Fault-related deformations and fractured-vuggy gas reservoirs in the Permian-Triassic marine carbonate strata in the Da’an area, southern Sichuan Basin

This study aims to elucidate the structural characteristics of third-order reverse faults within the Permian-Triassic marine carbonate strata in the Da’an area, southern Sichuan Basin and to explore both the controlling effects of these faults on fractured-vuggy gas reservoirs and the distribution pattern of fault-controlled gas reservoirs. By integrating well-to-seismic calibration, plan-section combined fine-scale structural interpretation, 3D structure-fault modeling, and comprehensive analyses of drilling-related oil and gas shows, we systematically investigate the characteristics of the deformation structures associated with the third-order reverse fault system and its control on reservoir formation and hydrocarbon accumulation. The results indicate that the third-order reverse faults in the Da’an area generally exhibit dip angles ranging from 10° to 20° and dip-slip displacements reaching up to 400~550 m, representing intralayer local adjustment faults formed under regional compression. These third-order reverse faults largely show lateral extensions of less than 6 km, while some extend for up to more than 20 km, resulting in a cumulative length exceeding 530 km. Carbonate fracture zones on their hanging walls or footwalls cover areas ranging from 360 km² to 600 km². The tectonic fracture systems within the Permian-Triassic marine carbonate strata have strikes roughly consistent with those of the third-order reverse faults. They form fault-controlled fractured-vuggy reservoirs, with the distribution of gas reservoirs significantly governed by fault zones. Overall, the third-order reverse faults in the Permian-Triassic marine carbonate strata in the Da’an area represent a key geological factor in controlling the formation of fractured-vuggy gas reservoirs. The fault-controlled fractured-vuggy reservoirs hold great exploration potential. Nevertheless, tapping this potential requires technical breakthroughs in reservoir heterogeneity characterization, 3D geological modeling, and sweet spot prediction.