鄂尔多斯盆地延长组烃源岩中单甲基烷烃来源及意义

摘要

Monomethylalkanes are common branched alkanes in the saturated hydrocarbon fraction of lacustrine source rocks. However, there is no agreement about their original source. The abundance of monomethylalkanes, from the Triassic Yanchang Formation source rocks in the Ordos Basin is relatively high. Determination of TOC, Rock⁃Eval, compound specific carbon isotopic composition, and GC⁃MS analysis of saturated hydrocarbons show that the organic matter of source rocks from Chang7-Chang10 have a mixed source, and that the type of organic matter isⅡ-Ⅲ. The steroid hydrocarbon distributions of source rocks showed nearly equal abundances of C27 and C29 steranes, suggesting mixed organic matter. The monomethylalkanes have 12 to 32 carbons, having a similar distri⁃bution to the n⁃alkanes, and the 2⁃MMAs and 3⁃MMAs are predominant. Theδ13 C values of Pr and Ph are 1‰to 2‰heavier than corresponding n⁃alkanes. 2⁃MMAs and 3⁃MMAs with the same carbon number have almost the sameδ13 C value, indicating they have the same source. Theδ13 C values of monomethylalkanes range from-29.3‰to -32.8‰, which are higher than n⁃alkanes. Moreover, combining with the sedimentary environment information of the Ordos Basin, it appears that the monomethylalkanes might come from phytoplankton and heterotrophic bacte⁃ria, and they can be used as a biomarker to judge the predecessor of sediment and petroleum.%单甲基烷烃是湖相烃源岩饱和烃中常见的支链烷烃，其母质来源的研究一直存在争议。对鄂尔多斯盆地上三叠统延长组烃源岩样品进行了TOC测定、岩石热解、饱和烃GC－MS和单体碳同位素等分析，并在饱和烃中检出了丰富的单甲基烷烃化合物。地化分析表明，该区长7－长10烃源岩的有机质类型为Ⅱ－Ⅲ型，C29≥C27规则甾烷，高等植物的贡献略微大于低等水生藻类或细菌；单甲基烷烃碳数分布为C12－C32，甲基取代位置以C2和C3为主。饱和烃单体碳同位素组成数据显示，姥鲛烷和植烷的δ13 C比同碳数正构烷烃重1‰~2‰；同碳数2－和3－甲基烷烃具有相近的碳同位素组成，表明两类化合物具有相同物源；单甲基烷烃比正构烷烃更富集13 C，表现出更大的δ13 C值。结合古环境等信息表明，鄂尔多斯盆地检出的单甲基烷烃可能来自于浮游植物或者异养细菌，单甲基烷烃可以作为一种指示生油母质的生物标志化合物。