The effect of Cenozoic basin inversion on coal-bed methane in Liupanshui Coalfield, Southern China

Abstract

Coalbed methane (CBM) is an important energy source globally, thus understanding its origin and evolution is essential to the assessment of reserves and developing exploration strategies. This study aims to identify the origin of CBM in Liupanshui Coalfield (LPC) in southern China and evaluate the influence of basin exhumation on gas storage in different rank coals. Ten CBM samples were collected from four blocks that have different thermal histories and coal rank ranges. Based on the C1/(C2 + C3) ratio (16–971), δ13CCH4 (−41.6 to −34.9‰) and δDCH4 (−185 to −140‰), the methane is mainly thermogenic in origin. Systematic compositional differences between the four blocks are consistent with coal maturity. Noble gases in LPC CBM are a mixture of air-derived gas dissolved in local groundwaters and radiogenic gases that were generated in the coals and diffused in from deeper crust. The radiogenic 4He/40Ar⁎ and 21Ne⁎/40Ar⁎ are lowest in CBM from Qingshan block, strong evidence of diffusive loss of light noble gases. 20Ne/36Ar (0.23–0.70) in all gases are higher than local air-saturated water (0.16) and are most easily explained by re-dissolution of noble gases that exsolved from groundwater during basin inversion. 84Kr/36Ar and 132Xe/36Ar ratios are higher than predicted from this process and likely reflect desorption of Xe and Kr that has been trapped on the coal matrix at the time of coal formation. The noble gases are consistent with gas exsolution and loss from the reservoir during Mesozoic-Cenozoic basin inversion. Modelling the loss of both free and adsorbed gas during basin inversion we find that the Qingshan block has lost the highest proportion of free gas (77%), yet the total gas loss (free and adsorbed) is the lowest (25%) due to the strong adsorption capacity of the mature coals. The volume of groundwater that has interacted with the gas phase during the basin inversion is estimated according to the re-dissolution process model to evaluate the role of groundwater in gas preservation and gas extraction. Combining gas production data for the four blocks, high gas content and efficient gas extraction in LPC are expected for the high maturity coals that have seen low groundwater during basin inversion but more at shallow depth.