Gas supply from overmature source rocks in a marine cratonic basin: Constraints from strike-slip fault activity timing and diagenetic–hydrocarbon fluid records

Whether previously generated gas retained in overmature marine source-rock systems can be released during later fault reactivation and supplied to deep and ultradeep reservoirs remains poorly constrained by geological evidence. We examined the Middle Permian Maokou Formation gas reservoir in the central Sichuan Basin using in situ U–Pb dating of carbonate minerals, diagenetic sequence analysis, seismic interpretation, fluid-inclusion microthermometry, and laser Raman spectroscopy to constrain strike-slip fault activity and reconstruct pressure evolution. The reservoir contains matrix dolomite (267.4 ± 5.8 Ma), two vug-filling dolomite generations (251.3 ± 7.2 and 247.7 ± 6.8 Ma), two fracture-filling calcite generations (235.3 ± 8.0 and 136.2 ± 6.5 Ma), and late vug-filling quartz. Primary aqueous inclusions in quartz growth zones yield an estimated trapping interval of 119–100 Ma. The vug-filling dolomites and earlier calcite veins record episodic Late Permian–Triassic fault-related fluid activity, whereas the later calcite veins and quartz record Early to mid-Cretaceous reactivation. Fluid inclusions record Early Triassic oil charging (coeval aqueous-inclusion T h,aq = 73–142 °C), Middle Jurassic–Early Cretaceous oil cracking ( T h,aq = 152–194 °C), and Cretaceous high-density methane charging after the main oil-cracking phase ( T h,aq = 186–212 °C), with the highest-temperature FIAs approaching the Late Cretaceous peak burial. Reservoir pressure coefficients rose from approximately 1.0 during oil charging to 1.08–1.52 during oil cracking and then to 1.59–2.24 during the third-stage charging, demonstrating continued gas charging after oil cracking. Fault reactivation re-established vertical connectivity between the overmature Cambrian Qiongzhusi source-rock system and the overlying Maokou reservoir, enabling previously generated retained gas to recharge the reservoir, with an additional contribution from dynamically adjusted deep paleo-gas reservoirs. These findings indicate that remobilized retained gas can contribute to late gas accumulation in ancient cratonic basins where deep retained gas, fault reactivation, and effective reservoir–seal systems coexist.