Superimposed accumulation mechanism and model of vertical source rock–reservoir system of coal-measure gas is crucial to evaluate the exploration potential, and also the basis of co-exploration and co-production of coal measure gas. This work investigates the formation mechanism of various types of reservoirs (coalbed methane, shale gas, tight sandstone) in the Taiyuan Formation (Yushe-Wuxiang Block, eastern Qinshui Basin). Source rocks (coal seams and coal-measure mudstones) in the study area are characterized by type III kerogen, organic rich and over-mature, and reach a gas generation peak during the Early to Late Cretaceous. Coalbed methane mainly adsorbs on the surface of micropores, shale gas mainly occurs in micropores, macropores and micro-factures in adsorbed and free states, and tight sandstone gas mainly occurs in macropores in a free state. The combinations of successions are identified, coalbed methane, shale gas, and tight sandstone gas horizons are divided into a mudstone-sandstone reservoir (combination I), a coal-mudstone-sandstone reservoir (combination II), and a coal-mudstone reservoir (combination III). This division occurs from top to bottom in the succession and is identified on the basis of lithology, total organic carbon content (TOC) of mudstones, gas logging, superimposition relationships, and the source rock-reservoir-caprock assemblage. The strata thickness, continuity, and gas logging results of combination III comprise the most favorable conditions for fairly good development potential, followed by combination I. The development potential of combination II is poor due to the small strata thickness and poor continuity. The identification of superimposed reservoirs can provide an engineering reference for the exploration of coal-measure gas.

The production of shale gas varies greatly in different regions due to the way gas has accumulated and preserved. This work investigates the dynamic evolution of shale gas generation, accumulation, adjustment, and loss from the Longmaxi formation (S₁l) in the Changning area, southern Sichuan Basin, China. The factors controlling the preservation conditions and formation mechanism of the overpressure shale gas reservoir are also studied. The results show that shale gas generation reached its peak during the Middle Jurassic to Early Cretaceous. Furthermore, the gas occurs mainly in organic matter pores of nanometer size, clay mineral pores of nano- to micro-meter size, and microfractures of micrometer size. Then, in the Early Cretaceous, the reservoir was damaged due to uplift of the crust. Additionally, the evaluation scheme of the shale gas reservoirs is established according to the organic geochemical parameters, mineralogical composition, sealing capacity, thickness, burial depth, faults, pressure coefficient, and gas content, etc. Hence, the shale gas reservoirs may be divided into four grades, with Class I being the grade with best gas preservation and Non-economic grade with the worst gas preservation. The annular region in the Jianwu–Luochang synclines and the northeast limb of the Changning anticline have optimum preservation conditions, with a grade of Class I. The preservation conditions gradually deteriorate towards the two limbs, with Class II, Class III, and Non-economic area grades. The good preservation conditions correspond to a high pressure coefficient, and the pressure of the reservoir is mainly caused by hydrocarbon generation pressurization of organic matter (mainly the stage of oil cracking gas and dry gas), tectonic uplift pressurization, and to a minor extent, transformation dehydration pressurization of clay minerals. Furthermore, overpressure preservation is controlled by microporous overpressure, source rock–caprock vertical sealing ability, the spatial distribution of S₁l, and development characteristics of faults. Results from this investigation provide specific guidance for shale gas exploitation in the study area, and provide a reference for the evaluation of preservation conditions in shale gas reservoirs and formation mechanism of overpressure gas reservoirs.