Coalbed gases and hydrocarbon source rock potential of upper Carboniferous coal-bearing strata in upper Silesian Coal Basin, Poland

The Upper Silesian Coal Basin (USCB) is one of the major Upper Carboniferous coal basins in the world. Its coalbed gas reserves to the depths of 1,000 m are estimated to be about 350 billion cubic meters (about 12.4 TCF). Coalbed gases in the USCB are variable in both molecular and stable isotope composition [[delta][sup 13]C(CH[sub 4]), [delta]D(CH[sub 4]), [delta][sup 13]C(C[sub 2]H[sub 6]), [delta][sup 13]C(C[sub 3]H[sub 8]), [delta][sup 13]C(CO[sub 2])]. Such variability suggests the effects of both primary reactions operating during the generation of gases and secondary processes such as mixing and migration. Coalbed gases are mostly thermogenic methane in which depth-related isotopic fractionation has resulted from migration but not from mixing with the microbial one. The stable carbon isotope composition indicates that the carbon dioxide, ethane and higher gaseous hydrocarbons were generated during the bituminous coal stage of the coalification process. The main stage of coalbed gas generation occurred during the Variscan orogeny, and generation was completed after the Leonian and Asturian phases of this orogeny. The coals and carbonaceous shales have high gas generation potential but low potential for generation and expulsion of oil compared to the known Type III source rocks elsewhere. In general, the carbonaceous shalesmore » have slightly higher potential for oil generation, but probably would not be able to exceed expulsion thresholds necessary to expel economic quantities of oil.« less