Fluid pressure evolution of a deeply buried reservoir in Ediacaran dolomite via integrated fluid inclusion analysis and basin modeling

ABSTRACT Understanding reservoir fluid overpressure evolution is vital for characterizing petroleum accumulation and preservation in deeply buried reservoirs. This study integrates petrography, fluid inclusion (FI) microthermometry, Raman spectroscopy, thermodynamics, and basin modeling to constrain the diagenetic fluid history of the Ediacaran Dengying Formation, Sichuan Basin, South China. Petrographic observations reveal a complex cementation sequence from early calcite 1 and dolomite 1 to late-stage quartz and fluorite. In situ U-Pb dating of FI-hosted dolomite 2 (232.9 ± 7.4 Ma) identifies a 60-bar overpressure buildup during rapid late Permian burial. Intense peak overpressure (360–690 bar) occurred during quartz and fluorite precipitation under the Yanshanian compression regime, supported by fluorite dating of 106.8 ± 7.5 Ma. At trapping temperatures exceeding 200°C, thermochemical sulfate reduction (TSR) and oil cracking were responsible for fluid expansion and intense overpressure buildup. Since the Paleogene, major tectonic uplift and fluid drainage have caused overpressure to decrease from 367 to 0 bar. This integrated microscale–macroscale workflow links tectonic events to pore-fluid processes with global universality. The Triassic rifting triggered rapid source rock maturation and oil cracking, a mechanism that is highly similar to that of the Aquitaine Basin, France. Commonalities in methane reserves and H2S content between these basins suggest an intrinsic coupling between rapid maturation and TSR. Furthermore, the significant pressure attenuation observed implies that hydrocarbons may have remigrated to adjacent undiscovered reservoirs rather than simply leaking, offering a strategic new direction for deep-seated petroleum exploration deployment.