羌塘盆地马牙山沥青脉地球化学特征及其指示意义

Geochemical characteristics of bitumen in Mayashan area in Qiangtang Basin and its indicative significance

本文以沥青抽提物的饱和烃色谱—质谱分析为基础,结合野外露头剖面沉积相分析,剖析了羌塘盆地马牙山沥青的地球化学特征,探讨了其沉积环境、成熟度和油源。测试结果表明:马牙山沥青的正构烷烃不具备明显的奇偶优势,(nC21+nC22)/(nC28+nC29)值较高,Pr/Ph、Pr/nC17值明显较低,C27、C28、C29规则甾烷呈不对称"V"字形分布,4—甲基甾烷普遍存在,Ts/Tm、C29ββ/(αα+ββ)、C2920S/(S+R)等热成熟度构型转化参数比值较低。饱和烃色谱质谱特征表明,沥青母源岩以海相水生生物输入为主,有机质处于低成熟演化阶段,有机母质形成于一定盐度的还原环境。油源对比显示,中侏罗统夏里组烃源岩与马牙山沥青的正构烷烃、甾烷、萜烷等生物标志化合物分布特征具有很好的对比性,反应了二者具有较强的亲缘关系。

Objectives:The purpose of this paper is to discuss the indicative significance of sedimentary environment and maturity,reveal the source of asphalt,and provide the basis for oil and gas exploration in the Qiangtang Basin.Methods:Based on analysis of saturated hydrocarbons chromatogram-mass spectrometry for bitumen extracts,the geochemical characteristics of the bitumen in Mayashan area were researched,as well as its indicator significance of sedimentary environment,thermal maturity and oil source were discussed,too.Results:The test results show that the n-alkanes in Mayashan bitumen show no obvious odd—even carbon dominance,the ratios of(n C21+n C22)/(n C28+n C29)are relatively high,while the ratios of Pr/Ph,Pr/n C17 are obviously low,and C27—C28—C29ααα20 R distributes in asymmetric"V"shape.The values of maturity parameters,such as Ts/Tm,C29ββ/(αα+ββ)and C2920 S/(S+R)are relatively low,while the 4-methyl-sters are also widely detected.The GC—MS characteristics of saturated hydrocarbons show that the bituminous kerogen mainly derived from lower level marine hydrobiont,which are at low mature stage and formed in reducing environment with some salinity.Conclusion:The oil source correlation reveals that the biomarkers characteristics of Mayashan bitumen are consistent with those of the Xiali source rocks in terms of n-alkanes,terpanes and steranes,indicating that they have close genetic relationship.