MOLECULAR NITROGEN GENESIS IN NATURAL GASES AND RELATIONSHIP WITH GAS ACCUMULATION HISTORY IN TARIM BASIN

Molecular nitrogen (N-2) content of marine saporpelic type gas derived from the northern and central part of Tarim basin is higher, especially for the wet gas, whose N-2 content of wet gas ranges from 10.1% to 36.2%, while dry gas less than 10%, that is, N-2 content of wet gas is higher than that of dry gas. Why is there large difference of N-2 content between wet gas and dry gas derived from the same type source rock? What relationship it has with gas migration and accumulation? Based on the composition and isotope geochemical characteristics of associated gas and non-hydrocarbon gas, as well as rare gas in natural gas, N-2 genesis and origin are ascertained in this paper. The study shows that natural gas of middle-high N-2 content belong to organic genesis, which is originated from low Paleozoic marine hydrocarbon source rock. It is pointed out that N-2 content difference between wet and dry gas has close relationship with source rock, maturity and natural gas accumulation history. For the same type source rock, N-2 content in gas is controlled by source rock maturity and entrapment conditions.