The formation mechanism of multi-stage carbonate cements in tight sandstone: A case study of Chang 6₃ sub-member of Triassic Yanchang Formation in Huaqing area, Ordos Basin

In order to understand tight sandstone characteristics and formation mechanism, the authors analyzed the distribution and formation mechanism of carbonate cements in terms of petrological and geochemical characteristics of the samples by means of casting thin sections, cathodoluminescence and scanning electron microscopy in the samples, take the Chang 6₃ sub-member reservoir of Triassic Yanchang Formation in Huaqing area of Ordos Basin as the study object. The result shows that the main types of carbonate minerals are calcite, ferrocalcite, dolomite and ferrodolomite, and different areas show great difference in type distribution, being higher in content in southwest and lower in northeast. The content and type are related to the provenance and the source of calcium ions in aqueous medium, which can be divided into authigenic carbonate and exogenous carbonate. As the pore volume was plugged up by the carbonate cement on a large scale and the later dissolution was lacking, the sandstone of Chang 6₃ sub-member of Yanchang Formation became densified. On the basis of drawing the contour map of ferrocalcite and ferrodolomite cement in study area, the mechanism of carbonate cements was investigated. It is held that the carbonate cements were mainly formed by the dissolution-reprecipitation of carbonate rock debris and feldspar dissolution-precipitation. The results obtained by the authors can provide reference for favorable prediction of tight oil of Yanchang Formation in Huaqing area, and can also provide reference for microscopic characteristics study of tight hydrocarbon reservoir both in China and abroad.