Abstract: Recently, important exploration discovery of natural gas has been obtained in the eastern slope of Yinggehai Basin. Ultra high pressure and high temperature gas field L has been found. There are few researches on the differences of geochemical characteristics and the origin of natural gas in the this gas field, which restrict the exploration progress in this area. This work exhibits a comprehensive understanding of source rock sedimentation, inclusion and detailed gas geochemistry to study the formation of natural gas discovered. It is concluded that the geochemical characteristics of gas in L1, L2 and L3 gas reservoirs in gas field L has significant differences. Although the natural gas is coal type, it can be subdivided into M type and S type according to the carbon isotope values of methane, ethane and propane and δ13C1-δ13C2 parameters. Secondly, viewed with the distribution of different types of natural gas, from the near depression center L1 gas reservoir to the near uplift L3 gas reservoir, the natural gas types changed from both M-type and S-type to S-type. This difference is closely related to the hydrocarbon generation process of source rock, the spatial location of gas reservoir and the vertical charging process. Finally, from the formation process of gas reservoirs, the source rocks of Meishan Formation and Sanya Formation entered the maturation or over-maturation stage in the Holocene. On the near depression center side, the gas generated from Meishan formation and Sanya formation migrated to L1 gas reservoir in the Huangliu formation through faults and micro faults, while, on the near uplift side, the gas generated from Sanya formation migrated to L3 gas reservoir in the second member of Meishan formation.

Key words: natural gas, geochemical characteristics, genetic type, ultrahigh pressure and high temperature, Yinggehai Basin