Carbonate Cement And Their Formation Mechanism In The Sandstone Of Taiyuan Formation, Northeastern Of Ordos Basin

It is mainly talk about Carbonate cements and their formation mechanism in the sandstone of Taiyuan formation, northeastern of ordos basin in this paper.By use of thin-section analysis, cathodeluminecenme analysis, and isotope analysis, chemical analysis and so on, we research mineralogy, petrology and geochemistry of carbonate cements of the sandstone reservoir in Taiyuan formation, the northeastern of ordos basin, and discuss its formation mechanism.The petrology types of the sandstone in taiyuan formation of the northeastern of ordos basin include quartz sandstone, lithic quartz sandstone and lithic sandstone.It is notable features of the reservoir that much lithic and low feldspar.The component maturity is better in the research area (average value 4.48). However, the textural maturity is poor, and the average of matrix is 10.85%.Carbonates are the most abundant cements in sandstone of the Taiyuan formation, the northeastern of ordos basin, which mainly consist of dolomite, calcite and siderite. The extensive dolomite cemenet is manly form in euhedral and half euhedral structure, basal-pore cementation, seldom takes up the solutional pore of feldspar and before the quartz overgrowth. The content of dolomite cement is higher, and the average value ofδ13C of dolomite cement is -5.9‰. So, its material source is relation to marine carbonate. The average value ofδ18O of dolomite cement is also bigger than of calcite (-12.8‰and -13.32‰), compared with calcite cement, the forming tempture of dolomite cement is lower and the forming time is earlier. Calcite cement is shaped in granularity, replaces clastic, fills pore, and widly occupies the solutional pore of feldspar and later than the quartz overgrowth.On the basis of geologic setting, province and buried history, we study in the solutional medium in deposition process, the material resource and the formation mechanism, which is of carbonate cement. We have the following suggestions: the carbonate cement in study area is forming in the middle diagenetic stage. The forming temperature of dolomite is lower (103.8℃to 138.7℃) . The negative cement porosity of dolomite cement rocks is higher, so the buried depth of dolomite cement is small. The forming temperature of calcite is higher (104.1℃to 154.8℃), and the forming time is later. The calcite has lower value ofδ13C (the average value is -7.6‰) (close to -10‰), which is close relation to organic carbon. Thanks to the petrology types of the study area are mainly of quartz sandstone, lithic quartz sandstone and lithic sandstone. And it is notable features of the reservoir that much lithic and low feldspar. So the material resource of these carbonate mineral is possibly related to the dissolution of feldspar.The porosity is most relevant to permeability in reservoir sandstone of northeastern Taiyuan formation, but the value of porosity and permeability are very small(average value 6.08% and 0.3681×10-3μm2) .The study area belongs to compact sandstone reservoir.Compaction processes and cementation, as the two most important diagenesis effect factors on reduction the porosity, are judged here, and showed that compaction processes is more important for the reservoir sandstone on reduction porosity than cementation. Relatively early cementation of dolomite cements improves the compression of rock solid ability, so part of the original pores are preserved. Late calcite cement plug some pores.