Precipitation mechanism of anhydrite cement in lacustrine tight sandstone: Implications for fluid activity, mass transfer and pore origin of open diagenetic system in tight sandstone

Abstract Anhydrite is a common type of cement occurring in the tight (low permeability) sandstone reservoirs in continental lake basins. The precipitation of anhydrite cement is governed by fluid activity during diagenesis and its precipitation controls the pore evolution in tight sandstones. It is important to understand the formation mechanism of anhydrite cement in order to forecast the quality of tight sandstone reservoirs ahead of drilling. Jurassic Shishugou Group lacustrine sandstones of the Fukang Sag of Junggar Basin, NW China, are important low‐permeability oil reservoirs. In the Shishugou Group sandstones, abundant anhydrite cement occurring mainly along a deep fault zone is associated with significant oil and gas shows. The electronic probe and laser ablation multi‐collector inductively coupled plasma mass spectrometry (LA‐MC‐ICP‐MS) were applied to investigate the formation time, precipitation conditions, material sources and precipitation process of anhydrite cement and provide new insights for fluid activity, diagenetic system and the formation of pores in the tight sandstone. Anhydrite cement is mainly formed by the dissolution and reprecipitation of early diagenetic calcite cements, feldspars and volcanic rock fragments at the mesodiagenetic stage. The dissolution of calcites and detrital grains (feldspars and volcanic rock fragments) provided sufficient Ca 2+ for both pore‐filling and grain‐replacive anhydrite. Organic acids can provide abundant SO 4 2− from the Permian petroleum source rock and transport it to the Shishugou Group sandstone through the deep‐penetrating faults. The sufficient SO 4 2− in an acidic environment is favourable for the precipitation of anhydrite at the mesodiagenetic stage. The precipitating process of the anhydrite cement in the Shishugou Group sandstone involved obvious mass transfer and dissolution in an open diagenetic system due to the hydrocarbon charging through deep faults.