The effect of organic matter maturation and porosity evolution on methane storage potential in the Baltic Basin (Poland) shale-gas reservoir

The CH4 adsorption in the Baltic Basin shales is predominantly controlled by OM micropore structure. The mesopore surface area and volume do not play an important role in CH4 sorption. The proposed adsorbed CH4 density equivalent (maximal absolute CH4 adsorption divided by micropore volume), revealed that the CH4 loading potential decreases in micropores with increasing maturity. The highest CH4 loading potential is linked to OM before metagenesis (Ro < 2%) where the adsorbed CH4 density equivalent was found greater than the density of liquid CH4. This suggests that in addition to physical adsorption, absorption (dissolution) of CH4 in OM occurs. When OM content was reduced by the treatment with NaOCl solution, CH4 adsorption decreased significantly, suggesting that OM microstructure has much higher adsorption potential than that of clay microstructure.