Diagenesis and reservoir quality of fluvial sandstones: a case study of outcropped Middle Jurassic Yungang Formation in Yungang area of Datong Basin, North China

Abstract

The Middle Jurassic Yungang Formation in the Datong Basin, located in North China, is a prime example of braided fluvial depositional systems, offering an alternative to the homogeneous subsurface oil and gas reservoirs prevalent across the country. In this study, mineralogical, petrographic, and geochemical analyses were conducted to identify the type and extent of diagenesis and its evolution in the sandstones of the Yungang Formation. The sandstones primarily consist of medium-grained, moderately sorted lithic arkose and feldspathic litharenite, featuring secondary dissolution pores as their dominant pore type. During the eodiagenetic stage, the primary processes include (i) mechanical compaction; (ii) cementation by early calcite, dolomite, limonite, chlorite and kaolinite, and (iii) feldspar dissolution by leaching from atmospheric precipitation. Mesodiagenetic processes include (i) further mechanical and chemical compactions; (ii) cementation by late calcite and dolomite; (ii) kaolinite and illitization of kaolinite; (iv) quartz overgrowth; and (v) feldspar dissolution. Compaction and carbonate cementation during both diagenetic stages are the primary factors controlling porosity reduction. Whereas, feldspar dissolution enhances porosity by creating valuable secondary pore spaces. Therefore, the relatively high-quality sandstones in the Yungang Formation are characterized by coarser-grained with better sorting and a high feldspar content. This research provides valuable diagenetic insights for the exploration and development of high-quality reservoirs for oil and gas resources.