辽河西部凹陷古潜山原油及其源岩的分子碳同位素地球化学

Molecular carbon isotopic geochemistry of buried-hill oils and source rocks of the West Liaohe Depression, China

以辽河西部凹陷潜山带原油及其源岩为例,系统研究了正构烷烃、异戊二烯烷烃、异构与反异构及藿烷系列化合物的碳同位素组成与分布特征,探讨了其相互关系及生源-环境因素的影响。各系列化合物的分子碳同位素组成的变化具有较好的相关性,反映了沉积体系中生源-环境因素的系统变化。应用各系列分子碳同位素组成相关分析有效地区分了不同潜山带原油的油源差异。原油正构烷烃较异戊二烯烷烃碳同位素偏正0.5‰～5.0‰、姥鲛烷较植烷偏正0.4‰～1.4‰,指示了甲烷生成菌对植烷的重要贡献;C21+长链异戊二烯烷烃较植烷系列碳同位素偏正0.5‰～2.0‰,表明其生源存在差异。异构及反异构烷烃与其他系列化合物碳同位素的对比显示了蓝细菌对这类化合物的重要贡献。C31+藿烷较C30藿烷富集13C达10‰～12‰,指示其生源不同,13C强烈损耗型C30藿烷(-61.2‰～-51.8‰)主要衍生于嗜甲烷菌,它们生存于Es4期强烈分层水体中的缺氧/有氧界面。Es3油源油中正构烷烃碳同位素特别偏正,且异构、反异构烷烃及藿烷碳同位素都明显偏正,综合反映了蓝细菌的重要贡献,而干酪根碳同位素明显偏正及4-甲基甾烷相对富集指示了沟鞭藻对沉积有机质的重要贡献。

The molecular carbon isotope composition （（δ^13Cmole） of n-alkanes, isoprenoid hydrocarbons, isoalkanes, anti-isoalkanes and hopanes in the oils and source rocks from the West Liaohe Depression are systematically studied. The variations in the δ^13Cmole values of the four series of biomarkers are well correlated and indicate that the biological and environmental constraints on δ^13Cmole values in various oils vary systematically and the δ^13Cmole values predominantly present the features of sedimentary-organic facies： Thus the oils from the West Liaohe Depression are well classified based on δ^13Cmole values of these series of biomarkers and the differences between Es4 and Es3 sources are described in detail. The n-alkanes are enriched in 13C by 0.5‰ - 5.0‰ related to isoprenoid HCs, and pristine is enriched in 13^C by 0.4‰ - 1.4‰ related to phytane. These indicate methanogenic bacteria should make important contribution to phytane. Regular isoprenoid HCs, relatively enriched in 13^C by 0.5‰ - 2.0‰, should be different from that of the series of phytane in source. Isoalkanes and anti-isoalkanes are probably related to cyanobacteria input in some extent as evidenced by their relatively positive 13^Cmole values. The important differences in δ^13Cmole value between C31＋ and C30 hopanes （10‰ - 12‰） strongly indicate that the precursors of hopanes were derived from two or more organisms, such as cyanobacteria, methyltrophic or some other heterotrophic bacteria. The most 13^C-depleted C30 hopanes （ -61.2‰ - 51.8‰） in Es4 oils and source rocks were derived from methyltrophic bacteria living at the anaerobic/aerobic boundary of the lake with strong water stratification, while the most 13^C-enriched C31＋ hopanes （ -37.5‰-33.4‰） in Es3 oils and source rocks should be contributed partly by cyanobacteria. The abnormally positive δ^13C n-ark, δ^13Cn-alk as well as δ^13CHop in the oils sourced from Es3 Formation may well indicate that cyanobacterial biomass input to the Es3 source rocks is very important. In addition, the relative high δ^13Ckero values and enrichment of 4-methyl-steranes in Es3 oils well indicate that the blooming of dinofragellates occurs seasonally and makes an important contribution to the organic matters.