Gas geochemistry and methane emission from Dushanzi mud volcanoes in the southern Junggar Basin, NW China

• Published in: Journal of Asian earth sciences 2017-11, Vol.149, p.184-190
• Main Authors: Zheng, Guodong, Ma, Xiangxian, Guo, Zhengfu, Hilton, David R., Xu, Wang, Liang, Shouyun, Fan, Qiaohui, Chen, Wenxing
• Format: Article
• Language: English
• Subjects: Carbon cycle; Gas origin; Greenhouse gases; Mud volcanoes; Southern Junggar Basin
• ISSN: 1367-9120; 1878-5786
• DOI: 10.1016/j.jseaes.2017.08.023

•Gas origin & flux were studied for the Dushanzi mud volcano in southern Juugaar Basin, NW China.•Emitted gas is typically thermogenic & sourced from the Jurassic coal-bearing system.•The total methane flux is about 22.6tona−1 with micro-seepage dominated. There are many mud volcanoes in the southern margin of the Junggar Basin, northwest China, of which the Dushanzi area is the most typical and active one, emitting large amount of greenhouse gases associated with water and mud. The emitted gas is dominated by methane (average 90.1%), together with other gases, such as ethane (4.84–5.46%), propane (0.06–0.90%), CO2 (0.67–1.0%), and N2 (2.8–3.3%). The carbon (δ13C1) and hydrogen (δD) isotopic ratios of methane are in the ranges of −40.6‰ to −45.0‰ and −221‰ to −249‰, respectively, whereas carbon isotope ratios of ethane (δ13C2) are −25.2‰ to −27.6‰. Based on δ13C values, the released gas is characterized asa thermogenic coal-type and possibly originated from the middle-low Jurassic coal-bearing sequences according to the gas-source correlation and regional geology. Helium isotopes show a crustal source. The methane flux of Dushanzi mud volcanoes from both macro-seepage (craters/vents) and micro-seepage (ground soil exhalation) ranged over the orders of magnitude, from 0.4–2.7kgd−1 and 4950mgm−2d−1 on average, respectively. Positive CH4 fluxes from dry soil were widespread throughout the investigated areas. The total CH4 emission from Dushanzi mud volcanoes is estimated to be at least 22.6tonsa−1, of which about 89% is from micro-seepage surrounding the mud volcano vents.