| Using X-ray diffraction (XRD) and scanning electron microscopy (SEM) techniques, the qualitative and quantitative analyses of the mineral composition of shale in the Lower Silurian Longmaxi Formation in northwestern Hu’nan were conducted. The main mineral components of the shale are quartz (48.1%), clay minerals(34.0%), and feldspar (11.5%), containing small amounts of calcite, dolomite, and pyrite. The content of illite and illite/montmorillonite mixed layer in clay minerals is relatively high, averaging 64.7% and 21.7% respectively. The clay mineral is the combination of illite, ordered illite/montmorillonite, and a small amount of chlorite and trace kaolinite, reflecting that the Longmaxi Formation shale reached the stage of late diagenesis. In a transitional sedimentary environment from shallow-water to deep-water continental shelf, the shale organic matter of Longmaxi Formation is easy to be enriched and preserved. The content of brittle minerals in shale core samples ranged from 34.4% to 79.6%, and the average brittleness index reached 65.4%, showing a good condition for fracturing transformation. Compared with the surrounding Jiaoshiba shale gas field, the composition of the shales in the Longmaxi Formation is similar, reflecting the geological potential of shale gas in northwestern Hu’nan. |
