Controls on Gas Content and Carbon Isotopic Abundance of Methane in Qinnan-East Coal Bed Methane Block, Qinshui Basin, China

摘要

Qinnan-East Block is one newly planned coal bed methane (CBM) exploration area in the Qinshui Basin, China, and research on the accumulation of gas in this block is quite limited. In this study, on the basis of a series of laboratory experiments and the latest exploration data, the gas content and carbon isotopic ratio of methane [delta C-13(CH4)] in the No. 3 coal seam of Permian Shanxi Formation in Quinan-East.Block were investigated, and then the correlations between delta C-13(CH4) and vitrinite reflectance, burial depth, gas content, reservoir pressure, and gas adsorption saturation were analyzed; then, the mechanism controlling the gas bearing properties and the distribution of delta C-13(CH4) in the study area were addressed. It turns out that the gas content in the study area ranges from 2.87 to 24.63 m(3)/t with an average of 13.78 m(3)/t, the CBM reservoir pressure ranges from 0.86 to 5.96 MPa with an average of 3.09 MPa, the reservoir pressure gradient ranges from 0.11 to 1.06 MPa/100 m with an average of 0.49 MPa/100 m, and the gas saturation ranges from 12.34 to 117.76% with an average of 67.58%; the gas content, CBM reservoir pressure, and gas saturation increase with an increase in burial depth.delta C-13(CH4) of naturally desorbed methane from the No. 3 coal seam varies from -28.89 to -53.27%o with an average of -36.48%o and increases with an increase in vitrinite reflectance and burial depth. The carbon isotopic abundance of CBM is mainly controlled by the thermodynamic equilibrium fractionation in the formation of CBM and the kinetic disequilibrium fractionation in the transport of CBM delta C-13(CH4) rises in a nearly logarithmic form with the increase in CBM content, reservoir pressure, and gas saturation. The geological controls on gas bearing properties of the coal seam are vitrinite reflectance and effective burial depth, which are similar to those of the carbon isotopic abundance of methane. The outcomes may benefit the exploration of CBM in the study area.