Distribution of methane carbon isotope and its significance on CBM accumulation of No.2 coal seam in Yanchuannan CBM block, Ordos Basin, China

Abstract

Distribution and origins of methane isotope (δ13C(CH4)) in coalbed methane (CBM) block of southeast-Ordos Basin are related to the accumulation and distribution of CBM in Yanchuannan CBM block, which is of great significance for the evaluation of CBM resources. In this paper, the stable isotope ratios of coalbed gases (δ13C(CH4), δD(CH4) and δ13C(CO2)) of the No.2 coal seam associated with water samples collected from Permian Shanxi Formation in southeast of Ordos Basin were investigated. The data set reveals that stable isotope ratios vary within the following ranges: δ13C(CH4) from −54.1‰ to −29.6‰ with an average of −37.73‰, δD(CH4) from −230‰ to −158‰ with an average of −188.39‰, and δ13C(CO2) from −20.6‰ to +2.2‰ with an average of −14.47‰. There are four dominant factors that decides the distribution of methane carbon isotope, including burial depth, Ro,max, hydrodynamic conditions and biogenic gases. The values of δ13C(CH4) naturally desorbed from No.2 coal seam increase with an increase with burial depth, Ro,max and reservoir pressure, which indicates that δ13C(CH4) of almost the whole studied area is controlled by the thermodynamic equilibrium fractionation, except the Xibaigou gentle slope belt (A4). The stable isotope compositions in Xibaigou gentle slope belt (A4) characterized by shallow and up-dip areas suggest that late-stage biogenic methane was generated via CO2-reduction associated with meteoric water recharge, which significantly contributes to δ13C(CH4) stripping and higher gas saturation than expected. Furthermore, it is summarized a variation model of gas-bearing properties and δ13C(CH4) values in Yanchuannan CBM block, which shows that the geological controls on gas-bearing properties are consistent with that on stable carbon isotope of methane, and normally lighter δ13C(CH4) ratios corresponds to lower gas-bearing properties domain. Significantly, this result can be used to evaluate and instruct the enrichment law of CBM and benefit its exploration and development in research area.