Effect of the properties of Silurian shales from the Barrandian Basin on their methane sorption potential

- High-pressure methane isotherms were measured on Czech Silurian shale samples.
- Maximum measured excess sorption and Langmuir sorption capacity were used.
- The methane excess sorption shows a positive correlation with TOC and clay contents.
- The highest sorption capacity was in shales with the lowest volume of micropores.
- No correlation was observed between maturity and methane sorption capacity.

High-pressure methane sorption isotherms were measured on seven representative samples of Silurian shales from the Barrandian Basin, Czech Republic. Excess sorption measurements were performed at a temperature of 45 °C and at pressures up to 15 MPa on dry samples, using a manometric method. Experimental methane high-pressure isotherms were fitted to a modified Langmuir equation. The maximum measured excess sorption parameter (nmax) and the Langmuir sorption capacity parameter (nL) were used to study the effect of TOC content, organic maturity, inorganic components and pore size distribution on the methane sorption capacity. The values of nmax ranged from 0.050 to 0.088 mmol.g−1, and the values of nL ranged from 0.068 to 0.133 mmol.g−1. The studied shale samples with random reflectance Rr of graptolite 0.56-1.76% had a very low TOC content from 0.34 to 2.37 wt% and dominant mineral fractions. Illite was the prevailing clay mineral (0-51%). Organic matter of the Silurian black shales was consisted of residues of graptolites, chitinozoans, two types of bitumen including dispersed and massive bitumens, recycled organic matter, and organic detritus. In the shales, the occurrence of fractures parallel with the original sedimentary bending was highly significant. A greater proportion of fragments of carbonaceous particles of graptolites and bitumens in the Barrandian shales had a smooth surface without pores. The sample porosity Porcalc ranged from 4.6 to 18.8%. In most samples, the micropore volumes were markedly lower than the meso- and macropore volumes. No relation has been proven between TOC-normalized excess sorption capacities or the TOC-normalized Langmuir sorption capacities and thermal maturation of the shales. The methane sorption capacities of shale samples show a positive correlation with TOC and a positive correlation with the clay content. The assumption that the sorption capacity is a function mainly of the microporous system of shales was not confirmed. The highest sorption capacity was observed in shale samples with the lowest volume of micropores and the highest TOC content, indicating that the organic matter content and the microporosity of clay minerals are the principal factors affecting the sorption capacity of shale samples.