The hydrogen isotopic characteristics of the Upper Paleozoic natural gas in Ordos Basin

摘要

Upper Paleozoic natural gas in the Ordos Basin is typical coal-associated gas. Natural gases in Sulige, Yulin and Zizhou gas fields were systematically analyzed in this study. Based on the hydrogen isotopes of CH_4, C_2H_6 and C_3H_8 components in natural gases, and combined with the natural gas compositions and carbon isotopes and fluid inclusions homogenization temperature measurement of reservoir rocks, we have discussed the source rock depositional environment, gas origin and maturity, and gas migration and accumulation. The results show that methane hydrogen isotope of natural gas not only can indicate the type of precursor organic matter and depositional redox condition, but also is closely associated with the maturity of natural gas. The difference between δD_(C2H6) and δD_(CH4) values is a good measure of the organic matter input of source rock. The value for marine sapropelic gas is normally < 12 and that for terrigenous humic gas > 10. The hydrogen isotopes of methane in Upper Palaeozoic natural gas was closely related to the vitrinite reflectance (%Ro) of the Upper Palaeozoic source rocks, displaying a short migration and accumulation feature. Sulige gas field is continually charged with coal-derived gas at different thermal maturation stages and the carbon and hydrogen isotopic reversals of alkanes result from the continuous charging and near-source migration and accumulation.