The significance of organic matter–mineral associations in different lithofacies in the Wufeng and longmaxi shale-gas reservoirs in the Sichuan Basin

Abstract The occurrence and enrichment of organic matter (OM) are influenced by the deposition of minerals in shale reservoirs. However, inadequate research has been published on the origin, spatial relationships, and significance of organic matter–mineral associations (OMMA) for predicting “sweet spots” in shale gas reservoirs. We describe OMMA in the Wufeng Formation and the Longmaxi Formation in the Sichuan Basin, which is the largest commercial shale gas accumulation outside of North America. We characterize the presence of solid bitumen (SB) and graptolite fragments (the most important kinds of OM in these reservoirs) with four important minerals: i.e., quartz, clay minerals, carbonates, and pyrite. The occurrence of OMMA is influenced by the physical (e.g., morphology, texture, and density) and chemical properties of the associated minerals, which influences the accumulation of OM and the development of porosity. By carefully studying the type and relative amount of OM, we have determined how the surface porosity, pore diameters, and amount of porosity in each kind of OMMA influence the quality of shale gas reservoirs: e.g., diagenetic quartz or clay minerals encased in SB is the most favorable kind of OMMA. The quality of OM–carbonate associations is lower because they contain a relatively low amount of OM. Similarly, the relatively low abundance and dispersed distribution of OM–pyrite associations do not improve the quality of shale gas reservoirs. We identified four important lithofacies using the color, composition, and sedimentary structures of samples. The heterogeneous distribution of OMMA has important implications for shale gas reservoirs by influencing hydrocarbon generation and gas storage conditions in each lithofacies: i.e., siliceous shale > argillaceous siliceous shale > calcareous shale > silty shale.