Predictive distribution of high-quality reservoirs of tight gas sandstones by linking diagenesis to depositional facies: Evidence from Xu-2 sandstones in the Penglai area of the central Sichuan basin, China

The second member of the Upper Triassic Xujiahe Formation (Xu-2) tight gas sandstone reservoirs is the main exploration target for hydrocarbons in the central Sichuan basin in China. The Xu-2 sandstones are characterized as tight with low porosity, low permeability and strong heterogeneity. Heterogeneity in diagenetically induced reservoirs hinders the economic success of Xu-2 tight gas sandstones. Analysis of these tight reservoirs by linking diagenesis to depositional facies provides a powerful tool to predict the distribution of high-quality reservoirs. This study investigates the composition, sedimentary facies, and diagenetic history and their impact on reservoir quality of the Xu-2 sandstones. Reservoir property, lithological components, depositional micro-facies, diagenesis, and diagenetic minerals were determined through petrographic studies including X-ray diffraction, scanning electron microscope (SEM), backscattered electron detector (BSE) image analysis and core analysis. The results show that the sandstones are dominated by feldspathic litharenite, litharenite and sublitharenite. The reservoir properties of the Xu-2 sandstones are generally poor with a porosity ranging from 0.68% to 15.94% and permeability ranging from 0.001 to 274 mD, which are attributed to the significant compaction and cementation. Porosity systems of the Xu-2 sandstones are of primary and secondary origins. The primary intergranular porosity ranges from trace levels to 7.50%, with an average of 2.50%. The secondary porosity (ranging from trace levels to 8.5%, averaging 2.25%) was derived from partial to complete dissolution of framework grains and is of great importance for the Xu-2 sandstones. By linking diagenesis to depositional facies, it can be concluded that the high-quality reservoirs in Xu-2 tight gas sandstones are associated with medium to coarse-grained sandstones of mouthbar or underwater distributary channel facies, development of secondary porosity, and low content of cements such as carbonate and clay minerals. The best reservoir quality is potentially encountered in the sandstones at deeper depths, which experienced a higher degree of dissolution due to closer proximity to the underlying Xu-1 source rocks.