吐哈盆地侏罗纪煤系油气主力源岩探讨

Main Source Rocks of Petroleum from Jurassic Coal-bearing Strata in the Turpan-Hami Basin, Northwest China

煤系地层中的石油主要来源于煤系泥岩还是煤层的问题是目前石油地质学家和地球化学家们争论的热点。本文以吐哈盆地台北凹陷为例,深入研究了侏罗纪煤系源岩和原油的地球化学特征,发现煤系泥岩很发育,中、下侏罗统暗色泥岩厚达400～1000 m,且生油潜力较高,主要生油层八道湾组热解生油潜量平均在4 mg/g左右,西山窑组也在2 mg/g左右,目前钻井获得西山窑组—八道湾组的成熟度R_0=0.6％～1.3％,具备了形成油气田的物质基础和生成成熟原油的热演化条件。大多数煤系原油密度为0.75～0.85 g/cm^3,含蜡量5％～20％,硫含量<0.5％,饱和烃含量70％～90％,低碳数正构烷烃含量较高,C_(21)^-/C_(22)^+>3,饱和烃ααα-C_(29)甾烷S/(S+R)值>0.4,芳烃甲基菲指数在0.4～0.8之间,原油碳同位素在-28‰—-25‰之间,总体看来以轻质、中蜡-低高蜡、低硫成熟原油为主。煤系原油与煤系泥岩具有十分相似的生物标志物分布特征,三环萜烷含量低且C_(22)以前低碳数高于C_(22)以后高碳数,Ts/Tm、C_(29)Ts/C_(29)藿烷、C_(30)重排藿烷/C_(30)藿烷值相对较高,而Tm/C_(30)藿烷、C_(31)藿烷/C_(30)藿烷值相对较低。煤和碳质泥岩的这些比值均与煤系原油及泥岩相反,甾烷和藿烷的分布也有明显差异。笔者认为煤系泥岩是主要油源岩,煤和碳质泥岩?

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.