Sedimentary evolution and shale gas exploration potential of the Lower Silurian strata in Hunan

The Lower Silurian strata are developed in the northwestern and central-southern parts in Hunan. The representative sedimentary facies may be classified into the shelf, tidal-flat and delta facies in the northwestern part, and the deep-water turbidite facies in the central-southern parts in Hunan. Controlled by the tectonic movements, sea-level changes and sediment supply, the black shales in the basal part of the Silurian strata are restricted only in the shallow-sea environments in the northwestern part of Hunan, where the shales, sandy shales and argillaceous sandstones are observed. The organic carbon contents generally vary between 0.12% and 3.86%, and maximum organic carbon contents occur in the Longshan-Yongshun zone. The organic carbon contents and the thickness of the source rocks sharply decrease toward the southeastern part of Hunan.