--> --> Genesis of Coalbed Methane in Hancheng, Southeastern of Ordos Basin, China, as Revealed from Chemical and Isotopic Compositions of Gas

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Genesis of Coalbed Methane in Hancheng, Southeastern of Ordos Basin, China, as Revealed from Chemical and Isotopic Compositions of Gas

Abstract

Coalbed methane and coal cores from exploratory and productive wells in Hancheng coalbed methane field in southeast of Ordors basin were sampled to ascertain the gas composition characteristics and origin of coalbed methane. This investigation emphasizes on 3# coal seam in Permian Shanxi Formation, 5# and 11# coal seams in Carboniferous Taiyuan Formation that are commercially targeted for coalbed methane production. From the results of sample measuring, methane is identified as the major component for well production gas which accounting for >97.5% of bulk coalbed methane with lesser amount of C2H6, CO2 and N2. Methane content of desorption gas from coals comparatively has a larger range of 20.210% to 95.00%. High N2 content varied from 2.630% to 75.180% is defined to be a predominated characteristic for desorption gas. The stable carbon isotope values of methane δ13C are characterized by having a trend to be depleted in 13C with a range from −42.978‰ to −32.200‰. Four possible reasons are illustrated to explain the methane δ13C isotopically lighter including biogenic gas generation (CO2 reduction), isotopic fractionation during gas desorption-diffusion process, carbon isotope exchange in the CO2-CH4 system and hydraulic solution. Ethane δ13C are isotopically heavier, ranges from −21.619‰ and −9.751‰ in value. Isotopic composition indicates that there is a thermogenic source for coalbed methane. Finally, considering geological events and sedimentary evolution process, it demonstrates that, in origin, coalbed methane is sourced from thermogenic gas which has been experienced secondary alteration with isotope fractionation.