Distribution of methane contents and coal rank in the profiles of deep boreholes in the Upper Silesian Coal Basin, Poland

Abstract

The central, western and southwestern parts of the Polish Upper Silesian Coal Basin (USCB) are characterized by elevated methane contents and coal ranging in rank from high-volatile bituminous to anthracite. Two patterns of methane distribution with depth have been recognized. The northern pattern involves a naturally outgassed zone and a primary methane interval below 600–800 m. The southern pattern comprises a secondary peak immediately below Miocene cover rocks and a deeper primary peak. A variety of factors influencing present-day methane contents make the distribution of methane at depth complex even in individual boreholes. An analysis of the depth trend of average methane contents reveals that, in some cases, the primary methane peak is divided into two smaller sub-peaks, the first coinciding with medium-volatile bituminous coal and the second typically with the transition from low-volatile bituminous coal to anthracite. The sub-maxima are separated by an interval of gradually decreasing methane content, usually in the direction towards low-volatile bituminous coals. Rapid increases in methane content with depth are probably related to the occurrence of medium-volatile bituminous coals that are, in turn, related to the second coalification jump at which coal loses ca. 10% of its volatiles, producing, inter alia, methane which is adsorbed by the coal. Low-volatile bituminous coal and anthracite are characterized by high methane contents due to further methane generation at this stage of the coalification process. Anthracite has a high methane content because of its high sorption capacity and high coal rank. Though basin evolution, temperature, and pressure (stress) are the key factors influencing the process of methane generation, coal sorption capacity for gases and present-day depth distribution of methane, other processes of gas migration and accumulation contribute to distribution patterns that are both variable and complex.