--> Factors Limiting Reservoir Quality of Pericratonic Silurian Shales in Poland — New Evidence From Cores in Lublin Basin

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Factors Limiting Reservoir Quality of Pericratonic Silurian Shales in Poland — New Evidence From Cores in Lublin Basin

Abstract

With estimations of GIP at 5.2 Tcm, the Silurian succession along the western margin of East European Platform in Poland ranks among major emerging gas plays. However, production tests on a still small number of horizontal wells did not yield satisfactory results. Sedimentology and regional correlations, based on high-quality cores through ca 300-thick Rhuddanian-Ludfordian section in the Lublin Basin, constrain better geological factors, which contributed to a higher than expected reservoir heterogeneity. Silurian mudrocks form distal belt in the Caledonian foredeep and interfinger cratonwards with forebulge-shelf carbonates. The Rhuddanian-Aeronian “hot-shale” interval consists of black, organic-rich (TOC 5-8%), banded, argillaceous mudstones. They rest on Hirnantian glaciomarine muddy diamicts, or contact across ravinement surface with Ordovician platform carbonates, and wedge out by onlap both cratonwards and axially to the SE. In contrast to the largely undeformed nature of the succession, this interval abounds in low-angle faults, shear fractures, small overturned folds, and slickensides, suggestive for stratal decoupling along the basal unconformity, possibly during later (Variscan) compression. This, together with stratigraphic pinch-outs, introduces uncertainty as to the reservoir integrity and continuity and may also be responsible for the observed alongstrike departures from the basinward increase in maturity trend. The organic-rich mudstones become interbedded on a cm-scale with bioturbated mudstones in the overlying Telychian, which documents periodic sea-floor ventilation leading to drastic reduction in TOC content (< 0.5%). Both Rhuddanian and Telychian mudstone were deposited in a high-accommodation and very low-supply setting (sedimentation rate 4 mm/ka), which favoured slow-suspension settling and the development of clayey matrices reducing fractability. The overlying Sheinwoodian-Gorstian interval is dominated by dark grey, planar laminated, calcitic-dolomitic, locally siliceous, argillaceous mudstones, which abound in fecal pellets, organic matter (TOC 0.80-2.5%) and pyrite, contain calcisiltite starved ripples and laminae, and reveal good permeability (up to 243 nD). It reflects increased mud deposition (sedimentation rate 13-29 mm/ka) on anoxic to dysoxic outer shelf affected by distal storm‵ currents. This moderate-quality reservoir has basinwide extent and is perspective for a further production testing.