Brittleness evaluation of the Upper Ordovician Wufeng–Lower Silurian Longmaxi shale in Southern Sichuan Basin, China

Abstract

Rock brittleness is now one of the significant parameters for geological evaluation of shale gas reservoirs due to the wide application of hydraulic fracturing in shale gas development. Currently, the evaluation of shale brittleness is generally based on mineral composition or elastic parameters, which both derive from the exploration experiences in North America. Due to the easier access of mineral composition data through x-ray diffraction method than rock mechanical data through array sonic logging method, the mineral-based brittleness indices have stronger practicability. However, there is not a unified definition of brittle minerals yet, which now usually refers to quartz or quartz and carbonate minerals. Considering the specific conditions of Wufeng–Longmaxi Formation in Southern Sichuan Basin; depositional environment of calcareous shelf; and higher content of calcite, dolomite, and pyrite, we redefined brittle minerals according to the elastic parameters of each mineral, taking quartz, dolomite, and pyrite which best satisfy the standard of “Young’s modulus>30 GPa and Poisson’s ratio<0.25” as brittle minerals to establish a new brittleness index, which was proved to be more accurate and applicable than the current mineral-based brittleness indices. Then we applied the new evaluation model to Well W201, and selected “sweet spot” intervals combining with lithology, total organic carbon (TOC), thickness, and other geological parameters. The results showed that the “sweet spot” interval of this well is the black siliceous shale developed in Rhuddanian–middle Aeronian stage with an average brittleness index of 50.66%, TOC of 2.3–6.2%, and total thickness of about 19 m.