Origin of giant vein-type bitumen deposits in the northwestern Junggar Basin, NW China: Implications for fault-controlled hydrocarbon accumulation

摘要

Giant vein-type bitumen deposits in the Wuerhe region within the northwestern Junggar Basin of NW China are representative examples of similar bitumen deposits that are found in a number of other locations in both China and elsewhere. These deposits can provide information on regional tectonics as well as insights into hydrocarbon and bitumen exploration, but the origin of the deposits remains unclear. This study focuses on determining the origin of the Wuerhe bitumen deposit using field, petrological, and geochemical data. Results show that the bitumen veins are generally hosted by shallow high-angle fault systems located within the nose of the Wuerhe fault-fold. The bitumen is of low maturity and has an organic origin, having formed by secondary alteration (most likely biodegradation) of crude oil sourced from the lower Permian Fengcheng Formation (P(1)f). The bitumen contains high concentrations of beta-carotane and gammacerane, low concentrations of Ts and diasteranes, and comprises (in ascending order of concentration) C-20-C-21-C-23 tricyclic terpanes and C-27-C-28-C-29 regular steranes. The bitumen was most likely formed during the Early Cretaceous under the control of faults. A fault-controlled model is presented for hydrocarbon migration and accumulation and the subsequent formation of this bitumen within the northwestern Junggar Basin. This model suggests that future hydrocarbon exploration should focus on the cores of fault-fold noses and areas of minor-fold nose near deep-seated fault zones, especially where these areas correspond to potential moderate- to shallow-depth Jurassic-Cretaceous reservoirs. The new model should be able to guide exploration in other basins with similar tectonic settings of this study, i.e., superimposed thrusting, compression, and transtension.