Organic geochemical and petrophysical characteristics of transitional coal-measure shale gas reservoirs and their relationships with sedimentary environments: A case study from the Carboniferous-Permian Qinshui Basin, China

The Carboniferous-Permian Qinshui Basin has shale gas resource potential. In this study, eighteen shale samples from two representative wells were analyzed to investigate the coal-measure shale reservoir characteristics and their relationships with the transitional sedimentary environments. The results show that controlled by sedimentary environments, the tidal flat and barrier-lagoon (TFBL) system is prone to developing more organic matter (OM) than the pro-delta and delta front (PDDF) and delta plain and fluvial (DPF) systems. Shale porosity is positively correlated with clay mineral content, but shows no obvious relationship with quartz content. However, biogenic quartz content exhibits a negative correlation with shale porosity, but has a positive correlation with shale brittleness in the PDDF system, while no obvious correlation between detrital quartz and reservoir quality is found in the TFBL system. In terms of its better preservation of pyrite, which improves shale reservoir brittleness, there is more enhancement of the shale reservoir quality within the TFBL system than the PDDF system. The coal-measure shale samples exhibit a positive logarithmic correlation between reservoir porosity and permeability due to the pore size distribution. In terms of micropores (pore size < 2 nm), interlaminar OM micropores and clay mineral micropores are mainly controlled by hydrodynamic intensity, while columnar OM micropores are dominated by TOC content. Compared with the PDDF and DPF systems, the TFBL system is prone to forming laminar structures of shale in response to the sedimentary environment changes, which favor pore network development. The results of this study aim to provide a basis for shale gas exploration in coal-measure intervals in the Qinshui Basin and other similar transitional basins.