Main Source Rocks of Petroleum from Jurassic Coal-bearing Strata in the Turpan-Hami Basin, Northwest China
Chen Jianping, Huang Difan, Li Jinchao(Research Institute of Petroleum Exploration and Development, CNPC, Beijing, 100083)Qin Yong(China University of Mining Technology, Xuzhou, Jiangsu, 221008)
It has been debated whether oil and gas from coal measures are mainly derived from mud-stones or coals in a coal-bearing basin. This paper is aimed at probing into the problem concerning the main source rocks of petroleum related to Jurassic coal-bearing basins in Northwest Chi- na, taking the Turpan -Hami coal-bearing basin as a typical analytic model. In the Turpan -Hami basin dark mudstones are well developed in the Lower-Middle Jurassic with a total thickness ranging from 400 to 1000 m in different parts of the basin. Geological and geochemical studies of the source rocks suggest that these dark mudstones have a higher petroleum potential. The average Rock-Eval pyrolysis potential (S1+S2) of the source rocks from the main source rock formations of Badaowan and Xishanyao is around 4 mg/g and 2 mg/g respectively. The vit-rinite reflectance R0 of the source rocks, sampled from a great number of drill wells, ranges from 0.6% of the upper Xishanyao Formation to 1. 3 % or higher of the bottom of the Badaowan Formation. It shows that these source rocks in the basin have gotten to the thermal maturity to generate large amount of commercial oils. The density of most oils from the Jurassic coal measures in the basin is 0. 75-0. 85 g/cm3. Their wax content is 5%-20% and the sulphur content is less than 0. 5%. Their saturated hydrocarbon fraction is 70%-90% and is dominated by low-carbon-number hydrocarbons, and C21-/C22+ ratios of them are usually great than 3. The ααα-C29 ster-anes isomerization ratios 20S/( 20S + 20R ) are generally great than 0.4. The methylphenanthrene index is in the range of 0. 4 to 0. 8. Stable carbon isotope ratios are commonly about - 28‰to - 25‰- In general, the crude oils from the coal-bearing basin belong to the low-density, medium to lower high wax-bearing, low-sulfur and mature oils. The biomark-ers in the crude oils are similar to those in the mudstones in the coal measures, that is, low tri-cyclic terpane content and that of low-carbon-number tricyclics before C22 tricyclic higher than that of high-carbon-number tricyclics, higher ratios of Ts/Tm, C29Ts/C29 17a(H)-hopane and C30-diahopane/C30 17α(H)-hopane, but lower ratios of Tm/C30 17α(H)-hopane and C31 17α(H)-hopane/C30 17α(H)-hopane. These biomarker ratios and the distribution of steranes and terpanes of the crude oils obviously differ from those of coals and carbonaceous shales. The authors consider that the mudstones in the coal-bearing basin are main source rocks and that the coals and carbonaceous shales are secondary source rocks. Coal beds in Jurassic coal-bearing formations are well developed in the Turpan-Hami basin. However, the crude oils in the basin are mainly derived from the mudstones in coal measures. Thus, the conclusion in this paper is significant to petroleum exploration in coal-bearing basins in Northwest China.
【CateGory Index】： P618.11