Coupling effect between tectonic inversion of sag-controlling fault zone and differential hydrocarbon accumulation in back-arc basin: a case study of Pinghu area, Xihu Sag, East China Sea Basin

The Pinghu Oil and Gas Field, as a core area for reserve increase and production enhancement in the East China Sea Basin, has important strategic significance for ensuring China's energy security. Significant differences in hydrocarbon enrichment layers and accumulation scales exist among different reservoirs in this oil and gas field, but the key factors controlling this differential hydrocarbon accumulation and distribution are still unclear. Utilizing newly acquired 3D high-resolution seismic, drilling, and logging data, the study systematically analyzed fault types and combinations, the superposition and evolution process of tectonic inversion and extensional structures, and the main controlling factors of hydrocarbon accumulation. Furthermore, it investigated the controlling mechanism of tectonic stratification and zonation on differential hydrocarbon accumulation. The results showed that the class Ⅰ sag-controlling fault and class Ⅲ associated faults together formed the Pinghu boundary fault zone, and multiple class Ⅱ plate-like faults formed the Pinghu fault-step structural zone. Tectonic inversion from the late Eocene to Miocene superimposed and compounded with the boundary fault zone, forming a series of bead-like faulted anticlines nearly parallel to the boundary faults. These faulted anticlines were small in scale and exhibited significant differences in hydrocarbon accumulation, with hydrocarbon enrichment mainly occurring in the middle and lower sections of the Eocene Pinghu Formation. The superposition of tectonic inversion onto the fault-step structural zone formed a dual-layer structure with faulted anticlines dominant in the lower part and anticlines dominant in the upper part. Hydrocarbons were concentrated in the lower fault-fold structural layer. The anatomy of typical oil and gas reservoirs further revealed that the opening and sealing characteristics of faults were critical in controlling hydrocarbon accumulation. It is comprehensively concluded that the superposition of fault extension in the rift period and post-rift tectonic inversion alters the configuration of hydrocarbon accumulation elements, thereby controlling the migration, storage, and differential accumulation of hydrocarbon fluids.