Abstract: Marine shale gas resources are abundant in Southern China,and nitrogen (N₂) content accounts for a larger proportion in the composition of some shale gas.Analytical data of the geological and geochemical characteristics of shale gas in southeastern Sichuan,Three Gorges Region of Yangtze River,and Cengong in Guizhou Province,and the thermal simulation experiment of gold tube,are integrated in the study of the origin of nitrogen in shale gas and its indicative significance.The results show that N₂ in shale gas is typically of organic crustal origin,shale gas with higher N₂ content is mainly derived from the late pyrolysis of kerogen,and the higher content of N₂ is just an embodiment of the early gene-ration of crude oil cracking gas under no migration and accumulation or under destruction after accumulation.Therefore,the fact that shale gas resources are limited may be verified by the following three aspects.First of all,the shale kerogen is typically of saprolite.Under good preservation conditions,the shale gas is supposed to be dominated by crude oil cracking gas.However,the identification of shale gas types indicates that the shale gas with higher N₂ content belongs to the kerogen pyrolysis gas or a mixture of crude oil cracking gas and kerogen pyrolysis gas.Secondly,the carbon isotope value of shale gas with higher N₂ content is significantly heavier than that of the secondary cracking gas of crude oil obtained by thermal simulation experiment,through the gold tube thermal simulation experiment of the crude oil samples taken from the Cambrian.Finally,with the growth of organic matter evolution,the N₂ content keeps increasing,and the shale gas wells with better preservation conditions are usually lower in N₂ content but better in gas-bearing properties.Therefore,the N₂ content is of indicative significance to the preservation conditions.

Key words: nitrogen (N₂), organic crustal origin, kerogen pyrolysis, gold tube thermal simulation experiment, marine shale gas, Southern China