New model of structure and evolution of the Lower Paleozoic Baltic Basin in N Poland based on regional seismic data

Abstract

The western edge of the East European Craton in Poland has recently been the focus of intense exploration efforts for unconventional hydrocarbons. Results of early exploration wells clearly demonstrate that there are still many unknowns regarding various aspects of the unconventional petroleum system, including structure and depositional architecture of the Lower Paleozoic succession. Seismic data from a recent high-effort regional deep reflection survey has allowed for a better understanding of the complex tectono-sedimentary history of the Baltic Basin.

The Lower Paleozoic basin in Poland is located above the southwestern edge of the East European Craton, northwest from the Teisseyre – Tornquist Zone which is separating the cratonic plate from the West European Platform. During the Precambrian/Cambrian, the cratonic edge underwent extension and rifting of the Rodinia supercontinent, while Cambrian – Ordovician subsidence was driven by a post-rift lithospheric thermal cooling. In the Silurian times, the cratonic edge was incorporated into the Caledonian foredeep basin. This has been very well documented on newly acquired regional seismic data that precisely imaged entire Mesozoic - Paleozoic sedimentary cover.

Within the Baltic Basin the Cambrian – Ordovician passive margin succession is plunging towards the southwest to the depth of 9 – 10 km. It contains one potential conventional reservoir: Upper Cambrian sandstones deposited in the shoreface-offshore setting.

Caradoc shales form one of two main unconventional targets in the Baltic Basin. They were deposited within the distal part of the basin during the final stages of development of the passive margin of the Tornquist Ocean, are characterized by low sedimentation rate and could be interpreted as highstand system tract deposits. As proven by seismic data, they are characterized by subtle lateral thickness variations and seismic amplitude changes, possibly reflecting also lateral TOC variations.

Cambro-Ordovician passive margin succession is unconformably covered by the Silurian foredeep sequence of up to 6–7 km (present-day i.e. after substantial Late Paleozoic erosion) total thickness. Silurian deposition was dominated by fine-grained organic rich shales, generally derived from the eroded orogenic wedge. Seismically defined large-scale geometry of the entire Silurian (Llandovery – Pridoli) succession reflects progressive progradation of the foredeep infill towards the east-southeast.

Llandovery basal shales form second unconventional target of the Baltic basin. They were deposited in a very distal part of the basin and are characterized by small thickness; only within the most southwestern part of the basin some thickening towards the – already non-existing - orogenic belt could be observed on seismic data. Wenlock, Ludlow and Pridoli are characterized by larger thickness related to enhanced subsidence triggered by increased flexure of the foreland plate beneath the Caledonian orogenic belt.

Regional fault pattern within the Lower Paleozoic succession identified on seismic data could be attributed to minor Cambrian – Ordovician faulting within the passive margin of the Tornquist Ocean, Silurian flexural extension of the lower plate associated to the evolution of the foredeep basin, Late Paleozoic reverse faulting related to uplift of the Mazury High and finally regional Late Triassic normal faulting within the northeastern flank of the Permo-Mesozoic Mid-Polish Trough that developed above the Teisseyre – Tornquist Zone.

Regional PSDM seismic data allowed for the quantitative reconstruction of the consecutive stages of development of the Baltic Basin, including the passive margin stage and the foredeep basin stage.

The geometry of Caradoc and Llandovery shale gas reservoirs mentioned above has been, despite their small thickness, successfully imaged using the recently-acquired regional seismic data. Their identification was based on a precise well-to-seismic tie derived from synthetic seismograms calculated using pseudo-sonic and pseudo-density logs created for key deep research wells located within the study area. Regional interpretation has provided insight into geometry, tectonics and subtle lateral thickness variations of Cambrian conventional reservoirs as well as Ordovician and Silurian unconventional reservoirs, and has enhanced the understanding of the play.

AAPG Datapages/Search and Discovery Article #90192 © 2014 European Regional Conference and Exhibition, Barcelona, Spain, May 13-15, 2014