鄂尔多斯盆地奥陶系烃源岩支链烷烃特征及其地质意义

Characteristics of Branched Alkanes in the Source Rocks of the Ordovian from the Ordos Basin and Their Geological Significance

通过对鄂尔多斯盆地奥陶系马家沟组6口井烃源岩岩芯样品饱和烃的GC/MS分析,系统论述了烃源岩中支链烷烃的鉴定方法和依据,检测到碳数分布范围为C_(15)~C_(21)的中等链长的支链烷烃,主要包括2-甲基(异构)、3-甲基(反异构)、高位取代单甲基支链烷烃、双甲基支链烷烃和无环类异戊二烯烷烃;其中异构烷烃和反异构烷烃碳数分布范围较广且短、中、长链都有分布。结合研究区的沉积特征及其他有机地球化学指标,认为鄂尔多斯盆地奥陶系马家沟组中等链长的单甲基支链烷烃来源于一些特殊细菌和藻类,其中蓝细菌为最可能直接输入母质;其次一些异养细菌对原始沉积物的改造也是中等链长支链烷烃富集的重要原因之一;长链的2-甲基异构和3-甲基反异构烷烃可能与热催化重排有很大的关系。

Taking source rock core samples collected from 6 wells of Majiagou Formation of Ordovician in Ordos Basin as the target,branched alkanes in the saturated hydrocarbons were investigated by GC/MS. The results show that there is relatively high abundance of medium-chain branched alkanes of which carbon number range from 15 to 21,including 2-methyl（ iso-alkanes）,3-methyl（ anteiso-alkanes）,high-methyl branched alkanes,dual-methyl branched alkanes and acyclic isoprenoid alkanes,in which the carbon number of iso-alkanes and anteiso-alkanes has a wide distribution,including short,medium and long-chain. According to sedimentary characteristics and other organic geochemical indexs in the study area,methyl-branched-chain alkanes with medium-chain length of Majiagou Formation of Ordovician in Ordos Basin are considered deriving from some special bacteria and algae,and the cyanobacteria is the most likely direct input parent material. Meanwhile,some heterotrophic bacteria＇s transformation to the original sediments is one of the important reasons that the accumulation of medium-chain branched alkanes. The long-chains isoalkanes and anteiso-alkanes may have much to do with thermal catalytic rearrangement.