River channel pattern controls on the quality of sandstone reservoirs: A case study from the Jurassic Shaximiao formation of western Sichuan Basin, China

Abstract

The fluvial sandbodies from river channels with different patterns have remarkable heterogeneities. This study suggests that the pattern of river channel controls on the sandbody stacking pattern, resulting in the heterogeneities of physical properties of sandstone reservoirs. The study area is located in the Gaomiaozi and Zhongjiang areas of western Sichuan depression in China. Based on the observation of cores and interpretation of well logging data of sandstones, three types of lithofacies assemblies and three types of vertical facies stacking patterns are identified to determine the sandbody stacking patterns. The formation of sandbody stacking patterns and ratios of sand/mud were influenced by the sinuosity of river channels, as well as the hydrodynamic condition and the frequently migration of river channels. Three sandbody stacking patterns were identified as homogeneous type composed of massive sandstones with grain size changing vertically (HT), mixed type formed by massive sandstones alternating with thin-bedded mudstones (MT1), and mixed type formed by thin-bedded sandstones wrapped in thick-bedded mudstones (MT2). HT patterns occur in the relatively high-sinuosity river channels (sinuosity index of 1.15–1.25), with sand/mud ratios of 0.65–1.74. MT1 and MT2 patterns occur in the low-sinuosity river channels (sinuosity index of 1.00–1.04), with sand/mud ratios of 0.29–0.91. Because of the long transportation distance before deposition influenced by the loop current, the sands in the relatively high-sinuosity river channels are of good sorting of grain size, with the large grain size and the high compositional maturity. As a result, sandstones deposited in the high-sinuosity river channels have the homogeneous massive pattern and relatively high porosity and permeability. This study proposed that the pattern of river channel controls the quality of fluvial sandstone reservoirs, which can be used to explore the new gas field in the similar geological setting.