The Interplay of Sedimentation, Eustasy, and Tectonics in Controlling Vertical TOC Variations in Ordovician to Silurian Shales, Eastern Poland

Lis, Pawel ¹; Kaufman, Jon ²; Bohacs, Kevin ³; Hardy, Martine J.²
(1)Polish Geological Institute, Warsaw, Poland. (2) ExxonMobil International Limited, Leatherhead, United Kingdom. (3) ExxonMobil Upstream Research Company, Houston, TX.

The application of sedimentologic analysis to unconventional shale gas/oil plays has been shown to improve prediction of source rock distribution and matrix properties, and highgrade potential “sweetspots” for acreage capture and management. To test the utility of this analysis in an unproven shale gas/oil play in eastern Poland, a joint technical study between the Polish Geological Institute and ExxonMobil was conducted in the Podlasie and Lublin basins. The main target of the Poland shale gas/oil play is organic-rich, Ordovician to Silurian shales deposited along the margin of Baltica. Tectonically, this period spans the transition from a passive margin (Ordovician to Llandoverian) to a foreland basin influenced by an advancing deformation front (Caledonian Orogeny, Wenlockian to Ludlovian). This study shows how integration of core observations with regional tectonics and eustasy has improved prediction of source rock quality and distribution.

Five cored wells were described at the cm-scale for key sedimentologic and diagenetic features. Twelve lithofacies were recognized in the Caradocian to Ludlovian succession; these lithofacies were further organized into three lithofacies associations based on similarity of rock properties. Important sedimetological and organic geochemical changes are recognized in vertical profiles through the Ordovician to Silurian shale succession. The Caradocian to Llandoverian sediments consist of dark-colored, laminated to bioturbated claystone/mudstone with highly variable TOC (up to 17% present-day TOC). Elevated TOC is found mainly in laminated claystone of the basal Llandovery, which overlies an unconformity developed at the close of the Ordovician. The Wenlockian to lower Ludlovian sediments consist of mm-scale, parallel to wavy laminated, mudstone to silty mudstone with generally low TOC; bioturbation is rare to absent.

The stratigraphic change in lithofacies and TOC abundance, in part, reflects the effects of the Caledonian Orogeny. As sedimentation rate increased due to the advancing deformation front, dilution of organic carbon resulted, and well-bedded sediments with generally poor source quality were deposited. The richest source rock (basal Llandoverian) formed during initial transgression of a karsted, top Ordovician surface, following a eustatic sea-level fall. This transgression may have triggered localized algal blooms, contributing to the high TOC and excellent source rock quality seen in this interval.

AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.