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

Abstract 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 m 2 /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 −3 μm 2 and 0.36–3.51% in average, they are still higher than the lower limit of favorable condition of shale gas accumulation. Based on the shale gas evaluation and exploration experience conducted in this geologically complex area, we suggest that the shale gas generation conditions are good in the Dabashan area, but the preservation conditions are quite different from place to place in the tectonically complex area. In general, the preservation conditions are of the utmost importance for shale gas enrichment in Dabashan area. A comparison of failed to the successful cases of exploration wells in the frontal margin of Dabashan suggests that relatively stable box-shaped anticlines, like Tianba and Manyue anticlines, are beneficial for shale gas storage. Information obtained from this study can be used as a good reference for shale gas reservoir evaluation and the selection of shale gas Sweet Spots in further study of this region. Furthermore, the research conducted in this area provides useful experience for shale gas exploration in the tectonically complex region in the world.