Lower Paleozoic Oil and Gas Shales in the Baltic-Podlasie-Lublin Basin (Poland)
Since most of US and Canadian gas/oil shale reservoirs are of the Devonian-Carboniferous age, new exploration shale targets of that age are of particular interest for oil industry. In a case of Poland exploration for the Devonian-Carboniferous shale and tight gas/oil focused so far mainly on the Lublin Basin (eastern Poland) and the Pomeranian basin (Northern Poland). However, both the basins miss proper quality, thick, organic rich shale. On contrary, some of the Upper Devonian-Carboniferous Variscan foreland basins in the Western and Southern Poland, which for different reasons remains poorly recognized with exploration wells, according to the current study could have significant potential for presence of shale and tight gas.
In the Upper Silesian Basin (USB) the Upper Carboniferous complex contains numerous thick coal seams, being excellent and mature source rock for dry gas. In the deep part of the basin coal source rock is buried as deep as 4500-5000 m at maximum. Sandstone interbedded with coal and mudstone loses porosity and permeability with increasing depth, and deeper than 2500 m in general remains tight. Therefore at least few hundred meters thick section might be saturated with dry gas due to permeability jail trapping mechanism. Thus a classical Basin Centered Gas System might be developed in the deeper part of the USB Upper Carboniferous (3500-4500 m). The concept remains untested since this part of the USB avoided exploration wells deeper than 1500-2500 m. The key exploration risk factor is an uncertain time of gas generation (Carboniferous versus Mesozoic) and a lack of porosity/permeability data for the reservoir buried to the proper depth (3500-4500 m).
Deeper in the stratigraphic section of the USB and further west (Moravo-Silesian Zone – MSZ), the Upper Devonian to lower-most Carboniferous is developed as an organic rich, thick, homogenous shale. Lateral distribution of the shale formation is based on regional facies model, and due to lack of deep wells remains partly unverified. However north the USB a few wells proofs presence of that shale in a shallower zone at the basin flank. Analysis of core samples documents high TOC contents (2-5 %) and II type of kerogen. Regional thermal maturity model indicates that the formation might be reservoir for dry shale gas. The formation might be regarded also as a conventional source rock.
Further NW of the USB and MSZ the Mississippian flysch (Kulm facies) is thrusted over lower foreland plate. This region, referred to as Fore-Sudetic Monocline (FSM), is a classical petroleum province with dry Nitrogen-rich gas accumulated in the Rotliegend reservoir. The Lower Carboniferous (Upper Visean/NamurianA) flysch of the FSM locally contains tight gas accumulations (NW of Wroclaw city). The source rock is few hundred meters thick shale (2.0-2.5 % TOC at average) interbedded with tight sandstone. Gas is dry (source rock maturity – 1.3-1.8 % Ro). The concept originally based on legacy wells was currently verified with one new vertical exploration well (no fracturing test yet). The mayor risk factor in this case remains tectonic deformation (fold and thrust belt) and uncertain lateral continuation of the paly. The same type of sediments continues however on a large area of western and southern Poland, including previously discussed the USB and the MSZ.
In the FSM region the early Mississippian pre-flysch organic- and kaolinite-rich shale of the lower plate might be regarded as a target for shale gas exploration. This formation was deposited on the basin slope of Laurussia passive margin, however due to competent rheology during deformation become one of the detachment levels and thus is present both beneath thrust complex, as well as is tectonically incorporated into it. Very high thickness (c.a. 1000m), good geochemical characteristics, dry-gas window thermal maturity and large lateral extension are advantages of this formation as a shale gas exploration target. However high burial depth and/or tectonic deformation might locally limit its potential. Due to here proposed interpretation this shale formation could be a source rock for conventional gas accumulated in the Rotliegend at the FSM, rather than previously anticipated for this role the flysch shale or the local small remains of Upper Carboniferous coal.
AAPG Datapages/Search and Discovery Article #90226 © 2015 European Regional Conference and Exhibition, Lisbon, Portugal, May 18-19, 2015