Guodong Zheng1,2*, Wang Xu3, Xiangxian Ma2, Zhongping Li2, Mingliang Liang4, Huayun Tang1, Georgii Chelnokov5, Vassil Lavrushin5, Zhi Chen6
1School of Environmental Studies, China University of China, Wuhan 430074, China (*correspondence: zhengguodong@cug.edu.cn)
2Key Laboratory of Petroleum Resources, Gansu Province/Northwest Institute of Eco-Environment and Resources, China Academy of Sciences, Lanzhou 730000, China
3Tianfu Yongxing Laboratory, Chengdu 610213, China
4Institute of Geomechanics, Chinese Academy of Geological Sciences, Beijing 100081, China
5Geological Institute Russian Academy of Sciences. Pyzhevsky lane 7. bld. 1. 119017. Moscow. Russia
6CEA Key Laboratory, Institute of Earthqauake Forecasting, China Earthquake Administration, Beijing 100036, China
Absract: As one kind of geofluids, especially gas emission systems on the Earth surface, petroleum-related mud volcanoes are significant to energy resources evaluation, environmental assessment, natural hazards prediction, and even tourism resources development. Therefore, field investigations and geochemical researches have been attractive for a lot of attention in recent decades. A series of samples including emitted gases, formation water, erupted mud, and the host rocks were collected from the mud volcanos in southern margin of the Junggar Basin, NW China, some related samples also collected from surrounding rivers, drilling-wells of the nearby oil and gas fields for comparisons. Their mineral and chemical compositions and iron species were systematically determined using XRD, XRF, GC, GC-MS, and Mössbauer spectroscopy, respectively. The analytical results indicate a series of oil-gas-water-rock interactions occurred in the mud volcano systems marked with the following main variations: 1) chemical and isotopic variations for emitted gases compared with well-gas samples from the corresponding reservoirs nearby, both emitted gas and well-gas are mostly orginated from the same source, the oil and gas reservior; 2) some conversion of clay minerals from smectite into chlorite and illite, and also the precipitation of newly formed metals carbonate precipitant including calcite and siderite, which induced the colour change of the host rocks; 3) silicon depletion and significant elemental enrichment of Fe, Mg, Mn, Ca and P etc. in the bleached rocks comparied with rock samples far from the mud volcano sysytems; 4) clearly transformation of iron species from ferric iron in hematite and smectite into ferrous species in siderite, chlorite, illite and even pyrite. These geochemical alterations likely induced various colour change of the original reddish Neogene argillite to the mud breccia in grey or black, resulted in elemental reduction and/or mineral alteration along with hydrocarbons oxidation, which is so-called as bleaching effect. Such a complex process might be positively contributed to the reduction of greenhouse effort of erupted gases from mud volcanic systems compared to direct emission of natural gas from the corresponding petroleum reservoirs as so-called relatively fixation of carbon, and eventually significant to a better understanding of physical properties such as porosity and permeability of the reservoir rocks and also the migration pathways, and even applicable as a reference to the evolution of long-term stability of geologically sequenced carbon dioxide. In addition, the conversion of carbon dioxide into methane occurred in the mud volcanic systems, mainly based on some specific microbe’s activity in the migration and emission pathway, which should be benefit for CCUS because converted methane could become a new source for energy. These findings with a series of publications could be also significant for tracing migration pathways of crude oils and natural gases underground, especially when the main migrating period is over.
Key words：mud volcano systems; emitted gases; mud breccia; brine water, host rocks; organic-inorganic interactions, bleaching effect
Author Profile (first author):
Guodong Zheng, Male, Professor, PhD, mainly engaged in gas geochemistry and iron speciation research. E-mail: zhengguodong@cug.edu.cn; gdzhbj@mail.iggcas.ac.cn
Acknowledgments: This study was supported by the 100-Tallent program Chinese Academy of Sciences, , the joint research project between Natural Science Foundation of China and Russia Science Foundation (NSFC 42261134534 and RSF 23-47-00035), the National Key Research and Development Program of China (2019YFA0708501), Natural Science Foundation of China (41572352; 41402298), CAS “Light of West China” Program.
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