Further discussion on the connotation and significance of the natural gas relaying generation model from organic matter

The natural gas relaying generation model (abbreviated as relay model) of organic matter focuses on the potential of gas generation and accumulation derived from the dispersed liquid hydrocarbons retained in source rocks during high to over mature stage (Ro>1.6%). It means the gas generation converts from thermal degradation of kerogen to oil cracking with the increase of thermal maturation, and it forms relaying gas generation processes at different thermal maturation. According to the supplementary kinetic studies and anatomization of discovered gas reservoirs, it was found that source rocks still contain a large amount of liquid hydrocarbons within oil window even after expulsion, which can serve as the effective gas source kitchen during the high to over mature stage. The maturity for the gas largely generated from oil cracking is greater than 1.6% vitrinite reflectance (Ro), and the content of methyl cyclohexane can be utilized as a specific index for identification of such natural gas. Conventional outside source natural gas reservoirs with considerable scale could be formed due to the existence of migration pathway, driving force and process for effective expulsion under high diagenetic environment. The significance of the natural gas relay model is reflected at the following three aspects: (1) focusing on natural gas accumulations of dispersed liquid hydrocarbons retained in source rocks, (2) determination of the main gas generation periods of retained liquid hydrocarbons, and (3) evaluation on the conventional gas accumulation potential of the cracking gas outside of the source kitchen from the detained liquid hydrocarbons during the high to over mature stage.