Effects of cementation on physical properties of clastic rock-originated weathering crust reservoirs in the Kexia region, Junggar Basin, NW China

Abstract

Cements are widely developed in clastic rock-originated weathering crust (CWC) reservoirs in the Kexia region along the northwestern margin of the Junggar Basin and significantly affect reservoir physical properties and oil and gas distribution in this area. Focusing on the CWC reservoirs at the top of both the Permian Jiamuhe Formation and the Triassic Karamay Formation, this study analyzed the types and characteristics of cements in the reservoirs and explored their effects on reservoir physical properties based on thin sections, SEM images, XRD results, and tests of physical properties. The main results are as follows. The cements in the CWC reservoirs in Kexia region mainly consist of carbonate minerals (41.5%), clay minerals (27.8%) and zeolite minerals (30%), as well as small amount of siliceous minerals. Among them, the carbonate minerals are dominated by siderite and calcite, the clay minerals mainly include kaolinite, interstratified illite/smectite (I/S) and chlorite, and the zeolite minerals primarily comprise heulandite and laumontite. These different types of multiphase cements are generally paragenetic or associated and affect reservoir physical properties to different degrees. Specifically, the carbonate and clay cements of the early diagenetic stage reduced the reservoirs’ average porosity from 21% to 15%. The dissolution of some carbonate and zeolite cements in the early A substage of the middle diagenetic stage restored the average porosity to 18%, and the cementation in the late A substage decreased the average porosity to 13% again, of which about 4% was reduced by carbonate cements. The average porosity of the CWC reservoirs gradually decreased to the current value of approximately 10% in the B substage of the middle diagenetic stage. The impact of cementation on the CWC reservoirs can reach as far as 70 m below the unconformity. Moreover, the types and contents of cements vary with their depth below the unconformity surface, leading to the development of multiple zones with high cement content and the differentiated oil and gas distribution.