The timing of petroleum generation, migration, and accumulation in the central Junggar Basin has been evaluated using basin modeling technology to decipher the processes of petroleum secondary migration and accumulation in the Jurassic carrier beds during the Yanshan and Himalayan orogenies. During the early Yanshan orogeny (J2–J3), crude oils, sourced mainly from the Permian source rocks, migrated laterally from depocenters to adjacent structural high points. During the late Yanshan orogeny (Cretaceous–Eocene), the hydrocarbon secondary migration pattern changed significantly. Specifically, in the Cretaceous, a large-scale regional second phase of generation and migration occurred in the western part of the central Junggar Basin; in the eastern part of the central Junggar Basin, crude oils emplaced in the depressions migrated mainly to the eastern uplifts. In the early Tertiary, oils in the source kitchens generally began migrating to the north and mixed with the Jurassic-generated hydrocarbons. During the Himalayan orogeny (Neogene–Quaternary), this migration was strengthened continually. The modeled results agree with migration-associated geochemical information of the Jurassic accumulations. Integration of the regional geological setting and some petroleum geology data suggest that the Jurassic tectonic high points and facies-change- and unconformity-associated subtle traps are of crucial importance for current oil exploration. Both overpressured bodies in the Triassic and Permian systems and volcanic rock reservoirs of the Carboniferous basement in the uplifts have important implications for future natural oil and gas exploration. This work should help reduce exploration risk in this region.

A shale condensate gas reservoir with a high clay content and a high formation pressure was found in the Jurassic shale of the Dongyuemiao Member in the Fuxing area of the eastern Sichuan Basin. Reservoir characteristics and formation pressure have a significant influence on optimal development. The present study investigated the continental shale of the Dongyuemiao Member in Well F. The petrological properties, physical properties, and pore structure of the Dongyuemiao Member were studied using X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), N₂ adsorption, and mercury intrusion porosimetry (MIP). The permeability stress sensitivity characteristics of the shale reservoirs are discussed based on the change in shale porosity and permeability under overburden pressure. The tested shale samples yielded total organic carbon (TOC) and S₁ + S₂ values ranging mainly from 1.0 wt.% to 1.5 wt.% and from 0.39 to 2.28 mg/g, respectively, which was in the high maturity stage of the thermal evolution of organic matter (OM). The shales of the Dongyuemiao Member were found to contain high average clay mineral contents (more than 50%) of calcite and quartz, as well as albite, pyrite, dolomite, and halite. The main developments were identified as silica-rich argillaceous shale lithofacies, argillaceous shale lithofacies, and mixed argillaceous shale lithofacies. The pores were found to mainly be plate-like and flake-like interlayer pores of clay minerals and OM pores with various shapes. The pore size was mainly concentrated below 110 nm, and the pore volume increment increased in flakes with pore diameter. The average porosity and permeability of shale were found to be 4.827% and 0.243 mD, respectively. Clay minerals and quartz are beneficial for improving the porosity and permeability of reservoirs, while carbonate minerals have the opposite effect. The permeability of the shale showed a negative exponential change with increasing effective stress under overburden pressure. When the effective confining pressure was greater than 20 MPa, the decline rate of the shale permeability decreased with increases in the effective stress. The higher the clay mineral and TOC content, the stronger the stress sensitivity of shale permeability. The higher the carbonate mineral content, the weaker the stress sensitivity of shale permeability. The porosity sensitivity exponent indicates that matrix pores and micro-fractures are both developed in the Dongyuemiao Member, and the development of internal fractures is the main factor in the strong stress sensitivity of the shale permeability in the study area.

Multistage fluid activities and hydrocarbon accumulation processes have occurred in the Dengying Formation of the Sichuan Basin during its long geological history. Petrography and cathodoluminescence observations; in situ microanalysis of rare earth elements, carbon, oxygen, and strontium isotopes; fluid inclusion microthermometric experiments; laser Raman experiments; burial history; thermal history; and hydrocarbon generation history simulation have been applied to study the characteristics of vein-forming fluid in the Dengying Formation reservoirs in the southeast and southwest of Sichuan Basin and to analyze in-depth the multistage fluid activity and hydrocarbon accumulation process. The results show that two stages of dolomite and one stage of quartz are developed in the 4th member of the Dengying Formation in the southeast of Sichuan Basin, and three stages of dolomite are developed in the 2nd member of the Dengying Formation in the southwest of Sichuan Basin. The source of the dolomite veins is mainly reservoir marine diagenetic fluid. Dolomites developed in the Hercynian period were affected by hydrothermal activity to a certain extent which may have been caused by the activity of the Emei mantle plume. The diagenetic mineral sequence of the 4th member of the Dengying Formation in the southeast of Sichuan Basin is quartz (432 Ma)/dolomite I (421 Ma)/dolomite II (288 Ma), and the 2nd member of the Dengying Formation in the southwest of the Sichuan Basin is dolomite I (425 Ma)/dolomite II (283 Ma)/dolomite III (262 Ma). The main hydrocarbon accumulation period was during the Hercynian–Indosinian stage which was related to the thermal influence of the Emei mantle plume activity on the source rock of the Qiongzhusi Formation. Combined with petrography, inclusion thermometry, burial history, and hydrocarbon generation history simulation, the fluid activity and hydrocarbon accumulation evolution sequence in the southeast and southwest of the Sichuan Basin are determined comprehensively.