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.