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.

中文翻译：

---

川南长宁地区龙马溪组页岩气藏保存条件及潜力评价

由于天然气的聚集和保存方式不同，页岩气产量差异很大。本文研究了四川盆地南部长宁地区龙马溪组（S1l）页岩气生成、聚集、调整和流失的动态演化。对超压页岩气藏保存条件及形成机制的控制因素进行了研究。结果表明，中侏罗世—早白垩世页岩气产量达到顶峰。此外，天然气主要赋存于纳米级有机质孔隙、纳米级至微米级粘土矿物孔隙以及微米级微裂缝中。然后，在早白垩世，由于地壳隆升，储层遭到破坏。此外，根据有机地球化学参数、矿物成分、封闭性、厚度、埋藏深度、断层、压力系数、含气量等建立页岩气藏评价方案，将页岩气藏分为四类：等级，其中Ⅰ级为气体保存最好的等级，非经济级为气体保存最差的等级。建武—洛昌向斜环形区和长宁背斜东北翼保存条件最佳，为Ⅰ级；向两翼保存条件逐渐恶化，为Ⅱ级、Ⅲ级、非经济级。地区等级。良好的保存条件对应着较高的压力系数，储集层压力主要由有机质生烃增压（主要是石油裂解气和干燥气阶段）、构造抬升增压以及少量粘土矿物转化脱水增压引起。此外，超压保存还受微孔超压、烃源岩垂向封闭能力、S1l空间分布、断裂发育特征等控制。该研究成果为研究区页岩气开发提供了具体指导，为页岩气藏保存条件评价和超压气藏形成机制提供了参考。在较小程度上，粘土矿物的转化脱水加压。此外，超压保存还受微孔超压、烃源岩垂向封闭能力、S1l空间分布、断裂发育特征等控制。该研究成果为研究区页岩气开发提供了具体指导，为页岩气藏保存条件评价和超压气藏形成机制提供了参考。在较小程度上，粘土矿物的转化脱水加压。此外，超压保存还受微孔超压、烃源岩垂向封闭能力、S1l空间分布和断裂发育特征的控制。该研究成果为研究区页岩气开发提供了具体指导，为页岩气藏保存条件评价和超压气藏形成机制提供了参考。