Formation and diagenetic characteristics of tight sandstones in closed to semi-closed systems: Typical example from the Permian Sulige gas field

Abstract

In a closed to semi-closed diagenetic environment, the migration efficiency of diagenetic fluid is low, material exchange is weak, and the diagenetic processes of dissolution and cementation differ greatly from those in an open diagenetic environment. Previous research on closed to semi-closed diagenetic systems has focused on the formation mechanism of tight sandstones and the factors that control the formation of high-quality reservoirs. In this study, we obtained data from measured physical properties, petrographic microscope images, scanning electron microscope images, nuclear magnetic resonance, and electronic CT scans from the Permian Sulige gas field, located in the Ordos Basin, North China. We comprehensively analyzed the diagenetic characteristics, diagenetic evolution, compaction mechanism of tight sandstones in closed and semi-closed diagenetic environments, as well as the relationship between compaction and accumulation within this environment. The findings showed that sandstone densification in this area is characterized by multiple stages of siliceous and calcareous cementation. The diagenetic evolution of the Sulige field suggests that the sandstone densification process was not the result of conventional compaction/reduction of porosity and simple cementation, but rather was caused by compaction and quasi-continuous solution-cementation. The migration of organic acids in this process was instrumental for dissolution and pore development, and induced the densification of sandstone. During the dissolution process within the reservoir, the dissolving fluid was retained insitu, thus forming different cements that corresponded to particular periods. We propose that the quasi-continuous dissolution and cementation that occurred during diagenesis was the main cause of densification in the sandstone reservoirs of the Sulige field closed to semi-closed systems. The findings of our study provide a new direction for ongoing research in the field of development of high-quality reservoirs.