柴达木盆地西部原油地球化学特征对比

Geochemical characteristic comparison of crude oil samples from the western Qaidam Basin

柴达木盆地西部地区各油田所产原油在成因上大多属咸水湖相，因而具有咸水湖相成因原油某些共同的地球化学特征，如低的姥植比和正构烷烃的偶碳优势等。对比后发现，柴西南区所产原油在正构烷烃系列分布特征上基本都具有nC37优势、姥植比相对较低且伽马蜡烷含量丰富，而柴西北区所产原油则在正构烷烃系列分布特征上大多没有nC37优势、姥植比相对偏高且伽马蜡烷含量明显偏低，这表明柴西南区与北区的原油其烃源岩具有不同的地球化学特征。依据原油中的nC37优势与姥植比之间负相关、与伽马蜡烷指数之间正相关这一现象，推测原油中正构烷烃系列的nC37优势主要受控于沉积环境的性质，高盐度和强还原的沉积环境是导致原油中出现nC37优势的主要地球化学因素。

Crnde oils produced from the oilfields in western Qaidam Basin are sourced from saline lacustrine fa- cies,thus they share certain geochemical characteristics such as lower Pr/Ph ratio and predominance of even carbon in normal alkanes. However, a comparison of the samples from the south and north parts of the western basin reveals that crude samples from the south part feature in an obvious nC37 predominance in normal alkanes, lower Pr/Ph ratio and high gammarcerane index, while those from the north part has no nC37 predominance in normal alkanes, but higher Pr/Ph ratio and lower gammacerane index, indicating differences in geochemical characteristics of the source rocks between the two parts of the basin. It is noted that a negative correlation be- tween nC37 predominance and Pr/Ph ratio, and a positive correlation between nC37 predominance and gammace- rance index could mean that the nC37 predominance be related to the nature of depositional environment and that a strongly reduced and highly saline environment is the main factor causing an obvious nC37 predominance in the crude samples.