Fluid-rock interaction and its effects on the Upper Triassic tight sandstones in the Sichuan Basin, China: Insights from petrographic and geochemical study of carbonate cements

Tight sandstone gas is an important unconventional hydrocarbon resource, but its exploration is made difficult by the compactness and heterogeneity of the sandstone. Fluid-rock interaction and subsequent cementation play key roles in developing this heterogeneity. We conducted a petrographic, trace element, and isotopic study of the Upper Triassic Xujiahe sandstones from the Sichuan Basin, to investigate fluid-rock interactions and associated diagenetic effects. Carbonate fragments and cements are widespread in these sandstones, and are the primary control on the nature of the Xujiahe sandstone reservoirs. The carbonate cements primarily comprise of calcite and are typically distributed around the margins of sandstone bodies and along fine-grained beds. Carbonate minerals in the centre or at the margin of sandstone units have heavy delta C-13(carb) values of ca. 0 parts per thousand., whereas delta C-13(carb) is markedly depleted in the transition zone with values as low as -10 parts per thousand, reflecting a contribution from organic carbon. Manganese, Strontium and Barium generally become enriched in the carbonate cements with diagenesis, and their concentrations correlate with porosity and delta C-13(carb). Redox sensitive Mn, and fluid-mobile Sr and Ba were analysed to reveal the diagenetic processes and environment of carbonate precipitation. We propose that fluids flowed preferentially along the central parts of sandstone units, associated with mineral dissolution, whereas cementations took place near the margins of the units. Fluid-rock interaction, especially that involving late hydrocarbon-related fluids, greatly enhances the heterogeneity of sandstones and can potentially create high-quality reservoirs in the central parts of tight sandstone units. (C) 2019 Elsevier B.V. All rights reserved.