柴达木盆地北缘西段石炭系烃源岩和油气地球化学特征

THE CARBONIFEROUS SOURCE ROCKS IN WEST PART OF NORTH MARGIN OF QAIDAM BASIN AND THEIR PETROLEUM GEOCHEMICAL CHARACTERIZATION

长期以来,由于柴达木盆地北缘发现的原油大多来源于侏罗系烃源岩,所以油气勘探始终以侏罗系含油气系统为目标。石炭系在柴北缘西段分布零星,学术界一直认为其生烃能力低下而未引起重视。冷湖五号构造冷科1井钻探后,对4666 m以下的地层归属产生极大争议,通过对岩性、电性、古生物等资料进行系统分析,现已确定这套地层属于石炭系,并由此认识到冷湖地区深86井也钻遇了石炭系;2004年绿梁山前钻探的尕丘1井钻遇了较厚石炭系,进一步揭示了石炭系的勘探潜力。通过分析石炭系烃源岩的地球化学特征,发现石炭系烃源岩成熟度高而重排甾烷和重排藿烷含量低,这与侏罗系烃源岩存在明显不同;结合岩性、电性及古生物资料,可以很好地区分石炭系和侏罗系。油源对比研究还发现在柴北缘西段存在少量来源于石炭系烃源岩的油气,从而揭示了柴北缘油气勘探的新领域。

The Jurassic petroleum system has been being the exploration target because most oils found in north margin of Qaidamu Basin are thought to sourced from the Jurassic source rocks for a long time. The Carboniferous distributed scatteredly in west part of north margin of Qaidam Basin has not been minded for the thought of poor capability of hydrocarbon generation. There are some disputes on the at- tribution of the stratum deeper than 4 666 m after the drilling of Well Lengkel in Lenghu 5th structure. The stratum is the Carboniferous based on study on lithology, electricity, paleontology, and it is realized that what has been drilled to in Well Shen86, Lenghu area is also Carboniferous. Thick Carboniferous in Well Gaqiul drilled before Luliang Mountain in 2004 suggested the exploration perspective of the Carboniferous. It is realized that there is not only one set of source rock, the Jurassic for the discovery of the Carboniferous in the west part of north margin of Qaidam Basin. The Carboniferous and the Jurassic source rocks are very different based on their geochemical characterization. The most remarkable difference is high maturity with low abundance of disasteranes and diahopanes for the Carboniferous source rocks. The Carboniferous and the Jurassic can be distinguished well by geochemical characterization, combing with litbology, electricity and paleontology. Correlation results of oil and source indicate that some petroleum is sourced from the Carboniferous in west part of north margin of Qaidam Basin rather than all petroleum is from the Jurassic, indicating new petroleum exploration area in north margin of Qaidam Basin.