Development characteristics and main controlling factors of tight sandstone reservoirs in the Shaximiao Formation in Zitong area, Sichuan Basin, China

Abstract

The shallow water delta sand bodies within the Jurassic Shaximiao Formation in the Sichuan Basin play a crucial role in tight gas exploration. The efficient exploration of these areas is hindered by the establishment of the favorable reservoirs’ model. The Zitong area, situated on the northwest of the Sichuan Basin, is of particular interest. The Shaximiao Formation in this region can be subdivided into the first and second members, separated by dark leaf limb shale. The first member primarily features delta-lacustrine sediments, while the second member is dominated by fluvial-lacustrine sediments. In this study, we focus on the tight sandstone clastic reservoir within the Shaximiao Formation in the Zitong area. Through core observations from four wells and a comprehensive analysis involving conventional slices, casting section, scanning electron microscopy, high-pressure mercury injection, nuclear magnetic resonance, and core plunger porosity and permeability, we explore the reservoir's development characteristics and controlling factors from both sedimentation and diagenesis perspectives. The research reveals the following findings: The reservoir components in the Zitong area are mainly composed of medium to fine-grained feldspar lithic sandstone and lithic feldspar sandstone. The primary reservoir space is intergranular pores, followed by intragranular dissolution pores. Porosity is mainly distributed in the range of 7-10%, and permeability ranges from 0.1-1.0 mD. While different subsegments exhibit varying physical properties, the overall characteristics indicate low porosity and ultra-low permeability, with the predominant reservoir type being pore type. The advantage of relatively high-energy and stable sedimentary facies is crucial for high-quality reservoir formation. The underwater distributary channel in the delta front of the Shaximiao Formation serves as a favorable facies zone for high-quality reservoir development. Variations in particle size, sorting, and mud content contribute to the reservoir's properties, with the middle sandstone presenting optimal conditions for reservoir development. Compaction and cementation emerge as the primary mechanisms reducing porosity. Utilizing the Ehrenberg (1989) model, calculations indicate that cementation contributes approximately 25.42% to porosity reduction, while compaction contributes 65.07%. Vertical distribution differences in zeolite and calcite contribute to variations in reservoir properties. The dissolution of feldspar and the development of chlorite crust are identified as key factors in high-quality reservoir formation. Factors such as the preservation of primary pores, strong dissolution, weak cementation, and the role of chlorite thin films influence the constraints on high-quality reservoir development. The preservation of primary pores is particularly associated with the formation of chlorite thin films. These findings contribute valuable insights into understanding and optimizing the exploration and development of tight gas reservoirs within the Shaximiao Formation in the Zitong area.