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Diagenesis of tight sandstone reservoirs of Xujiahe Formation (Upper Triassic), the Xinchang Gas Field, western Sichuan Basin, China

The tight sandstones of the Upper Triassic Xujiahe Formation are important gas reservoirs in the western part of the Sichuan Basin, China. However, their diagenesis and the factors controlling diagenetic processes are poorly constrained. The petrological and geochemical nature of diagenetic minerals of the Xujiahe sandstone reservoirs was analysed from 105 core samples of three wells in the Xinchang Gas Field. The lower member, Xu‐2, sandstones have an average framework composition of Q65F16R19, and 47% of the total lithic fragments are volcanic rock fragments (VRFs). In contrast, the upper member, Xu‐4, sandstones have an average framework composition of Q53F0R47 with a near absence of feldspar and VRFs. The main phase of carbonate cementation in the Xu‐2 sandstones is mesogenetic ferroan dolomite, whereas in the Xu‐4 sandstones, it is intergranular poikilotopic calcite. A fair amount (0.97% in bulk rock and 29% in clay fraction) of authigenic kaolinites appear in the Xu‐4 sandstones, but they are completely absent in the Xu‐2 sandstones. The observed textural occurrence of kaolinites, in combination with the positive relationship between δ13C and δ18O in the calcite cements, suggests that meteoric water flushed the Xu‐4 sandstones, resulting in the dissolution of feldspar to a negligible amount. Quartz cementation is more extensive (1.4 vol%) in the Xu‐2 sandstones than in the Xu‐4 sandstones (0.4 vol%). The fluid inclusions within the quartz cements in the Xu‐2 sandstones have a higher homogenization temperature interval (84–217°C) than those in the Xu‐4 sandstones (81–147°C), indicating that quartz cementation continued to affect the Xu‐2 sandstones during deep burial but ceased relatively early in the Xu‐4 sandstones. High‐intensity chlorite authigenesis, feldspar overgrowth, and albitization occur only in the Xu‐2 sandstones, suggesting that the source of these sandstones was primarily from the alteration of VRFs during late‐stage diagenesis. These clear variations in diagenetic features between the Xu‐2 and Xu‐4 members were most likely controlled by the significant variations in their depositional environments (marine vs. non‐marine) and in the original framework compositions.