热力作用对塔里木盆地海相原油中甾烷类化合物组成和分布的影响

为了探讨热力作用对原油中甾烷类化合物形成和分布的影响,对塔里木盆地26个典型海相原油样品和1个热模拟样品进行系统的地球化学研究。结果表明:不同演化阶段原油中甾烷类化合物组成和分布的变化较为明显。高熟阶段原油的热裂解作用导致了甾烷类化合物含量大幅降低,过熟阶段甾烷类化合物含量的短暂升高则与原油中高分子量的烃类化合物裂解有关。随着热演化程度的增加,C₂₇和C₂₉规则甾烷在生成和裂解速率上的不同,致使C₂₇/C₂₉规则甾烷的值呈现先减小后增大的趋势,所以C₂₇/C₂₉规则甾烷的值在成熟-高熟阶段不能用于物源方面评价。同样由于热稳定性差异的影响,原油C₂₉甾烷异构化参数在高熟阶段发生“倒转”,因此可利用甾烷异构化参数倒转的特征来判断原油的热演化阶段。而原油中重排甾烷/规则甾烷的值与成熟度具有较好的正相关性,可作为原油在成熟-高熟阶段有效的成熟度评价参数。

Influence of Thermal Action on Composition and Distribution of Steranes in Marine Crude Oil,Tarim Basin

Based on systematic geochemical analysis, a suite of 26 marine crude oil samples and 1 simulation sample from the Tarim Basin (western China) were analyzed to investigate the influence of thermal action on the formation and distribution of steranes in crude oil. The results reveal distinct changes in the composition and distribution of steranes in different thermal evolution stages. Thermal cracking of crude oil results in decreasing sterane contents in the highly-mature stage. Meanwhile, the transient sterane content increase in the over-mature stage is likely related to the broken-down of hydrocarbons (with high molecular mass) in the crude oil. With increasing thermal maturity, the C₂₇/C₂₉ ratios of regular steranes shows an initial decrease and then an increase, which is likely controlled by the different generation and cracking rates of C₂₇ and C₂₉ regular steranes. Therefore, the C₂₇/C₂₉ ratios of regular steranes cannot be used to evaluate the sediment source in mature to highly-mature stage. In addition, the isomerization parameters of C₂₉ steranes in crude oil exhibit a significant maturity reduction or “reversal” in the highly-mature stage, which can be used to determine the thermal evolution stage of crude oil. Meanwhile, the value of diasteranes/regular steranes shows good positive correlation with maturity, and can be used as an effective maturity evaluation parameter of mature to highly-mature crude oil.