Preservation conditions and potential evaluation of the Longmaxi shale gas reservoir in the Changning area, southern Sichuan Basin

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.