Porosity evolution in tight gas sands of the Upper Triassic Xujiahe Formation, western Sichuan basin, China

Abstract

A recent exploration verified a significant gas potential of tight sands of the Upper Triassic Xujiahe Formation from the western Sichuan basin of southwest China. The mechanism for gas entrapment in basincentered areas needs to be further studied, and porosity evolution is a key factor for understanding this issue. In order to assess the porosity evolution in those sands, an integrated approach was applied consisting of: plane-light and ultra-violet fluorescence petrography, fluid inclusion microthermometry and properties, scanning electron microscopy, X-ray diffractometry, basic sandstone texture and mineralogical compositions, diagenetic features, pore types, and porosity and permeability. An overall diagenetic history of the Xu2 Member of Xujiahe Formation was deduced based on petrographic textural relationships, diagenetic minerals, fluid inclusion homogenization temperatures, organic matter maturation and burial history curve. Further, we constructed a porosity prediction model for the evolution history of the Xu2 Member tight gas sand after considering the initial porosity prediction model, the role of compaction and cementation on porosity loss, and the impact of dissolution on secondary porosity. The study revealed that the diagenetic process of the Xu2 Member tight sand occurred in five phases. Up to the late depositional age of the Xu4 Member, mechanical compaction was the principal factor for the loss of porosity during the Eogenesis A phase, which decreased the initial porosity by 10–12%. During the Eogenesis B phase (up to the Late Triassic), mechanical compaction, early quartz growth and carbonate cements generally caused the porosity to decrease by 6–7%. The phases Mesogenesis A and B, which lasted up to the middle Cretaceous, were the key stages for porosity loss because of the intensive compaction, cementation and formation of authigenic minerals. At the end of Mesogenesis, the remaining porosity in the tight gas sand of the Xu2 Member decreased by 6–10%. From the late Cretaceous to the present, the remaining porosity sharply decreased, and the tight sand was totally consolidated. The stimulating results show that the time of densification of the tight gas sand would have varied in the different gas fields, but the main stage of sandstone consolidation can be placed in the Middle Jurassic to the Late Jurassic.