Origin and Distribution of Carbonate Cement in Tight Sandstones: The Upper Triassic Yanchang Formation Chang 8 Oil Layer in West Ordos Basin, China

Abstract

Two generations of carbonate cement as Type I (microcrystalline calcite and dolomite) and Type II (mainly Fe-calcite and Fe-dolomite) are recognized in Chang 8 sandstones, Ordos basin. Carbonate cement in Chang 8 sandstones is closely related to the dissolved carbon from thermal maturation of organic matters. Carbonate cement in the loosely packed framework grains precipitated shortly after deposition, and late-stage ferroan calcite and ferroan dolomite formed with progressive burial. The early diagenetic carbonate cement is partially to completely replaced by late-stage ferroan calcite and ferroan dolomite. Carbonate cement is much more commonly observed in sand bodies adjacent to Chang 7 source rocks. With increasing distance from the Chang 7 oil layers, the carbonate cement content gradually decreases. However, some tight carbonate cemented zones also occur at the sandstone-mudstone interfaces. Dissolution of Ca-feldspars by organic acids-rich fluids, together with clay mineral transformations such as illitization of smectite, would provide Ca 2+ and Mg 2+ ions for carbonate cementation. Organic acids and CO 2 rich fluids would charge into the reservoirs with the hydrocarbons, and when the CO 2 and acids were buffered by the framework grain dissolution, carbonate cement would precipitate with a decrease in CO 2 concentration.