Petrologic and geochemical characteristics of carbonate cements in the Upper Triassic Xujiahe Formation tight gas sandstone, western Sichuan Basin, China

Early calcite, when dissolved by organic acids, can generate porosity in sandstones. Mineralogical, petrographic, and geochemical analyses determined the timing and origin of carbonate cements in the fourth sandstone member of the Upper Triassic Xujiahe Formation in the western Sichuan Basin. Two phases of carbonate cement identified are the crystalline carbonate and carbonate filling of secondary pores, which consists of calcite, dolomite, ankerite, and ferrocalcite. Early calcite precipitated during the Middle Jurassic with low Na+ and K+ contents and the carbon isotope compositions show that early calcite precipitated from alkaline fluid after deposition. The 818O values of the fluid indicate an involvement of meteoric and alkaline water. The burial history and fluid inclusions indicate that late calcite formed during the Late Jurassic. The relatively high Na+ and K+ contents and carbon isotope compositions suggest that organic acids participated in the formation; the 818O values of the fluid are slightly higher than that of early calcite because of the reduction in the influence of meteoric water as burial depth increased. Dolomite cement precipitated from the Late Jurassic to Early Cretaceous and has the same carbon source as late calcite. The transformation of smectite to illite likely sourced the calcium and magnesium. Active water-rock interactions in an open diagenetic system are the leading cause of carbonate mineral precipitation.

Automated summary

The study identified two phases of carbonate cement in the fourth sandstone member of the Upper Triassic Xujiahe Formation in the western Sichuan Basin: crystalline carbonate and carbonate filling of secondary pores. Early calcite formed during the Middle Jurassic with low Na+ and K+ contents, while late calcite precipitated during the Late Jurassic with relatively higher Na+ and K+ contents, suggesting the involvement of organic acids in its formation. The presence of dolomite cement from the Late Jurassic to Early Cretaceous indicates similar carbon sources to late calcite, likely sourced from the transformation of smectite to illite. Active water-rock interactions in an open diagenetic system were the main cause of carbonate mineral precipitation.