Tight sandstone diageneses, evolution, and controls on high-graded reservoirs in slope zones of foreland basins: A case study of the fourth Member of Xujiahe Formation, Tianfu gas field, Sichuan Basin

ABSTRACT: The Triassic Xujiahe Formation in the slope zone of the Sichuan foreland basin is a new field of continental tight gas exploration in recent years. The fourth member of the Xujiahe Formation (Xu4 Member), the major interval in the Jianyang Block of the Tianfu gas field in the basin, is characterized by considerable buried depth, tight reservoirs, and strong heterogeneity. By using cast thin section, X-ray diffraction (XRD), scanning electron microscopy (SEM), fluid inclusion thermometry, and core analysis, the reservoir rock types, dominant diageneses, diagenetic history, and controls on high-graded reservoirs were investigated. It is found that the Xu4 Member in Jianyang mainly consists of lithic feldspar sandstones and feldspar lithic sandstones, followed by lithic quartz sandstones. High-energy hydrodynamic conditions in the microfacies of underwater distributary channels and mouth bars are beneficial to the preservation of primary pores and the occurrence of secondary pores, and there are no significant differences in petrophysical properties between these two microfacies. Compaction and calcareous cementation are the dominant controls on reservoir porosity decrease in the Xujiahe Formation; corrosion is the major contributor to porosity increase by generating secondary dissolved pores, e.g. intragranular dissolved pores and intergranular dissolved pores, as major reservoir space in the study area. Fracture zones around the faults inside the Xujiahe Formation (fourth−order faults) are favorable for proximal tight gas accumulation, preservation, and production. The research findings have been successfully applied to explore the Xujiahe Formation in the slope zone of the Sichuan foreland basin. They can be referential for other similar tight sandstone gas accumulations.