Łupki sylurskie platformy wschodnioeuropejskiej w Polsce : wybrane problemy poszukiwawcze

The pericratonic Silurian shale succession in Poland, despite its reasonably well-constrained geological framework, entails a number of contentious issues that need to be resolved before this emerging shale gas play will enter a stage of successful development. The succession is thought to have originated in a Caledonian foredeep encroaching distally onto a pericratonic shelf ramp. However, the geochemical signature of the mudrocks is consistent with a cratonic rather than orogenic sourcing, the proximal part of the foredeep basin-fill is apparently missing, and the shale succession juxtaposes in part across the Teisseyre-Tornquist Zone against suspected ter- ranes with no evidence of Silurian tectonism. Organic-rich Llandovery-Wenlock shales form a NW-SE striking central belt that is increasingly calcareous toward the craton (NE) and more silty toward the inferred orogen (SW), with the TOC content decreasing in both directions perpendicular to the strike. The TOC trend seems consistent with the deep-downlap model of black shale deposition suggested for many Paleozoic foredeep basins, but does not quite agree with the outer neritic to upper bathyal depths assumed for the shale deposition. Preliminary results from three wells drilled by Orlen Upstream in the Lublin Basin indicate that the Llandovery-Wenlock shales were deposited on a distal shelf ramp sheltered from the craton by shelf carbonate shoals and periodically affected by weak storm-generated currents. The impact of storms on water column resulted in intermittent rises of oxygen content in the otherwise anoxic to dysoxic near-bottom conditions. The prospective interval is dominated by calcite-cemented clayey mudstones showing moderate to good reservoir qualities. It is cut locally by ENE- or NE-dipping, steep fractures favourable for fluid transmissibility, and a NE or SW direction is most advantageous for artificial fracturing. This interval is capped with a Ludlow calcite-cemented, laminated siltstone that forms a regional correlation marker and shows soft-sediment deformations attributable to gravitational collapse on a NE-dippingpaleoslope. If correct, this interpretation might imply the encroachment of orogen-fed clinothem system onto the SW-inclined craton-margin shelf ramp.