Chapter 12 - Diagenetically induced heterogeneity of tight gas reservoirs near Zizhou (Ordos Basin, eastern China)

Abstract

The Early Permian Shanxi Formation around Zizhou in the Ordos Basin can be divided into two members, informally named the Shan 1 and Shan 2 Members; because the members were distinguished during drilling activities, the upper member is coded Shan 1 and the lower member Shan 2. The latter can be further divided into three sub-members (Shan 21 through Shan 23). The lowermost sub-member (Shan 23) comprises sandstones that are targets for tight gas exploration. The influence of diagenesis on the heterogeneity of the sandstones of the Shan 23 Sub-member has been investigated to identify the diagenetic processes that control the reservoir quality. It appears that the sandstone heterogeneity is controlled primarily by the cement and the pore-throat characteristics, which differ regionally due to composition-dependent diagenetic processes. In particular, compaction causes primary porosity losses; quartz and clay cementation also contributed. Residual intergranular pores, quartz overgrowths, and authigenic kaolinite occurred mostly in sandstones rich in detrital quartz. Dissolution formed secondary pores and contributed slightly to reservoir permeability. In particular, the pore types controlled the reservoir quality. The intercrystalline micropores in the clays result in small pore throats. They are responsible for the extremely low permeability of the sandstones.