Hidden Pennsylvanian Coals as Potential Source for Natural Gas in Central European Basin in Poland
Pensylvanian coal-bearing strata occur in Poland in foredeep, intramontane and peripheral, intracratonic basins. Coal maturity is relatively low (locally up to 2% (Rr) and related to a Pennsylvanian/Early Permian intrusive heating. All these basins host small accumulations of natural gas, which occurs as CBM or fills post-Pennsylvanian siliciclastic reservoirs. However, as also noted elsewhere across the northwest Europe, major economic gas accumulations occur in Lower Permian redbeds that are located in the Central European Basin beyond the limits of known coal basins. It is hypothesized here that these accumulations were charged from so far unrecognized, deeply buried foreland or thrust-top basin fills composed of Pennsylvanian coal-bearing strata. The pre-Permian basement of the Central European Basin in Poland is deeply buried (> 6 km) and penetrated by wells that were mostly terminated in thick, red-oxidized crusts forming the topmost part of Carboniferous succession.
An indirect evidence for gas-prone source rocks present at depths is the Central European Magnetotelluric Anomaly, which may record meta-anthracite coals or marine organic-rich shales. However, the preservation of pre-Pennsylvanian generated gases is likely to be low due to fast exhumation and deep erosion of the whole Variscan accretionary prism.
The composition of natural gases, which are dry and nitrogen-rich, points to their derivation from source rocks rich in terrestrial organic matter (Type III kerogen). Thus, Pennsylvanian coal-bearing strata appear the best possible candidate as source rock for gas and, moreover, subjected for a prolonged, post-Permian resdence in the gas window. Compositionally, the gases group into two distinctive domains; one comprising uniform, dry, isotopically heavy (δ13C -35 to 30‰ and δD -130 to -150‰) and nitrogen-rich (up to 66%) thermogenic gases (CH4/ N2 ratio 0.5), and the other grouping compositionally less homogenous gas with variable contribution of biogenic component. Most of the dry thermogenic gases fall into the evolution path of Type III kerogen-derived gases. In addition, such gases reveal isotopically heavy nitrogen characterized by δ15N values up to 5 ‰, suggesting a significant release of nitrogen (as NH3 and/or N2) from a pool enriched in the residual nitrogen.
The above results, though still premature, clearly indicate a need for a critical reassessing the Central European Basin in terms its hydrocarbon potential.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009