Formation, Evolution, and Preservation Mechanisms of Overpressure in the Wufeng–Longmaxi Shale, Changning Area, Sichuan Basin

ABSTRACT Abnormally high pressure is widely developed in the Wufeng–Longmaxi (WF–LMX) reservoirs in the Changning (CN) area; however, the paleo‐pressure state, pressure evolution and genetic mechanisms during burial, and uplift remain unclear. In this study, present‐day formation pressure data, basin modeling, core‐based fracture observations, fluid‐inclusion microthermometry, laser Raman analysis of methane inclusions (MI), and PVT‐based paleo‐pressure restoration were integrated to systematically investigate the overpressure evolution and its genetic mechanism. It shows that the present‐day pressure coefficient of the study area is generally 1.2–2.1, indicating a moderate to strong overpressure system. Basin modeling indicates that the WF–LMX shale has undergone continuous burial, deep‐burial heating and subsequent uplift and erosion since deposition. As a result of burial down to nearly 4800 m in the Middle–Late Yanshanian, the shale entered the high‐ to over‐mature phase, during which large quantities of cracking gas were produced. The fracture‐filling veins comprise primarily single‐phase MI, coexisting methane and two‐phase brine inclusions, and isolated two‐phase brine inclusions. The homogenization temperatures (Th) of brine inclusions are mainly concentrated at 166.3–190.5°C. The restored paleo‐pressures of MI range from 64.97 to 134.67 MPa, yielding pressure coefficients between 1.35 and 2.31, indicating that the shale reservoir had already reached a moderate to strong overpressure state near the maximum burial stage or during the early uplift stage. Integration of basin modeling, inclusion‐derived paleo‐pressures and logging responses suggests that overpressure in the CN WF–LMX shale was mainly caused by fluid expansion related to hydrocarbon generation and residual liquid‐hydrocarbon cracking during deep burial, whereas late tectonic uplift, local pressure release and top‐bottom sealing conditions jointly controlled the preservation of present‐day overpressure. The pressure evolution is summarized as a model of “deep‐burial hydrocarbon‐generation overpressure, tectonic‐uplift pressure reduction, and residual‐overpressure preservation.” Present‐day overpressure preservation indicates favorable preservation conditions, high gas‐bearing capacity, and strong production potential and thus constitutes a key criterion for evaluating shale gas enrichment and identifying sweet spots in structurally complex regions.