Geological controls on gas saturation in the Yanchuannan Coalbed Methane Field, Southeastern Ordos Basin, China

Abstract

The paper investigates lateral variation of gas saturation and its geological controls of the No.2 coal seam in the Permian Shanxi Formation based on coal samples from 21 exploration wells in the Yanchuannan Coalbed Methane (CBM) Field, Southeastern Ordos Basin, China. The data set reveals that gas saturation of the No.2 coal seam shows a high lateral variation from 32.44% to 89.69%, and that there is an overall trend in gas saturation with depth overprinted by four other factors: 1) influence of faults, 2) secondary biogenic gas generation, 3) local top seal conditions and 4) variations in coal properties. The reverse faults F1 and F2 are effective barriers horizontally for the hydraulic communication and divide our study area into relatively closed and recharge zones. Gas saturation of the relatively closed zone is generally higher than that in the recharge zone. In the recharge, gas saturation increases generally along the flow pathway of groundwater with the exceptions of the areas affected by secondary biogenic gas. The stable isotope compositions suggest that late-stage biogenic methane was generated via CO2 reduction associated with meteoric water recharge, which significantly contributes to higher gas saturation than expected. Meanwhile, the biogenic gas generation was/is restricted to shallow depths. The apparent variations in local areas of lithology of the overlying stratum next to the coal seam suggest that the top seal conditions are deteriorative, which can cause gas dissipation and reduce gas saturation remarkably. Integrally, for the variations in coal properties, gas saturation has a general positive relationship with coalification, and weak negative correlations with moisture content and ash yield but the relationships lacks statistical significances. Gas saturation is not associated with coal maceral in the study area.