Geochemical characteristics of natural gas from mud volcanoes in the southern Junggar Basin

A number of mud volcanoes exist in the southern Junggar Basin. To date few systematic studies on natural gas geochemistry of mud volcanoes have been conducted in China. In June 1991 and August 2010, the authors investigated the mud volcanoes in the southern Junggar Basin twice, and the mud volcanism weakened gradually as seen from the variations such as the decrease of gas pressure and output, the downthrow and dry up of the mud pool. The volcanic intensity was significantly weaker than that in Taiwan. The natural gas from the mud volcanoes in the southern Junggar Basin has similar geochemical characteristics, indicating the same source or origin. The main component of the mud volcano gas is alkane gas with contents of 91.15%-97.49%, and the gas is high-quality commercial gas since methane dominates in the alkane. The delta C-13(1) values are -49.1 parts per thousand-40.6 parts per thousand, which are in accordance with the peak delta C-13(1) frequency of mud volcano gas around the world, and the alkane gas displays positive carbon isotopic series, i.e., delta C-13(1)<delta C-13(2)<delta C-13(3), suggesting typical thermogenic origin. The helium in the mud volcano gas is typically crust-derived due to the low R/Ra values of 0.011-0.054. The mud volcano gas is coal-derived since the delta C-13(2) values are all greater than -28 parts per thousand, and C-1/C1-4 and delta C-13(1) values are in accordance with those of natural gas derived from the Lower-Middle Jurassic coal-measures. Therefore, alkane gas from mud volcanoes in the southern Junggar Basin is mainly sourced from the Lower-Middle Jurassic coal-measures.