Sedimentary facies and palaeogeography during the middle and late Darriwilian to the early Katian (Ordovician) and hydrocarbon exploration in central and southern Hunan

The areas of central and southern Hunan sandwiched between the Yangtze and Cathaysian landmasses are believed to be the favourable sedimentary environments for the formation of source rocks due to the sea-level changes and global palaeoclimatic changes during the Early Palaeozoic. The middle and late Darriwilian to the early Katian (Ordovician) sedimentary strata dominantly consist of the Yanxi and Modaoxi Formations in central Hunan, and Baimachong and Shuangjiakou Formations in southern Hunan, and may be divided, on the basis of sedimentary facies and palaeogeography and petroleum geology, into continental shelf facies, basin facies and continental shelf-margin facies. The continental shelf facies is made up of sandy slate, sandstone, black shale and siliceous rocks. The continental shelf-margin facies is built up of silty to fine-grained quartz sandstone and feldspathic quartz sandstone, intercalated with banded slate, carbonaceous slate and siliceous rocks. The basin facies is assembled by a succession of black rock series including carbonaceous shale and siliceous rocks, representing the stagnant and anoxic environment. The above-mentioned rocks have higher contents of organic matter, and greater hydrocarbon potential, and thus are believed to be the key horizons for further petroleum exploration.