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.