Coal-measure tight sandstone reservoir evolution under the control of the depositional–diagenetic process of the Middle–Lower Jurassic Shuixigou Group in Taibei Sag, Turpan-Hami Basin, Northwest China

Abstract

Abstract The previous studies on coal-measure tight sandstone reservoirs often focused on the single diagenetic evolution and pore structure characterization. The understanding of reservoir evolution under the comprehensive control of the sedimentary–diagenetic process was insufficient. In this paper, the coal-measure tight sandstone reservoir evolution mechanism under the control of the depositional–diagenetic process is studied through sequence stratigraphy, sedimentology, petrology, mineralogy, and geochemistry analysis. The study found that during the depositional process of coal-bearing strata, there were differences in sedimentary environments, sand body types, and coal-mud-sand combinations in different sedimentary stages. These differences result in different diagenetic evolution processes and ultimately different reservoir qualities. In general, the coal-bearing strata underwent a depositional evolution process from the lowstand filling stage to the lacustrine transgressive-maximum flooding stage, and finally highstand filling stage. In this process, sand-coal-mud combinations of thick coal-mud covering thick sand, thick coal-mud thin sand, and thick sand thin coal-mud are formed. The sedimentary environment and coal-mud-sand combination control the type and content of carbonate cement. In the rapid lacustrine transgressive-maximum flooding stage, with a reducing-high salinity sedimentary environment, strong organic matter fermentation leads to the production of large amounts of CO2 and reducing metal cations. It is conducive to the formation of a large number of early carbonate cements (siderite and ankerite). And the fluids and substances often migrate from the thick coal seam to the underlying thick lowstand filling sandstone. That is the reason why carbonate cement of lowstand filling sandstone is dominated by siderite and ankerite. In the highstand filling stage, with an oxidized-low salinity sedimentary environment, ferro calcite is the main carbonate cement in tight sandstone. Overall, the lacustrine transgressive-maximum flooding sandstone has a fine grain size and low content of rigid minerals, resulting in strong compaction and weak dissolution. And there is a large amount of siderite and ankerite developed. The reservoir quality is poor. Sandstones in lowstand and highstand filling stages have a coarse grain size and high content of rigid minerals, resulting in good pore preservation and strong dissolution. The reservoir quality is good. Siderite and ankerite are developed in reservoirs of lowstand filling stage. Due to the dissolution of early carbonate cements, its pores are particularly well preserved.

Keywords

• Coal-measure strata
• Tight sandstone reservoirs
• Depositional process
• Diagenetic process
• Reservoir evolution