GENETIC TYPES, MIGRATION AND ACCUMULATION OF GAS IN THE NORTH MARGINAL BASINS OF SOUTH CHINA SEA AND NEW EXPLORATION TARGETS

In the north marginal basins of the South China Sea, the gas discovered could be classified into three types, e.i. biogenic and sub-biogenic gas, matured hydrocarbon gas, and over-matured hydrocarbon gas, based on the previous research and exploration. In addition to it,there is great amount of non-hydrocarbon gas, such as CO₂ and N₂. Gas hydrates, as the origin of gas is concerned, could also be grouped into three types:thermocatalytic and pyrolytic gas, biogenic gas and sub-biogenic gas, with the sub-biogenic dominating. Biogenic and sub-biogenic gas occurs on the sea bottom and/or the upper part of sediments, and the gas is from the marine Pliocene and Quaternary sediments. The matured hydrocarbon gas mainly distributed in the Beibuwan Basin and the Pearl River Mouth Basin is from the mixed sapropel dominated source rocks of Eocene lacustrine deposits. The over-matured coal-related hydrocarbon gas,mainly found in the Qiongdongnan Basin and the shelf area of the Pearl River Mouth Basin, is from the Oligocene coal beds and the Miocene marine sapropel type of source rocks. The deep water gas distributed in the southern Qiongdongnan Basin and some deep areas of the Pearl River Mouth Basin owe their origin to the gas sources from the Eocene deep lake source rocks, while the gas of gas hydrate is mainly from the highly matured source rocks from the deep. The non-hydrocarbon gas mainly occurs in the mud diapiric zones of the Yinggehai Basin, the Eastern Qiongdongnan Basin and some areas of the Pearl River Mouth Basin. It is from the Miocene marine calcareous mudstone with part from the mantle-derived volcanism in the deep crust. It will be more efficient and successful if the above factors are well considered in exploration.