The geological characteristics of the large- and medium-sized gas fields in the South China Sea

By the end of 2019, more than 220 gas fields had been discovered in the South China Sea. In order to accurately determine the geological characteristics of the large- and medium-sized gas fields in the South China Sea, this study conducted a comprehensive examination of the gas fields. Based on the abundant available geologic and geochemical data, the distribution and key controlling factors of the hydrocarbon accumulation in the South China Sea were analyzed. The geological and geochemical features of the gas fields were as follows: (1) the gas fields were distributed similar to beads in the shape of a “C” along the northern, western, and southern continental margins; (2) the natural gas in the region was determined to be composed of higher amounts of alkane gas and less CO2; (3) the majority of the alkane gas was observed to be coal-type gas; (4) the gas reservoir types included structural reservoirs, lithologic reservoirs, and stratigraphic reservoirs, respectively; (5) the reservoir ages were mainly Oligocene, Miocene, and Pliocene, while the lithology was mainly organic reef, with some sandstone deposits; and (6) the main hydrocarbon accumulation period for the region was determined to be the late Pliocene-Quaternary Period. In addition, the main controlling factors of the gas reservoirs were confirmed to have been the development of coal measures, sufficient thermal evolution, and favorable migration and accumulation conditions.