Characteristics and genetic mechanisms of secondary porosity in He 8th member and Shan 1st member in the west of Ordos Basin

Detrital feldspars are almost absent in sandstones from the late Paleozoic 8th member of lower Shihezi and the 1st member of Shanxi formations located in the West of Ordos Basin. Based on the field outcrop and core observa- tion,thin section samples,X-ray diffraction data, and reservoir porosity and permeability data are collected for the study area. Dissolution pores are the main reservoir space,followed by kaolinite intercrystalline pores and fissures. Feldspars and lithic fragments dissolved by the organic acids expelled because of thermal maturation of humic-type kerogen are the origins of secondary porosity. According to the kaolinite characteristics and the dissolution mechanism of feldspars,Organic acids and the volcanic activities happened on the edge of Ordos Basin in the late Paleozoic are the main causes of feldspars absence in the study area. Thick medium-coarse sandstone supplied by high-energy channels,hard lithic fragments and volcanic ash are fundamental for the secondary dissolution porosity. Compaction and ce- mentation are the main causes of reservoir quality reduction. Secondary dissolution enhanced reservoir quality. Sedi- mentary and diagenesis processes both control the secondary porosity development.