Characteristics of carbonate cements in sandstone reservoirs: A case from Yanchang Formation, middle and southern Ordos Basin, China

Authigenic carbonate cement is dominant in the Yanchang Formation sandstones of the middle and southern Ordos Basin. The main types of the carbonate cement are calcite, dolomite, ferrodolomite and ferrocalcite and the carbonate cement abundance decreases with depth increasing. Polarizing microscope and SEM reveal that carbonate cement occurs as pore-filling cement, replacement of feldspar, and thin rim surrounding the grains. The isotopic values of carbon and oxygen in the authigenic carbonates indicate that the precipitation of the carbonate cements was long, multi-phased, and varying in temperature. The origin of the substance for carbonate cements include the solution of the detrital carbonate, the illitization of smectite, the expelled fluid flow from the overlying shale, and the decomposition of the unstable dark-color minerals in reservoirs. The occluding of pores, the division of thick reservoir, the formation of calcium layers and the absence of solution of the carbonate cements significantly decrease the reservoir qualities all together. ： 自生碳酸盐矿物是鄂尔多斯盆地中南部延长组砂岩储集层中最重要的胶结物，其主要类型有方解石、铁方解石、白云石和铁白云石，在不同层位上分布类型有较大差别，总体含量随地层深度增加而递减。偏光显微镜及SEM研究表明，该区碳酸盐胶结物主要呈充填粒间孔隙、交代矿物以及泥微晶环边赋存。碳、氧同位素分析揭示各种类型自生碳酸盐胶结物具有多期次、长时间的沉淀过程，其沉淀温度范围也存在较大差异。该区碳酸盐胶结物的物质来源主要有海洋碳酸盐岩岩屑的后期溶解、蒙脱石的伊利石化、相邻泥岩的压释流体以及长石和暗色亚稳定矿物的溶解等。由于碳酸盐胶结物较大规模地堵塞了孔隙，在个别部位形成了分割储集层的致密钙质层，且缺乏后期溶解作用，使其成为延长组储集层致密超低渗的一个重要因素。图7表2参36