【摘要】 以碳—氧同位素特征为切入点辅助薄片镜下观察和沉积环境分析,对鄂尔多斯盆地长8段储层碳酸盐胶结作用进行系统研究。从镜下矿物观察可知:碳酸盐胶结物发育方解石、白云石、铁白云石、铁方解石和菱铁矿。矿物成分和分布特征受不均一流体化学活动影响强烈,碳酸盐胶结对孔渗主要起破坏作用。同时通过磷酸法测定氧—碳同位素值,由实验结果可知:碳同位素分布范围是-14.8‰^-1.5‰,氧同位素分布范围是-25.7‰^-9.9‰;样品的Z值大都小于120,指示样品中的碳酸盐胶结作用发生于淡水—微咸水环境;通过δ¹⁸O值测定得到古温度为68℃¹28℃,说明胶结作用发生于早成岩B期至中成岩阶段A期;长8上部层位碳酸盐胶结物含量比下部多,说明胶结作用来源于与有机酸脱羧作用有关的碳酸盐沉积;按照沉积亚相划分为三个区,氧—碳同位素由湖心相—重力流相—三角洲相逐渐减轻。最后推断同位素分布形成原因:压实作用下有机酸由高势区向低势区逐渐运移,经一系列化学变化后发生胶结作用有机碳被捕捉而成。更多还原

【Abstract】 Based on the study of the carbon oxygen isotopic characteristics and the observation of thin slices, we systematically studied the carbonate cementation of Chang-8 reservoir in Ordos Basin. According to the observation of the minerals under the microscope, it can be concluded that the carbonate cements were composed of the calcite, dolomite, ferrodolomite, ferrocalcite and siderite. The mineral composition and distribution are strongly influenced by the heterogeneous chemical activity of fluids. The carbonate cementation often damages the porosity and permeability. Meanwhile, the C-O isotope value was determined by the phosphoric acid method. The experimental results show that the range of carbon isotope distribution is -14.8 ‰ to -1.5 ‰, and the distribution range of oxygen isotope is -25.7 ‰ to -9.9 ‰. The Z values of the samples are mostly less than 120, indicating that the carbonate cements in the sample formed in the freshwater-brackish water environment. The paleo temperature was measured from 68 ℃ to 128 ℃ by oxygen isotope value, which shows that the cementation occurred mainly in the early diagenetic stage B to the middle diagenetic stage A. The carbonate cement content in the upper part of Chang-8 is much lower than that in the lower part, indicating that cementation was derived from carbonate deposits which were related to decarboxylation of organic acids. According to the sedimentary subfacies, the study area is divided into three areas, and the oxygen-carbon isotope gradually decreases in accordance with the half deep lacustrine subfacies-gravity flow subfacies-delta subfacies. Finally, the reasons for the present situation of isotope distribution in the basin are concluded: the organic acid moves gradually from the high potential region to the low potential region under the compaction, and the organic carbon is captured by chemical cementation after a series of chemical changes.更多还原