Integrated identification and determination of genetic types and accumulation time in the marginal basin of northern South China Sea

Abstract：Gas exploration has made rapid progress in the marginal basin of northern South China Sea since the 1990’s. In addition to YC13-1 gas field found in the early stage in Qiongdongnan basin, DF gas field belt and LD gas field belt were found in succession in Yinggehai basin, WC gas field belt and PY gas field belt were found in Zhujiangkou basin, and LW-LH gas field belt was found recently in deep water area of Zhujiangkou basin. The gas composition of these gas field belts is mainly hydrocarbons, with some blocks of the gas field belts in Yinggehai basin aslo rich in CO2 (carbon dioxide), N2 (nitrogen) and other non-hydrocarbon gas. According to geological and geochemical characteristics of the gas as well as known genetic identification criteria and indicators of N2 both in China and abroad, the types of the gas in this area can be divided into biogas, quasi- biogas, mature oil type gas, mature-high mature coal type gas and high mature-over mature gas. CO2 gas cloud be divide into three main types, i.e., shell type (organic / inorganic), crust-mantle mixed type and volcanic type. N2 gas cloud be divided into three main genetic types, i.e., atmosphere, crust and crust-derived organic type and organic-inorganic mixed type. The identification and determination of the formation and accumulation time of different gas reservoirs are mainly based on geochemical characteristics and dynamics of hydrocarbon production, dynamics of isotope and hydrocarbon-bearing brine inclusions and Ar-Ar isotopic dating of autogenetic illite, in combination with concrete geological conditions of accumulation so as to make synthetic judgment and decision. The formation and accumulation time of non-hydrocarbon gas is mainly based on the formation time of associated hydrocarbon gas in combination with double controlling factors of temperature and pressure in thermal simulation and geological conditions t so as to make synthetic judgment and decision.