Paleo-reservoir bitumen of the Middle Protozoic Jixian System in the southwest margin of the Ordos Basin, China

Solid bitumen was found in the outcrops of laminated algal dolomite of the Middle Protozoic Jixian System in the southwest margin of the Ordos Basin. Bitumen was distributed through the Jixian System and migrated upward into the overlying purple sandstone of the Lower Cretaceous Luohandong Formation. Bitumen was distributed on rock bed surfaces and in rock pores and fractures as lamina and small veins. Distribution characteristics of chloroform bitumen A normal alkanes and the analysis of terpene and steroid biomarkers show that the bitumen is sourced from lower aquatic and algal organisms. The laminated algal dolomite of the Jixian System is the most probable source rocks of the bitumen. The measured bitumen reflectance is 2.7%–3.6%, which is very inconsistent with the degree of thermal maturation of the coal beds of the Jurassic Yan'an Formation in the study area (where the vitrinite reflectance (Ro) is 0.6%–0.8%), suggesting that the bitumen maturity was controlled by local abnormal thermal events. The paleo-reservoir was destroyed by the Tongcheng alkaline magmatic intrusion and the oil and gas inside the paleo-reservoir was transformed to bitumen because of thermal cracking under high temperature. The study area has the source rock and accumulation conditions for the formation of oil and gas. The potential of hydrocarbon generation and exploration should be recognised and deserves further study.