal distribution with significance level 0.1. Under positive copula conditions, the maximum failure probability is 5.99%. Under negative copula conditions, the maximum failure probability is 4.58%. Different copula functions under positive and negative copula conditions have different failure probabilities. For the exponential correlation structure, the minimum failure probability is 3.46%, while the maximum failure probability is 6.19%. The mean failure probability of the EM, CF, and PR of deep shale reservoirs is 4.85%. Different random field-related structures and parameters have different influences on the failure risk.