鄂尔多斯盆地南缘长7油层组碳酸盐结核的特征及石油地质意义

摘要

探讨鄂尔多斯盆地南缘渭北地区上三叠统延长组长7油层组油页岩中的碳酸盐结核与优质烃源岩发育的关系.通过野外观察、薄片鉴定、扫描电镜、X射线衍射和碳、氧同位素等多种分析手段的研究,认为长7油层组灰岩和白云岩结核为成岩早期的产物,方解石和白云石圆球粒的形成时间较早,特征与藻类习性非常相似.δ13C明显重于晚三叠世δ13C的变化范围,表明碳酸盐结核的形成与产烷带微生物代谢活动引起的甲烷生成有关.结核中方解石、白云石圆球粒的形成可能是蓝细菌细胞方解石化或白云石化的结果.长7油层组中富含的碳酸盐结核指示其为优质的烃源岩,具有良好的生烃能力.%Carbonate concretions,usually the products of microbial activity,can provide the information of diagenetic processes.On the basis of field investigation,thin section observation,application of scanning electron microscopy,X-ray diffraction and carbon oxygen isotopes,the limestone and dolomite concretions,abundant in the oil shales of Chang-7 (Member 7 of Upper Triassic Yanchang Formation) in Weibei area of the southern Ordos Basin,are studied.It is considered that the limestone and dolomite concretions are products of early diagenesis and the formation of calcite pellets and dolomite pellets is formed early and their features are similar to the algae.δ13C,which evidently heavier than the variation range of δ13C during late Triassic,indicating that the formation of the carbonate concretions is related to the methane formation resulted from alkane producing and microbial metabolic activity.The formation of calcite pellets and dolomite pellets in the Chang-7 concretions is probably related to microbial activity and the carbonate nodules in the Chang-7 are indicator of favorable source rocks with good hydrocarbon generating capacity.