Distribution characteristics and geochemistry mechanisms of carbon isotope of coalbed methane in central-southern Qinshui basin, China

Coalbed methane (CBM) as an environmentally clean energy resource, its origin and geochemistry characteristics play critical roles for the exploration and development of CBM resource. In this paper, the carbon isotopic distribution laws and the geochemistry mechanisms of CBM were analyzed in the No. 3 coal seam of Permian Shanxi Formation and the No. 15 coal seam of Late Carboniferous Taiyuan Formation, central-southern part of Qinshui basin, China. The research results revealed that the delta C-13(1) values of the No. 3 coal seam distribute between -47.3 parts per thousand and -26.5 parts per thousand (average -36.4 parts per thousand), which are generally larger than those of the No. 15 coal seam that range from -48.6 parts per thousand to -27.5 parts per thousand(average -38.3 parts per thousand). As a whole, the delta C-13(1) values of CBM in the two coal seams show a gradually decreasing tendency from southeast to northwest. Then, due to the greater generation and the higher adsorption capacity of (CH4)-C-13 in the high rank coal, the delta C-13(1) values show a strongly positive correlation with the thermal evolution degree. The gas content and burial depth also have positive correlations with the delta C-13(1) values, but the hydrodynamic activity presents a negative relationship along with the delta C-13(1) values. Actually the burial depth, geological structure and hydrodynamic activity influence the distribution of CBM carbon isotopes by affecting the gas content. Moreover, there also exists an obvious fractionation phenomenon for CBM carbon isotopes under the post-reformation effects. On the one hand, owing to the preference for (CH4)-C-13 adsorption and postponement for (CH4)-C-13 desorption in the coal reservoir, the desorption-diffusion-migration effect that occurs during tectonic evolution contributes a weak vertical zone characteristic. On the other hand, the lateral groundwater circulation and the vertical thermal fluid circulation under an abnormally paleo geotemperature field have a good effect on (CH4)-C-13 dissolution. Finally it concluded that the geological period from the Late Jurassic to the Late Cretaceous is a crucial stage in causing this fractionation phenomenon and determining the thermal evolution feature and gas-bearing property, thus forming the current distribution of CBM reservoir and geochemistry characteristics in the central-southern Qinshui basin.