Geochemical Characteristics of Shale Gas in the Silurian Longmaxi Formation, Jiaoshiba Area, Southeast Sichuan Basin, China

摘要

On the basis of geochemistry tests, in combination with comparative studies of the Weiyuan-Changning shale gas field in the south Sichuan Basin and typical shale gas fields in North America (such as Barnett and Fayetteville), the geochemical characteristics of the Lower Silurian shale gas in the Jiaoshiba area were studied. The results show that (1) the average R-o value of Longmaxi shale in Jiaoshiba area is about 2.65%, which is at the dry gas stage. The content of CH(4)ranges from 94.02% to 98.95%, and the contents of ethane and propane are lower. The weight anomaly average values of delta(C-13(1)) and delta(C-13(2)) are -30.18 parts per thousand and -35.45 parts per thousand, respectively. (2) Non-hydrocarbon gases are dominated by CO2 and N-2. The CO2 in the Longmaxi shale gas in the Jiaoshiba has three origins: the thermogenic CO2, the abiogenic CO2, and the biogenic CO2. The N-2 in the Longmaxi shale gas comes from the released N-2 during the thermal evolution process of sedimentary organic matter under the control of maturity and kerogen type. (3) The natural gas genetic type discriminating signs and plates indicate that the shale gas in the Jiaoshiba area belongs to an organic pyrolysis oil type gas at high temperature. (4) With an increasing degree of thermal evolution, the wetness of the shale gas decreases gradually, the carbon isotopic composition of methane becomes heavier, and the carbon isotopic composition of ethane and propane will reverse. In the Jiaoshiba, a complete reversal phenomenon of carbon isotopes (delta(C-13(1)) > delta(C-13(2)) > delta(C-13(3))) is caused by the reaction among water, ferrous metals, and hydrocarbon gas which was mainly generated from the mixed gas caused during the secondary cracking, wet gas cracking, and Rayleigh fractionation at the high formation temperature.