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

Abstract

Supercritical CH4 and subcritical CO2 and N2 gas adsorption measurements, combined with scanning electron microscopy (SEM) have been used to determine CH4 sorption capacity and pore characteristics for immature, mature and overmature shales from the Baltic Basin (Poland).Organic matter (OM) maturity exerts a dominant control on porosity evolution in micro- and mesoscale. In the Baltic Basin shales, the initial formation of micro- (<2nm) and mesopores (2–50nm) occurs in the oil window (beginning of catagenesis, vitrinite reflectance Ro~0.5-0.9%) due to primary cracking of kerogen that left OM highly porous. The expelled liquid hydrocarbons turned into solid bitumen that is responsible for pore blocking and significant decrease in micro- and mesopore volume in late mature shales (middle catagenesis Ro~0.9–1.2%). Micro- and mesopores were regenerated in advanced catagenesis (Ro~1.4–1.9%) due to secondary cracking of OM. The micropore volume in the Baltic Basin shales is mostly controlled by the OM content while the influence of clay content is minor and masked by OM.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.