Abstract:

The mineral composition and diagenesis of shale play important roles in constraining the sealing efficiency as caprock of CO₂ sequestration and the fracability in shale gas. X-ray fluorescence spectrum, X-ray diffraction and scanning electron microscopy are employed in the determination of mineral composition of mudstone and the identification of submicron-sized authigenic quartz crystals in well HX, Qingshankou Formation, Upper Cretaceous. The type of host rock and weathering effect are in consistent with mudstone in Qingshankou Formation. The mudstone is composed of clay mineral, albite, quartz and less K-feldspar. The clay fraction consists mainly of mixed-layer I/S and minor amounts of chlorite. Brittle mineral (albite, quartz and feldspar) average content is 44.43%, and clay mineral content is 54.26% on average. Submicron-sized authigenic quartz crystals released from S-I relation occur as large amounts of extremely fine-grained micro-sized crystals embedded as discrete, small clusters/nest and locate in micropores in mixed-layer I/S. The caprock consisting of mainly mixed-layer I/S mineral and minor amount of micron-quartz will be conductive to long-term CO₂ geological storage. The content of brittle mineral meet the reservoir standard of industrial value shale gas, while the content of clay mineral is quite low.

Key words: south of Songliao basin, black mudstone, mineral composition, submicron-sized authigenic quartz crystals, clay minerals