Origin and Distribution of Carbonate Cement in Tight Sandstones: The Upper Triassic Yanchang Formation Chang 8 Oil Layer in West Ordos Basin, China. (30th July 2017)

• Record Type: Journal Article
• Authors: Lai, Jin; Wang, Guiwen; Chen, Jing; Wang, Shuchen; Zhou, Zhenglong; Fan, Xuqiang
• Other Names: Luo Xiaorong Academic Editor.
• Is Part Of: Geofluids. Volume 2017(2017)
• Publication Date: 2017-07-30
• Subjects: Hydrogeology -- Periodicals; Sedimentary basins -- Periodicals; Fluids -- Migration -- Periodicals; Groundwater flow -- Periodicals; Geothermal resources -- Periodicals; Fluid dynamics -- Periodicals; Earth -- Crust -- Periodicals; 551.49
• DOI: 10.1155/2017/8681753
• Languages: English
• ISSNs: 1468-8115

Abstract

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 CO2 rich fluids would charge into the reservoirs with the hydrocarbons, and when the CO2 and acids were buffered by the framework grain dissolution, carbonate cement would precipitate with a decrease in CO2 concentration.