The origin and alteration of calcite cement in tight sandstones of the Jurassic Shishugou Group, Fukang Sag, Junggar Basin, NW China: implications for fluid–rock interactions and porosity evolution

ABSTRACTThe Shishugou Group, which consists of Middle Jurassic Toutunhe Formation and Upper Jurassic Qigu Formation, is currently an important hydrocarbon exploration target in the Fukang Sag of Junggar Basin, China. The Shishugou Group sandstones experienced a complex diagenetic history with deep burial (3600–5800 m) to develop low–ultralow porosity and permeability reservoir with some high-quality reservoirs found in the tight sandstones owing to the reservoir heterogeneity. This integrated petrographic and geochemical study aims to unravel the origin and alteration of calcite cement in the Shishugou Group sandstones and predict fluid–rock interaction and porosity evolution. The Shishugou Group sandstones (Q43.8F7.4R48.8) have a dominant calcite cement with strong heterogeneity forming in two generations: poikilotopic, pore-filling masses that formed at an early diagenetic stage and isolated rhombs or partial grain replacements that formed at a late stage. The Shishugou Group, which are lacustrine sedim...