Methane Adsorption Capacities of the Lower Paleozoic Marine Shales in the Yangtze Platform, South China

The adsorption capacities of the Lower Silurian Longamxi and Lower Cambrian Niutitang marine shales in the Yangtze Platform in China were investigated through methane adsorption experiments. The correlations between the adsorption capacities and major factors, e.g., total organic carbon (TOC) contents, thermal maturity, mineral composition, moisture content, pressure, and temperature, were discussed. The isosteric adsorption heat was calculated according to the temperature dependency of the methane adsorption isotherms. The results show that, under the temperature of 30 °C and pressure range of 0–12 MPa, the maximum adsorption capacity of the Longmaxi shales ranges between 0.47 and 3.08 m3/ton of rock and that of the Niutitang shales ranges between 1.59 and 7.43 m3/ton of rock. The Langmuir adsorption capacity varies from 0.54 to 3.84 m3/ton of rock for the Longmaxi shales and from 1.98 to 9.73 m3/ton of rock for the Niutitang shales. The TOC content shows a significantly positive correlation with the adsorption capacity, indicating that organic matter is responsible for adsorbing gas in the shales. For these high mature shales, the thermal maturity shows no effect on the adsorption capacity. The clay minerals show little contributions to the adsorption capacity in the shales because of the effect of the water content. For the studied shales, the moisture exhibits no distinct correlation with the adsorption capacity. The influence of the pressure on the adsorption capacity varies from sample to sample, while the temperature shows a generally negative effect on the adsorption capacity. The isosteric heat of adsorption ranges from 8.48 to 27.35 kJ/mol, with an average of 17.59 kJ/mol, indicating a dominant physical adsorption behavior of the methane molecule in the shales.