中国西北地区侏罗纪煤系油气形成(之二)

Petroleum formation in Jurassic coal bearing basins, Northwest China (part 2).

西北地区已发现的侏罗纪煤系原油绝大多数为低密度、低硫、中含蜡，饱和烃含量一般为７０％～９０％，并且以Ｃ２１以前的低碳数正构烷烃为主，全油δ１３Ｃ值多为－２９‰～－２４‰。成熟度分析表明，绝大多数煤系原油为正常成熟原油，少数为低成熟原油，没有未成熟原油。天然气中的重烃含量高，Ｃ２以上组分平均达２０％，δ１３Ｃ１值一般为－４５‰～－３５‰，多属接近油型气的混合气。在侏罗纪煤系有机质和原油生物标志物分布中，类异戊二烯烷烃中姥鲛烷占明显优势，即具有较高的Ｐｒ／Ｐｈ值；含有较高的二环倍半萜烷，而三环萜烷含量较低、三环二萜烷含量很低；五环萜烷含量丰富，约占萜烷总量的８０％以上；煤和炭质泥岩中Ｔｓ、Ｃ２９Ｔｓ、Ｃ３０重排藿烷和伽马蜡烷含量很低，而煤系原油和泥岩中这些化合物相对较高；甾烷含量明显较低，且以Ｃ２９甾烷为主，占４０％～８０％；重排甾烷含量相对较高，也以Ｃ２９为主。总体上，煤系原油与泥岩的生物标志物特征较相似，而与煤和炭质泥岩有较大差异。煤系泥岩是西北地区侏罗纪煤系地层中的主力油源岩，煤和炭质泥岩是次要油源岩。这一结论在西北地区侏罗纪煤系含油气盆地具有普遍性。

Majority of crude oils discovered from the Jurassic coal bearing basins in the Northwest China is generally mature, low density, middle wax, low chill point and low sulfur content. Saturated hydrocarbon is about 70% 90%, and is dominated by normal hydrocarbon below C 21 in their chemical composition. Carbon isotope ratios of these crude oils range from -29‰ to -24‰. Most of these crude oils are normal mature, a few of them are low mature, and none is immature. The content of heavy hydrocarbon above C 2 is higher and in an average up to 20% in natural gases discovered in the area. The ratios of methane carbon isotope ( δ 13 C 1) of these natural gases are distributed from -45‰ to -35‰. Most of them are much more similar to the oil type natural gas at the liquid window stage. Biomarkers of saturated hydrocarbon in coal bearing organic matter and crude oils show abundant pristane in isoprenoid alkanes, that is, higher Pr/Ph ratios, high content of bicyclic sesquiterpanes, low tricyclic terpanes (cheilanthanes), few tricyclic diterpanes, abundant hopanes (above 80% of terpanes),a higher content of Ts,C 29 Ts,17 α (H)C 30- diahopane and gammacerane in the oils and mudstones while a low content of them in coals and carbonaceous shales, low concentration of steranes dominated by C 29 steranes (40% 80%), higher content of diasteranes contrasted with regular steranes and also dominated by C 29 diasteranes. Generally, the distribution of biomarkers in the crude oils is similar to that in mudstones and oil shales, but differs from that in coals and carbonaceous shales. Thus, the mudstones and oil shales are main oil source rocks, while coal and carbonaceous shale are secondary oil source rocks in these coal bearing basins. This is a common conclusion for the Jurassic coal bearing basins in the Northwest China.