Origins and hydrocarbon accumulation significance of bitumen in the deeply buried Jurassic reservoirs in the southern Junggar Basin

Despite its high potential, the degree of prospecting is low in the lower reservoir accumulated under the seal of the Lower Cretaceous Tugulu Group mudstones, in the southern Junggar Basin. The Dushanzi anticline is situated to the east of Sikeshu Sag. The first oil field in the southern Junggar Basin was found in the shallow Neocene sandstone reservoirs in the Dushanzi anticline. The hydrocarbon accumulation potential in the deeply buried reservoirs remains to be investigated. The DS-1 well was drilled down to the Upper Jurassic Qigu Formation (J3q) with the aim of reaching a huge oil pool. A lot of bitumen directed along the fractures was found in J3q cores at a depth of 6410~6418 m. The J3q bitumen had Pr/Ph values of 0.12~3.48, the δ13C values of -29.95‰ ~-25.3‰, high peaks of the gammacerane with isomorides, C29 steranes much more than C28 and C27 steranes, and plenty of diasteranes. According to the organic geochemical features, the bitumen should be a mixture of the Jurassic and the Cretaceous source rock generated hydrocarbons. Two stages of mature hydrocarbon migration were recognized by comprehensive experimental analysis of bitumen occurrence, Rb, Raman, GOI of hydrocarbon inclusions, and reservoir quantitative grain fluorescence. The Jurassic and Lower Cretaceous source rocks began to generate much hydrocarbon and tectonic traps developed in the Neogene in the Sikeshu Sag. Conditions for hydrocarbon accumulation were favourable in the late Cenozoic. However, the DS-1 well was in the southern slope of the Dushanzi anticline, which should be the hydrocarbon pathway instead of a charging trap. Therefore, it will be important to accurately identify the structural traps for the deep-strata oil and gas exploration in the southern Junggar Basin. © 2020, Science Press. All right reserved.