The shale characteristics and shale gas exploration prospects of the Lower Silurian Longmaxi shale, Sichuan Basin, South China

The Lower Silurian Longmaxi Formation is a key target for shale-gas exploration in South China and comprises organic-rich black shale unit deposited in an anoxic deep-water setting. Mineral components are dominated by quartz and clay minerals with minor amounts of plagioclase, potassium feldspar, calcite and pyrite. The brittle mineral content ranges from 45% to 80% by weight (average: 62.5 wt. %). Organic geochemical analyses (TOC, organic matter type, Ro and gas content) show that the shale has good source rock potential. Matrix pores include organic pores; inter-particle pores between clay floccules, faecal pellets, organic matter particles and quartz and framboidal pyrite; and inter-crystalline pores within clay minerals and pyrite framboids. Fractures are also present. The development of storage space is controlled by the TOC content, the thermal maturity and type of organic matter present, by mineral composition and structural deformation. Factors which support the occurrence of shale gas accumulations in the study area include a high TOC content together with thermal maturity and organic matter type; the presence of thick and widespread shales; high rock fragility; and a stable, anoxic deep-water environment during shale deposition.