Microscopic pore structures of tight sandstone reservoirs and their diagenetic controls: A case study of the Upper Triassic Xujiahe Formation of the Western Sichuan Depression, China

The study of microscopic pore structures is of central significance to the evaluation of the reservoir quality and well productivity of tight gas sandstone. The second and fourth members (abbreviated as T(3)x(2) and T(3)x(4), respectively) of the Upper Triassic Xujiahe Formation of the Western Sichuan Depression were used as cases to analyze microscopic pore structures and diagenetic controls in consideration of the disparities in reservoir quality and well productivity. Optical microscopy of thin sections, Micro-CT Scanning, scanning electron microscopy (SEM), High Pressure Mercury Injection (HPMI) and apex methods based on the HPMI results were used to determine the pore and throat geometries and the types and pore size distributions in terms of the quantity and levels of pore connectivity. Tight sandstone reservoirs of the study area are characterized by poor physical properties, with the T(3)x(2) reservoirs presenting relatively higher levels of permeability and lower porosity levels than those of the T(3)x(4) reservoirs. The pore spacing configurations, sizes and distributions and throat geometries reveal significant disparities. Primary pores and micro-fractures were found to be more developed in the T(3)x(2) reservoirs with bending and narrow pore throats measured as smaller than 0.5 mu m. Compared to those of the T(3)x(2) reservoirs, secondary pores including moldic and intracrystalline pores were more developed in the T(3)x(4) reservoirs with relatively larger, straighter and wider throats of 0.5-1.0 mu m. An evaluation of the pore connectivity levels shows that only 26.86% and 28.90% pores are closely connected to the whole for T(3)x(2) and T(3)x(4) reservoirs, respectively. Four major diagenetic processes were found to control reservoir quality and the microscopic pore structure. Mechanical compaction is the primary cause of the densification of reservoirs in the study area. Compared to that observed in the T(3)x(4) reservoirs, more intensive mechanical compaction was observed in the T(3)x(2) reservoirs, changing the configuration of the throats present, reducing the pore throat radii, and ultimately lowering the porosity of T(3)x(2) reservoirs as a whole. Quartz overgrowth and late period carbonate cements were observed in partly filled pore spaces, narrowing throats and reducing pore connectivity, but eogenetic cementation and chlorite cements can increase the pressure-resistance of sandstone and inhibit compaction quartz overgrowth. Thus, T(3)x(4) reservoirs are relatively more porous as a result of eogenetic calcite cementation, and primary pores are more developed in T(3)x(2) reservoirs with chlorite cements. Dissolution is a major factor that improves the tight sandstone reservoir quality because it generates a large volume of secondary pores. The more intensive levels of dissolution in T(3)x(4) reservoirs not only creates more intergranular, intragranular, moldic and intracrystalline pores but also further increases the porosity of T(3)x(4) reservoirs. Fracturing effects have developed in the study area to improve pore connectivity levels and thus the permeability of tight sandstone reservoirs, potentially explaining the pore connectivity and permeability of certain T(3)x(2) reservoirs. This work may be able to be used as a useful reference for the reservoir quality evaluation and exploration of tight gas sandstone reservoirs for similar oil fields worldwide.