鄂尔多斯盆地吴起—高桥地区延长组重排类藿烷分布特征及成因探讨

Distribution and origin of rearranged hopanes in Yanchang Formation,Wuqi-Gaoqiao area,Ordos Basin

鄂尔多斯盆地吴起—高桥地区三叠系延长组长7-长9段烃源岩中检测出异常高丰度的重排类藿烷化合物。通过对不同层段烃源岩生物标志化合物特征的对比和探讨,对研究区烃源岩进行了分类并对其成因机制进行了分析。GC-MS分析结果表明:研究区源岩中含17α(H)-重排藿烷、早洗脱重排藿烷以及18α(H)-新藿烷(Ts和C29Ts)这3个系列重排藿烷类化合物。根据重排藿烷相对丰度存在的差异,可以将烃源岩划为以17α(H)-C30藿烷(C30H)为主峰,以17α(H)-C30重排藿烷(C30*)为主峰和以18α(H)-C27新藿烷(Ts)为主峰的3种类型。此外,早洗脱重排藿烷常与高丰度的17α(H)-重排藿烷以及18α(H)-新藿烷相伴出现,且这3类重排藿烷富集的烃源岩中同时富含重排甾烷和重排补身烷,说明重排类化合物的形成可能受控于相似的机制。各项地球化学参数的对比分析表明,高黏土含量、氧化性淡水沉积环境和以陆源植物为主的母质来源更利于烃源岩中高丰度重排类藿烷化合物的形成,较高的热成熟度促进藿烷重排反应的发生,但要考虑高成熟度下化合物裂解速率不同而造成的含量差异。

A high abundance of rearranged hopanes has been detected from source rocks of the Triassic Chang 7,8,9 formations in the Wuqi-Gaoqiao area of the central Ordos Basin.We classified 3 types of source rocks and separately analyzed their genesis compared with the characteristics of biomarkers from different formations.The gas chromatography-mass spectrometry(GC-MS) analyses indicated that there were three series of rearranged hopanes in the source rocks,which are 17α(H)-diahopanes,early-eluting rearranged hopanes and 18α(H)-neohopanes.Moreover,early-eluting rearranged hopanes are commonly associated with high abundant 17α(H)-diahopanes and18α(H)-neohopanes,meanwhile,rearranged drimanes and rearranged steranes are detected in source rocks which also retain high amounts of those three types of rearranged hopanes.This indicates that the genesis and alteration of rearranged compounds are probably affected by similar processes.Many geochemistry parameters showed that,the formation of rearranged hopanes could be closely related with higher plant source materials.Sub-oxic,freshwater and clay-rich depositional environment are beneficial for forming a significant proportion of rearranged hopanes.Higher thermal evolution favors rearranged hopanes; however,the content differences of hopanes affected by different pyrolysis rates at high maturity must be considered.