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Salt tectonics within the central Mid-Polish Trough and its control on unconventional petroleum system

Abstract

Study area is located in central Poland, within the Kuiavian segment of the Mid-Polish Swell (MPS). MPS was formed as a result of regional uplift and inversion of the Mid-Polish Trough: Permo-Mesozoic basin that evolved above the Teisseyre – Tornquist Zone, one of the most fundamental crustal / lithospheric – scale boundaries in continental Europe. Following deposition of the Lower Permian (Rotliegend) siliciclastics and Upper Permian (Zechstein) evaporites, the Mid-Polish Trough was filled by several kilometers of the Triassic – Cretaceous sediments, mostly shallow water siliciclastics and carbonates, including also Upper Triassic evaporites. Mid-Polish Trough was completely inverted in Late Cretaceous – Paleogene.

Regional development of the Triassic – Cretaceous sedimentary cover was primarily governed by a regional subsidence related to deeper crustal processes such as basement faulting and thermal subsidence with secondary role – although in place very important – of salt tectonics.

Salt structures started to develop already in Triassic times. Salt movements have been at least partly triggered by regional basement faulting. In Late Triassic some of the salt pillows reached diapiric stage. After their further growth in Jurassic to Early Cretaceous times, salt structures were strongly compressionally reactivated during Late Cretaceous – Paleogene inversion of the Mid-Polish Trough. Continuous growth of salt structures strongly controlled Mesozoic depositional systems, with thinner sedimentary cover characterized by generally shallower facies developed above these structures, and larger thickness and deeper facies located within the synclines formed between salt structures. Most complex salt structures are known from the central MPT, Kuiavian segment of the Mid-Polish Trough, where the large Klodawa salt diapir is located together with salt pillows and other smaller diapirs. In this area, several targets for shale gas exploration have been identified: middle Jurassic mudstones, upper Jurassic mudstones and marls and Kimmeridgian fractured carbonate. Conventional reservoirs in Triassic through middle Jurassic sandstones offer attractive additional targets. Thermal modeling and Ro data indicate the shales are in the oil window in the syncline adjacent to the Klodawa diapir. Core data indicate excellent quality reservoirs within 500m thick Jurassic Dogger sand packages (10% – 20% porosity, up to 300mD permeability). Oil in open fractures has been frequently noted in the Tithonian carbonate (35m–116m thick) that lies directly above the organic rich Kimmeridgian shale. Seismic data calibrated by deep wells allowed to better constrain timing of growth of salt structures and, as consequence, to reconstruct evolution of the source rocks and better understand Mesozoic petroleum system.