Geochemistry and origin of gas pools in the Gaoqing-Pingnan fault zone, Jiyang Depression

摘要

In the surroundings of the Gaoqing-Pingnan fault zone are developed quite a number of gas reservoirs. Based on gas compositions, they can be divided into two groups, i.e., CO_2 and CH_4. Their composition and isotope geochemistry were dealt with in this study. The CO_2 contents range from 60.72%-99.99%, the δ~(13)C_(CO_2) values from -3.41‰- -9.8‰, and the ~3He/~4He ratios from 4.35 × 10~(-6)-6.35 × 10~(-6) (i.e. R/Ra=4.45-4.35). Based on the data on composition and isotope geochemistry, deep geological background, deep faults and volcanic rocks, it is shown that CO_2 .distributed in the Gaoqing area, mostly originated from mantle-source inorganic matter which is associated with magmatic rocks. The favorable tectonic environment for the formation of CO_2 reservoirs is the rift, which is related to great fault-magmatic activity, the formation of CO_2 gas pools and their space-time correlation to the most recent magmatic activities. Hydrocarbon gas pools occur in the Huagou area. The CH_4 contents are within the range of 88.83%-99.12%, and the δ~(13)C_(CH_4) values, -44.7‰--54.39‰. This indicates that the hydrocarbon gas resulted from the decomposition of oil-type gas at high temperatures. Volcanic rocks in the CO_2 gas pool- and CH_4 gas pool-distributed areas show significant differences in Fe_2O_3 and FeO contents. This has proven that the hydrocarbon gas may have resulted from various chemical reactions. Magmatic activities are the primary reason for the distribution of CO_2 and CH_4 gas pools in the Gaoqing-Pingnan fault zone.