The Sichuan Basin in southern China is well-known for its large natural gas resource potential stored in Sinian-Cambrian systems. Recently, high-yield industrial gas flow has been discovered from the Dengying Formation (Sinian System) and Canglangpu Formation (Cambrian System) in the Penglai gas area, preluding the multilayer stereoscopic exploration in Sichuan Basin. However, the origin of the natural gas and its preserving mechanics is still debated, and thus, in this study the geochemical characteristics of the natural gas are systematically analyzed, based on the data from gas composition and hydrocarbon isotope of a series of local wells. On this basis, the geochemical characteristics of natural gas in different regions and layers are compared, and the reasons for these differences from the origin and influencing factors are analyzed. The results show the following: (1) The natural gas of the Penglai gas field is dry gas dominated by CH₄, and the Sinian Dengying Formation gas has lower C₂H₆ content, larger dryness coefficient, heavier δ¹³C, and lighter δ²H_CH4 than the Cambrian gas, which is associated with the high proportion of hydrocarbons from the high-maturity Dengying source rocks. (2) The natural gas from some wells in the lower part of the structure is characterized by high H₂S content and low CH₄ content, and heavy δ¹³C in the components, which seems to be affected by the thermochemical sulfate reduction (TSR) effect. (3) The natural gas from the Penglai gas area has a relatively low maturity, which appears to be attributed to the continuous sealing ability of the caprock, which can preserve both the early generated gas and the late thermal-cracked gas.

The analysis of the chemical characteristics of the Middle Permian formation water in 10 wells in the northwest of Sichuan Basin indicates that: the TDS of the Middle Permian formation water in Northwest Sichuan increases longitudinally with the burial depth of the formation, and the formation water is mainly of CaCl₂ type (Surin classification), Cl-Na-Ca type and Cl-Ca-Na type (Shukarev classification). The rNa⁺/rCl⁻ and rSO₄ ^2-*100/rCl⁻ of the formation water are 0.05-0.95 and 0-5 respectively, reflecting strong water-rock interaction and strong sealing conditions. The analysis of the chemical characteristics of formation water and the preservation conditions of oil and gas shows that the overall preservation conditions of the Middle Permian in Northwest Sichuan are good, most of which is located in the “preservation zone”; longitudinally, the preservation conditions of Qixia Formation are better than those of Maokou Formation; areally, the preservation conditions of Shuangyushi and Jiulongshan regions are good; the sealing property of the strata in Hewanchang region is poor.

The muddy limestone in the Lei₃² sub-member of the Middle Triassic Leikoupo Formation in the well Chongtan 1 (CT1) of the Sichuan Basin has yielded promising industrial oil and gas production. This discovery has the potential to become a significant strategic reservoir in the future for oil and gas exploration in the Sichuan Basin. However, the understanding of hydrocarbon accumulation in the muddy limestone of the Lei₃² sub-member remains insufficient, which poses limitations on further exploration selection and deployment strategies. This study focuses on the analysis of core samples and laboratory data in the wells CT1 and Jianyang 1 (JY1), aiming to investigate the source rock and reservoir characteristics of the muddy limestone in the Lei₃² sub-member, as well as the primary controlling factors influencing the development of the source rock and reservoir. The results show that the Lei₃² sub-member in the Central Sichuan Basin is an evaporated lagoon deposition; the tight argillaceous limestone and limy mudstone exhibit the characteristic of source–reservoir integration, belonging to a new type of unconventional oil and gas reservoir. The reservoir space of the argillaceous limestone and limy mudstone in the Lei₃² sub-member primarily consists of inorganic and organic micro–nanopores and microfractures. The average porosity and permeability are 2.7% and 0.19 mD, indicating a low-porosity and low-permeability unconventional reservoir. The clay minerals and gypsum content are the important factors influencing reservoir porosity, and the fractures are key factors influencing permeability. This study will elucidate the specific features of hydrocarbon accumulation in the muddy limestone reservoirs of the Lei₃² sub-member and provide insights into its exploration potential.

Gas exploration has made major breakthrough firstly in Devonian strata in northwest Sichuan Basin, and its natural gas source have attracted much attention. In this work we developed comprehensive researches on natural gas geochemistry, reservoir bitumen and related source rock biomarkers of Devonian. Result shows: Natural gas in Devonian is secondary cracking gas dominated by hydrocarbon gas. It is major in sapropel type gas and comes from the source rock of Qiongzhusi Formation of Lower Cambrian and Middle Permian, mainly from the former. There are two reservoir accumulation model for gas in Devonian: lower source to upper reservoir and upper source to lower reservoir. Result is of important guidance significance for the exploration deployment of natural gas in this area.

The tight reservoirs in the Sichuan Basin generally contain water and have complex gas–water relationships. The dynamic changes and main controlling factors of natural gas injection are unclear, which has had a serious impact on the exploration and development of tight sandstone gas. This article selects samples from Yongqian and Qiulin gas fields to characterize the reservoir characteristics of the tight sandstone samples in the Xu-3 section. Nuclear magnetic resonance technology is applied to plan gas–water injection simulation experiments, and the dynamic changes in pore water and gas content during the natural gas injection of tight reservoir rock samples are characterized. The main controlling factors are analyzed based on the theory of nuclear magnetic resonance singlet and multifractal models. The results showed that material composition, pore type, structural characteristics, and physical properties cooperatively control the charging characteristics of natural gas. There was no significant difference in mineral content among the tight sandstone samples, and the pore morphology types were mainly parallel plate-like pores and fracture-type pores. There were significant differences in the pore structure characteristics of the samples with varying burial depths. The heterogeneity of gas-bearing pores is negatively related to the buried depth of tight sandstone, is a coupling relationship with quartz and feldspar content, and is negatively correlated with pore permeability. The stronger the sample heterogeneity, the more unfavorable it is for natural gas migration and accumulation.