The differential diagenesis controls on the physical properties of lithofacies in sandstone reservoirs from the Jurassic Shaximiao Formation, western Sichuan depression, China

The Jurassic Shaximiao Formation on the eastern slope of western Sichuan depression in western Sichuan Basin of China develops a single or overlapping channel sandstones with strong internal heterogeneity. This study aims to reveal the origin of the differences of physical properties within the sandstone reservoirs under different assemblies of lithofacies. Based on analysis of sedimentary structure and grain size, the lithofacies are classified into nine types. Based on the classification of lithofacies formed under different energy of sedimentary environment and mechanical differentiation, the effect of early compaction on physical properties of sandstone reservoirs is confirmed. It is suggested that the physical properties of lithofacies can be influenced by differential diagenesis in different lithofacies assemblies. The rate of early compaction depends on the grain sorting and compositional maturity. The massive bedded lithofacies are prone to be of good grain sorting and high compositional maturity with relatively low compaction rate of 70.91%-72.75%. There are relatively high porosities and permeabilities of sandstones in the lithofacies assembly containing massive bedded lithofacies. Because of the export of diagenetic fluid along the bedding plane of parallel bedded sandstones, there would be relatively intensive dissolution of sandstones developed with parallel bedded lithofacies. Even if the development of parallel bedded sandstones could enhance the dissolution, yet they are subjected to relatively strong compaction, leading to the low proportion of intergranular pores. The permeability of sandstones within the lithofacies assembly containing parallel bedded lithofacies seems to be low. The porosity and permeability of sandstones within the lithofacies assembly containing horizontal bedded lithofacies are both poorer than other assemblies, caused by strong compaction, cementation and weak dissolution. The direction of bedding, argillaceous contents, and the capacity of resistance to compaction are the key factors controlling the physical property of high-quality sandstone reservoirs. The research conclusions can be used to interpret the genesis of different physical properties of sandstone reservoirs in other similar gas fields.