Deep structural characteristics of the piedmont in the central Longmen Mountains and their implications for hydrocarbon exploration

The piedmont of the central Longmen Mountains, West Sichuan Basin, is one of the earliest explored zones in the basin. Recent years a series of new exploration breakthroughs have been witnessed. However, the complex structural deformation, involving multi-stage tectonic superposition, along with ongoing debates regarding deformation characteristics and mechanisms, have severely hindered the advancement of deep exploration. Based on newly acquired 2D/3D seismic data, drilling and logging data, remote sensing interpretations, and ground geological surveys, a fine quantitative geometric analysis of structures in the central Longmen Mountains was conducted to elucidate their deformation features and mechanisms. The study reveals that the Juyuan-Jinma structure in the piedmont of the central Longmen Mountains is influenced by Lower-Middle Triassic evaporitic layers. The evaporite layers exhibit decoupling, leading to deformation incoherence between the upper and lower beds. Beneath the evaporites, multiple sets of backthrust faults developed in the Permian-Lower Jialingjiang Formation, with the lower part sliding off Cambrian mudstone layers and the upper part terminating at the evaporite layers. These faults control the formation of multi-row backthrust structures in the Permian. Above the evaporites, deformation of the Middle Triassic-Cretaceous strata was controlled by the Pengxian fault, forming the main body of the Juyuan-Jinma anticline. This anticline exhibits typical characteristics of shear fault-bend folding, with displacement progression increasing from the Lower Leikoupo Formation to the Jurassic along the Pengxian fault. Structural restoration indicates that deformation in the foreland of the central Longmen Mountains initiated in the Late Triassic, weakened during the Jurassic, intensified in the Cretaceous, and was largely finalized during the Himalayan Period. In the deep subsurface of the piedmont central Longmen Mountains, the Permian below the evaporites develops multiple rows of structural traps, underlain by thick Cambrian hydrocarbon source rock systems. The favorable reservoir conditions suggest significant exploration potential.