Impact of depositional environment and diagenesis on the Upper Triassic Xujiahe tight-sand reservoir in Guang'an area, Central Sichuan Basin, SW China

The Upper Triassic Xujiahe tight-sand reservoir is characterized by low porosity and ultra-low permeability as well as intense heterogeneity. Investigation on depositional facies and their impact on reservoir quality has been proven to be a powerful tool to predict high-quality reservoirs. This study attempted to explain impacts of depositional environment and diagenesis on Xujiahe tight-sand reservoir quality in Guang'an area by: (1) identifying petrofacies and wireline logs patterns and analyzing the spatial variation of sedimentary facies, (2) determining diagenesis through detrital components, rock properties and their spatially variation as well. The results suggest that (1) T(3)x(2), T(3)x(4) and T(3)x(6) were mainly deposited in delta system, including braided river in delta plain, delta distributary channels, underwater distributary channel, and mouth bar, etc., (2) the Upper Triassic Xujiahe Formation in the Guang'an area is typical tight-sand reservoir with porosity of 0.68-15.94% and permeability of 0.001-274mD. Poor reservoir quality is attributed to intense compaction and dissolution during middle diagenetic stage A2, which are dominated by feldspathic litharenite and lithic sandstone. (3) Potential high-quality reservoirs in T(3)x(2), T(3)x(4) and T(3)x(6) are associated with delta plain distributary channel, delta front underwater distributary channel and mouth bar, since high-energy delta channel microfacies typically resulted in a large number of primary pores in sand body. Considerable secondary pores were resulted from organic acid derived from source rocks during middle diagenetic stage, improving reservoir quality significantly.