Shale Gas Potential of the Ordovician-Silurian Complex in the Narol Zone (SE Poland)

Abstract

Abstract

The Narol Zone is a tectono-facies unit of which the Lower Paleozoic section is closely associated with basins at the western slope of the East European Craton (EEC). The unit was transported to the present position for a limited distance along the margin of the EEC in a strike-slip mode. The Ordovician-Silurian section represent more proximal (with relation to collision zone) facies of the Caledonian foredeep basin than the facies of the EEC basins. Due to presence of organic rich shale the Ordovician and Silurian section of the Narol Zone was studied for its shale gas potential. The study included RockEval, thermal maturity, mineralogical composition and porosity analysis. The core material was available for 3 historical wells and 1 new exploration well. Together with previous studies the data from 9 boreholes where included in the current research.

Based on dense coverage of laboratory TOC contents measurements and wire log interpretation 3 intervals with elevated organic matter contents were identified: the Upper Llandovery to Lower Wenlock (net thickness – 30-53 m, aver. TOC – 2.0-3.5 %, max. TOC – 8.1 %), the Lower Llandovery (net thickness – 10-15 m, aver. TOC – 2.0-2.5 %, max. TOC – 11.5 %), and the Tremadocian (net thickness – 17-41 m, aver. TOC – 1.9-2.4 %, max. TOC – 3.7 %). All the shale formation contain II type of kerogen. Their thermal maturity is in a range of 1.6-2.3 % VRo, equivalent of dry gas window. This is confirmed by gas composition (85-98 % Methane). Recent burial depth of the organic rich shale increases from SW to NE, from 1830 m to 3250 m, respectively. The shale has low to moderate silica contents (10-53 %), as well as carbonate contents (3-80 %) – their brittleness factor varies significantly from 0.4 to 0.8. The shale total porosity is in a range of 1.1-6.7 %.

Gas saturation is relatively low, equal to 1.0-1.4 m³/tone at maximum, which is not coherent with high TOC contents of the samples for which it was determined. This is most probably related to Variscan uplift and erosion of a few thousand meter scale. This is confirmed by the results of performed modeling of burial/thermal and HC generation history, which indicate that gas generation predates the uplift. The other exploration risk factor is Variscan tectonic deformation, locally moderate to intensive.

The current research was financed by the projects: AGH Univ. Sci. & Techn. statutory fund (11.11.140.322) and BlueGas (BG1/LUPZAS/13).

AAPG Datapages/Search and Discovery Article #90260 © 2016 AAPG/SEG International Conference & Exhibition, Cancun, Mexico, September 6-9, 2016