Fault Development Characteristics in the Western Junggar Basin and Implications for Petroleum Geology

The western Junggar Basin (WJB) has undergone multi-stage tectonic evolution. However, there are still uncertainties regarding the deformation timing, reactivation dynamics, and reservoir-controlling effect of complex faults within the WJB, which restricts oil and gas evaluation and exploration efforts. In this paper, it clarified the development characteristics and reservoir-controlling mechanisms of faults at different stages through integrated structural analysis of intra-basin seismic data and basin-margin field outcrops. The research shows that the WJB has mainly undergone four stages of deformation, each controlled by differential dynamic settings. During the Hercynian, multi-directional plate collisions and compressions generated multi-episode thrust-nappe faults, including N-striking brittle-ductile, NW-striking ductile-brittle, and NE-striking brittle faults. During the Indosinian, thrust-transpressional faults developed in the WJB, driven by the counterclockwise rotation of the Junggar Block and EW-directed compression, with basin-margin dextral transpressional faults and near EW-striking conjugate shear faults formed. During the Yanshanian, the NNE-directed subduction of the Bangong-Nujiang Ocean and the subsequent Lhasa Block collision drove left-lateral strike-slip shearing of the Darbut Fault Zone, and its Riedel shears derived NW-striking and E-striking dextral secondary strike-slip faults. During the Himalayan, the rapid uplift of the North Tianshan Mountains, driven by the India-Eurasia continental collision, triggered the rapid southward tilting of the WJB, generating extensional normal faults within the WJB. In terms of hydrocarbon geological significance, Hercynian thrust-nappe faults were later intensely filled with hydrothermal fluids, forming boundaries that seal oil and gas in deep formations, and basic traps such as faulted anticlines and fault blocks developed concurrently. The Indosinian "fault-fault" migration system transported oil and gas from source rock areas to the Carboniferous, forming two types of paleo-oil reservoirs in the Carboniferous, namely weathered crust reservoirs and interior reservoirs, and developing composite traps concurrently.Yanshanian reticular faults dominated ultra-long-distance westward oil and gas migration, and differential strike-slip movement promoted the migration and accumulation of some oil and gas into the overlying Jurassic-Cretaceous strata. Himalayan normal faults channeled deep oil and gas upward to adjust and accumulate in the Neogene, with the concurrent formation of structural-stratigraphic traps. Multi-stage faults regulated differential hydrocarbon accumulation through reservoir modification, vertical-horizontal migration, and paleo-reservoir adjustment. The vertically ordered superimposition of multiple oil-bearing intervals forms a stereoscopic exploration framework, providing theoretical support for hydrocarbon exploration breakthroughs in the WJB.