Shale gas enrichment conditions in the frontal margin of Dabashan orogenic belt, south China

In this study, we carried out a survey on organic-rich mudstones of Wufeng-Longmaxi formations in the frontal margin of Dabashan orogenic belt where geological structure is complex, aiming to investigate the shale gas enrichment condition and their major controlling factors. Methods including optical microscope observation, Scanning Electron Microscopy (SEM), organic geochemistry and tests of porosity, permeability, gas-bearing property, and Specific Surface Area have been applied in this study. Based on these comprehensive information collected from both the field and the laboratory, the general geological characteristics and distribution of organic-rich mudstones of Wufeng-Longmaxi formations are summarized. In the Late Ordovician-Early Silurian epoch, organic-rich mudstones were probably deposited in the continental shelf, with the thicknesses ranging from 31.5 to 156.4 m in the frontal margin of Dabashan orogenic belt, and the averaged value of total organic carbon in these mudstones varies from 2.43 to 5.17 wt% in different localities. Values of BET specific surface area range from 1.57 to 29.15 m2/g. Thermal maturity is high because Ro values vary from 1.20 to 2.58% in this region. The mineral composition, i.e., low clay mineral content (16.14–27.88%) and relatively high brittle mineral content (larger than 72.12%), is proper for future fracturing. Although the permeability and porosity values are relatively low, respectively 0.0001–0.0472 × 10−3μm2 and 0.36–3.51% in average, they are still higher than the lower limit of favorable condition of shale gas accumulation.