Strike-slip superimposed Qaidam Basin and its control on oil and gas accumulation, NW China

Based on detailed field survey in the western Qaidam Basin, combined with petroleum exploration practices in recent years, this study suggests that the Qaidam Basin is a strike-slip superimposed basin jointly controlled by the left-lateral strike-slip Altyn Tagh and East Kunlun faults. The Altyn Tagh fault acts as the major controlling boundary, while the East Kunlun fault only controls the local evolution of the southern edge of the basin. To the east, the northern Qaidam-Qilian Shan thrust-fold belt passively accommodates the northeastward displacement along the Altyn Tagh fault through NE-SW directed shortening. In the India-Asia collision background, the left-lateral strike-slip faulting along the Altyn Tagh fault initiated from the early Eocene, forcing the Qaidam Basin to move northeastward, causing the thrust and slip deformation of the NW-SE faults in northern Qaidam margin-Qilian Mountain area, and the deposition of the Cenozoic since the Paleogene Lulehe Formation. During the Paleogene, the northern Qaidam Basin developed coarse-grained sediments like that in a foreland basin, forming poor quality source rocks; while the southwestern Qaidam Basin was an extensional sag basin where good quality source rocks deposited. By the early Miocene, left-lateral faulting along the Kunlun fault became active, leading to the formation of a series of en-echelon faults (e.g. the Kunbei fault, the Arlar fault and the Hongliuquan fault). These faults gradually migrated northward, their kinematics changing from left-lateral strike-slip motion to NE-SW transpression. Strike-slip-related structures controlled by those faults (e.g. the Yingxiongling structure) are ideal places for oil and gas accumulation from Paleogene source rocks. To sum up, the Qaidam Basin is a strike-slip superimposed basin jointly controlled by the left-lateral strike-slip Altyn Tagh and East Kunlun faults. The temporal and spatial superimposition of these two strike-slip faults during the Cenozoic controlled the evolution of the basin as well as the oil and gas accumulation.