Diagenetic System Control of Tight Sandstone Reservoir Quality: Case study of Xu2 Member, Xujiahe Formation, in Zhongtaishan area, Sichuan Basin, SW China

Abstract

In order to discuss the control mechanism of diagenetic systems on the quality of tight sandstone reservoirs， this study analyzed the genetic mechanism of the Xu2 member tight sandstone reservoir in the Zhongtaishan area of the Sichuan Basin by combining data from cores， thin sections， cathode luminescence （CL）， X-ray diffraction （XRD）， scanning electron microscope （SEM）， inclusion analysis， pore permeability and 3-D seismic data. It was found that mainly feldspathic quartz sandstone and lithic quartz sandstone were developed in the Xu2 member of well Zhongtai1 and well Wentan1， respectively， and the reservoir space is mainly composed of feldspars and rock fragment dissolution pores， with porosity 20%. No faulting is evident in well Wentan1； calcite and quartz cementation were mainly developed at the early stage； the diagenetic fluid has a paleosalinity >20%. Siliceous and clay cementation of the dissolution byproducts were evident in both well Zhongtai1 and well Wentan1. These are generally all present and contain intra- or intergranular pores， indicating a short migration distance of the dissolution byproducts. The results show that well Zhongtai1 was in an open diagenetic system during the early stage and in a closed diagenetic system in the late stage， and that well Wentan1 was always in a closed diagenetic system. The dissolution byproducts migrated outwards through fractures to produce a net porosity increment in the open system， which effectively improved the reservoir quality. However， the dissolution byproducts did not migrate outwards in the closed system. They cemented and filled reservoir pores， causing the redistribution of the reservoir pore space. As a result， there is no net increment in reservoir porosity in the closed system， and thus no obvious improvement in reservoir quality. Therefore， it was concluded that reservoir quality in the tight sandstone was controlled by the evolution of the diagenetic system. It was mainly improved in the open system during the early stage， then preserved in the closed system during the late stage. The opening and closure of fractures was the key factor impacting the diagenetic system.