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A geochemical study on mud volcanoes in the Junggar Basin, China |
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| Authors: | Ryoichi Nakada Yoshio TakahashiUrumu Tsunogai Guodong ZhengHiroshi Shimizu Keiko H Hattori |
| Institution: | a Department of Earth and Planetary Systems Science, Graduate School of Science, Hiroshima University, Higashi-Hiroshima, Hiroshima 739-8526, Japan b Division of Earth and Planetary Sciences, Graduate School of Science, Hokkaido University, Kita-10 Nishi-8, Kita-ku, Sapporo 060-0810, Japan c Key Laboratory of Petroleum Resources Research, Institute of Geology and Geophysics, Chinese Academy of Sciences, 382 West Donggang Road, Lanzhou 730000, China d Department of Earth Science, University of Ottawa, Ottawa, ON, Canada K1N 6N5 |
| Abstract: | A comprehensive study was performed to characterize, for the first time, the mud, water, and gases released from onshore mud volcanoes located in the southern margin of the Junggar Basin, northwestern China. Chemical compositions of mud, along with the geology of the basin, suggest that a source of the mud is Mesozoic or Cenozoic shale. Oxygen and H isotope compositions of the released water suggest a local meteoric origin. Combined with the positive Eu anomalies of the water, a large 18O shift of the water suggests extensive interaction with rocks. Gases discharged from the mud volcanoes are predominantly thermogenic hydrocarbons, and the high δ13C values (>+20‰ VPDB) for CO2 gases and dissolved carbonate in muddy water suggest secondary methanogenesis with CO2 reduction after oil biodegradation.The enrichments of Eu and 18O in water and the low thermal gradient of the area suggest that the water-rock interactions possibly occur deeper than 3670 ± 200 m. On the other hand, considering the relationship to the petroleum reservoir around the mud volcanoes, the depth of the gases can be derived from about 3600 m, a depth that is greater than that generally estimated for reservoirs whose gas is characterized by 13C-enriched CO2. Oil biodegradation with CO2 reduction likely occurs at a shallower depth along the seepage system of the mud volcano. The results contribute to the worldwide data set of gas genesis in mud volcanoes. Moreover, they further support the concept that most terrestrial mud volcanoes release thermogenic gas produced in very deep sediments and may be early indicators of oil biodegradation, an important problem in the petroleum industry. |
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