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Unconventional Petroleum System of the Lower Paleozoic Baltic Basin — Insight From the Regional High-Effort Seismic Data and Integrated Geological-Geophysical Analysis

Abstract

The Baltic Basin of northern Poland forms part of the broader Lower Paleozoic basin that developed above the southwestern edge of the East European Craton. The prospective unconventional shale reservoirs identified within the Baltic Basin are connected with the Upper Cambrian (Furongian) Piasnica formation, Upper Ordovician (Caradoc) Sasino formation and Lower Silurian (Llandovery) Paslek formation. All of these targets are located within the wet-to-dry gas and liquid windows. The Baltic Basin also contains a conventional exploration target - Upper Cambrian sandstones that are connected to onshore and offshore oil production. The Baltic Basin unconventional potential was assessed utilizing high resolution regional seismic reflection lines (PolandSPAN project) calibrated by deep legacy research wells and new recently-drilled shale gas wells. The Cambrian - Ordovician succession was deposited above the Baltica passive margin. Some extensional control on the regional depositional pattern might be inferred as minor thickness changes can be observed across some of the basement faults. The Silurian succession was deposited within the foreland flexural basin that developed due to the continental collision and formation of the Caledonian thrust belt. Deposition within the Caledonian foreland basin was dominated by fine-grained organic rich shales deposited in the distal foredeep basin. Seismically-defined depositional architecture was used to for the quantitative reconstruction of particular stages of development of the Silurian foreland basin. Maximum burial depth of the Ordovician and Silurian source rock was estimated as minimum 5 – 6 km in the southwestern part of the basin and 3 – 4 km in the northeastern part of the basin. Regional seismic data has also imaged fault pattern that may have formed due to the flexural extension triggered by continental collision. Some of these faults have been reactivated as reverse faults in the Late Paleozoic, and then again as normal faults in the Late Triassic. High resolution seismic imaging at the reservoir level allowed a very detailed stratigraphic interpretation to be performed. Rock physics and mechanical models were built at the well locations and used to predict lithology, TOC and mechanical property changes away from known wells utilizing calibrated seismic inversion results. The basin model was then updated with the new information and high resolution maps were generated for subsequent prospect ranking.