鄂尔多斯盆地长8储层碳酸盐胶结物成分组成与碳氧同位素特征研究

Study on composition,carbon and oxygen isotopic characteristics of carbonate cements in Chang-8 reservoir,Ordos Basin

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δ^(18)O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。

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 mi-croscope,it can be concluded that the carbonate cements were composed of the calcite,dolomite,ferrodolomite,ferrocalcite and sid-erite.The mineral composition and distribution are strongly influenced by the heterogeneous chemical activity of fluids.The carbon-ate 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 distribu-tion range of oxygen isotope is-25.7‰to-9.9‰.The Z values of the samples are mostly less than 120,indicating that the carbon-ate 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 sedi-mentary 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 iso-tope distribution in the basin are concluded:the organic acid moves gradually from the high potential region to the low potential re-gion under the compaction,and the organic carbon is captured by chemical cementation after a series of chemical changes.