Geochemical Study of the Upper Paleozoic Gas in the Northern Ordos Basin

The geochemical parameters of natural gas in the northern Ordos basin show the characteristics of coal-derived gas which has high or much higher methanation parameter, low or lower C2+ component concentration, low condensate oil concentration and the isotope of 13C is abundant. Besides, the content of high cycloparafinic hydrocarbon is equivalent in light hydrocarbon, indicating that the source rock is the gas-bearing coal seams of the cross-facies between marine and continental environment. The changing trend of methanation modelus on the surface or vertical direction, connected with the thermal evolution of gas lava, shows that the gas migration and accumulation is persistent and throughout in the low penetrative deposited lava. The gas accumulation period is in line with the hydrocarbon migration period (J2 - K1). But in the local area, the Shiqianfeng group accumulates gas after K1. The heterogeneity and maturity difference of parent substance, and the migrating and accumulating characteristic, these factors superimposed each other causing the variation of whole geochemical features, and it is complex in local area.