Chemical and isotopic features of seepage gas from mud volcanoes in southern margin of the Junggar Basin, NW China

The chemical and stable carbon isotopic composition of seepage gases from three mud volcanoes were analyzed and compared with their corresponding reservoir gases in the southern margin of the Junggar Basin. The results showed a clear alteration of the seepage gases with respect to their chemical and isotopic composition. The seepage gases released from vents/craters were relatively enriched in methane and had a decrease in ethane and propane contents when compared to their corresponding reservoir gases. Advective process during gas ascent to the surface was the dominant post-genetic process that induced such molecular fractionation. The diffusive process during gas migration through sandy rocks or liquid horizons and gas-water-rock-microbe interactions also contributed to the molecular fractionation. High gas flux and eruptive behaviors triggered by earthquakes can however restrain this kind of fractionation. The seepage gases had relatively lighter carbon isotopes in CH4, similar carbon isotopes in C2H6 and heavier carbon isotopes in C3H8 when compared to their corresponding reservoir gases. Diffusive process led to the lighter carbon isotopes of the seepage gases due to mass-dependent kinetic isotope effect, whereas gas-water-rock-microbe interactions contributed to the heavier ones. The high variability of delta C-13(CO2) values might be caused by the differently biodegraded seepage systems or by the high sensitivity of delta C-13(CO2) values to gas-water-rock interactions. Such molecular and isotopic fractionations caused by geochemical processes provide distinctive indicators for a better understanding of gas migration from reservoirs to the ground surface. It is necessary to take these geochemical processes into consideration when we assess gas origin and source rocks based on seepage gases.