Shale pore systems are the result of the geological evolution of different matrix assemblages, and the composition of gas shale is considered to affect the pore systems in shale reservoirs. This study aimed to investigate the impact of both organic and inorganic constituents on the shale pore system, including specific surface area (SSA) and pore volume in Wufeng–Longmaxi Shale. Multiple linear regression (MLR) was employed to examine the contributions of different components to shale pore structure. The pore structure parameters, including pore SSA and pore volume, were obtained by gas adsorption experiments in 32 Wufeng–Longmaxi Shale (Late Ordovician–Early Silurian) samples. Both pore SSA and pore volume were calculated by the density functional theory (DFT) model on shale samples, and the pore types were determined by high-resolution field emission scanning electron microscopy (FE-SEM). The results of the X-ray diffractometer (XRD) analysis indicate that the Wufeng–Longmaxi Shale is dominated by quartz, clays, carbonates, feldspar, pyrite, and organic matter. Four models were made using SPSS software, all of which showed significant correlation between shale pore size and organic matter (OM) and clays. The content of organic matter played the biggest role in determining the size and structure of the pores. Although the content of quartz is the highest and serves as a rigid skeleton in shale reservoirs, it has complicated effects on the pore structure. In this study, most of the quartz is biogenetic and part of it is transformed from clays in deep shale. Therefore, these two parts of quartz are, respectively, related to organic matter and clays. In essence, the pores related to these two parts of quartz should be attributed to organic matter and clays, which also support the conclusion of the MLR models.

Shale in the Wufeng Formation of the upper Ordovician and Longmaxi Formation of lower Silurian in the Sichuan Basin and its surrounding area is widespread. Shale gas resources are abundant. Shale gas in the Wufeng Formation and the Longmaxi Formation in the basin has been a major breakthrough. The basin margin transition zone in southeastern Chongqing is in an intense tectonic activity area, which is more complicated and special than the stable stratum in the basin. Therefore, it is necessary to put forward higher requirements for preservation conditions and enrichment pattern evaluation of shale gas. Therefore, in view of the complicated structural pattern of the basin margin transition zone in southeastern Chongqing, the preservation conditions and reservoir forming patterns are analyzed through structural evolution, uplift and denudation, fault development, structural styles, roof and floor conditions, and formation pressure. The results show that the main reason for the formation of normal pressure is the late uplift, denudation and fault development. The pressure coefficient from the basin to the outer layer is changed from overpressure to normal pressure, and structural transformation forms the preservation form of shale gas with narrow and steep residual anticline, wide residual syncline and residual slope. The preservation condition evaluation of normal shale gas should be based on structural factors such as structural evolution, structural style, uplift and denudation degree and fault development degree, with formation pressure coefficient as reference condition, combined with material basic conditions such as roof and floor conditions and formation thickness. The findings of this study can help for better understanding of the “sweet spot” prediction of normal pressure shale gas in complex structural area.

In contrast to the widely used sequence stratigraphic models for passive continental margins, the stacking patterns of strata within epeiric seas, which are influenced by regional tectonic activity, may display opposing characteristics during the same geological period. These variations serve as a record of basin evolution and also affect the accumulation of hydrocarbons within the strata. Our study investigated the development potential of the deep Longmaxi Shale in the southern Sichuan Basin by examining the sequence stratigraphy and sedimentary fill patterns. Using a combination of core observation, well-logging data analysis, and 3D seismic profile interpretation, we aimed to gain an understanding of the sedimentary fill history of the Longmaxi Shale during the Early Silurian. Our analysis revealed that deglaciation and regional tectonic events affected the sequence stratigraphy, resulting in unconformities that were identifiable using seismic data and wireline logs. Through an analysis of thirty wireline logs and two seismic profiles, we identified two third-order sequences suggested in the Lower Longmaxi Formation. Within the two third-order sequences were five systems tracts, with the first exhibiting a complete cycle of sea-level change and the second cycle being incomplete due to regional tectonic events. The graptolite succession on the upper Yangtze Platform provided a temporal view of the sequence stratigraphy and sedimentation rates of the Longmaxi Shale. The thickness trends of the systems tracts reflected the interplay of short-term eustasy fluctuations, subsidence, and uplift. Our analysis suggests that regional subsidence played a significant role in the deposition of the second transgressive systems tract (TST) in the Weiyuan and Luzhou areas, which represents a promising target for shale gas exploration, in addition to the first TST. However, the Changning area experienced a relative sea-level decrease due to the intense uplift of the Qianzhogn Paleo-uplift and the increased supply of sediment and is interpreted as a highstand systems tract (HST); it is not considered to have shale gas exploration potential.