EXPERIMENT RESEARCH ON AEROBIC OXIDATION OF MULTICOMPONENT HYDROCARBONS DECOMPOSED FROM MARINE GAS HYDRATES

Natural gas hydrate, as an enormous carbon reservoir, is mainly embedded in subsurface marine sediments. A large amount of hydrocarbons may release from the marine regions where gas hydrate deposits occur. Anaerobic or aerobic oxidation of dissociated hydrocarbon gas in its upward migration may cause hydrocarbon consumption thus decrease the carbon emission to atmosphere. Here, we performed experimental measurements on the aerobic oxidation process using marine sediments containing aerobic hydrocarbon-oxidizing bacteria to simulate the process of aerobic biodegradation for hydrocarbons (C₁+C₂+C₃) that decomposed from gas hydrate. The results show that the composition of methane, ethane and propane decreases together with carbon and hydrogen isotope fractionation during the aerobic consumption. An apparent preference for C₁ over C₂ and C₃ is observed during oxidation. The rates of oxidation are also in an order of C₁>C₂>C₃. At the same time, the carbon and hydrogen isotope of hydrocarbons show a various enrichment tendency. The enrichment amount of carbon isotope of C₁, C₂ and C₃ are 71.05‰, 12.03‰ and 4.61‰, and the average of ε_C are -11.219‰, -2.951‰ and -1.539‰, respectively. The accumulation amount of hydrogen isotope are 368.64‰, 156.00‰ and 111.97‰ for C₁, C₂ and C₃, as well as the average of ε_H are -56.092‰, -99.696‰ and -73.303‰ for C₁, C₂ and C₃ , respectively. The enrichment degree of carbon and hydrogen isotope fractionation are in an order of C₁>C₂>C₃ and C₂>C₃>C₁, respectively. Therefore, the aerobic biodegradation of hydrocarbons decomposed from gas hydrate may interfere with the origin discrimination of gas hydrate since the aerobic oxidation makes the composition and carbon and hydrogen isotope fractionation of hydrocarbon changed. Therefore, the influential factor should be considered appropriately to genesis study on gas hydrate when using decomposed hydrocarbons in headspace analysis.