Bogda nappe structure and its relations to hydrocarbon in Xinjiang

Located between the Turpan-Hami Basin in the south and the Junggar Basin in the north in the northern Xinjiang, China, the Bogda Mountain extends in the east-west direction, and can be divided into the east and the west segments from Qitai according to geomorphology. The Bogda nappe structure refers to the western segment of the Bogda Mountain, and more attention has been attracted due to its close relationship with oil and gas and other minerals, yet its composition and texture, formation mechanism and evolutionary history are not very clear. The comprehensive analyses of field outcrops, seismic sections and magnetotelluric data show that the Bogda nappe structure mainly consists of the EW-trending Chaiwopu Basin and the Bogda anticlinorium, with the Fukang fault as the frontal boundary while the western and eastern boundaries are defined by the Urumqi-Miquan and Dabancheng-Laoqitai adjustment faults, respectively. The main detachment lies in the middle crust and the root fault is the northern boundary fault of the Middle Tianshan orogenic belt. The Bogda nappe structure is divided by faults into several sub-units with remarkably different features from south to north. Structural pattern changes from basement-involved structure in the south to cover detachment structure in the north. The formation of the Bogda nappe structure is controlled by the deep subduction of the Junggar Basin into the Bogda Mountain. Three evolution stages are identified, as known: rudiment from the Late Jurassic to the Early Cretaceous, large-scale nappe to the north from Cretaceous to Paleogene, and again large-scale nappe to the north which resulted in the strong uplifting of the Bogda Mountain ever since Neogene. Located in the front of the nappe structure and with better settings than the Chaiwopu Basin, the Fukang Sag is a favorable area for middle to large size oil pool exploration.