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Control of Salt Tectonics on Mesozoic Unconventional Petroleum System of the Central Mid-Polish Trough

Abstract

The Permian to Cretaceous Mid-Polish Trough was filled with several kilometers of Permian and Mesozoic sediments, including thick Zechstein (approx. Upper Permian) salts, and was completely inverted in Late Cretaceous - Paleogene times. The presence of thick salts gave rise to the development of a complex system of salt structures. Salt pillows and reactive diapirs started to form in early Triassic, triggered at least in part by regional basement faulting. In the Late Triassic some of the salt pillows reached diapiric stage. After their further growth in Jurassic to Early Cretaceous times, salt structures were compressionally reactived during regional 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 salt structures, and larger thickness and deeper facies located within the intervening synclines. The most complex salt structures are known from the central, Kuiavian segment of the Mid-Polish Trough, where the large Klodawa salt diapir is located along with by several salt pillows and other smaller diapirs. In this area, various unconventional exploration targets have been identified. Middle Jurassic Dogger shales are 42m to 154m thick, with TOC's in the 1% – 3% range. The Upper Jurassic Kimmeridgian shale is between 72m and 123m thick, with TOC's up to 4.5%. Thermal modeling and Ro data indicate that shales are in the oil window in the syncline adjacent to the Klodawa diapir. Oil in open fractures has been frequently noted in the Tithonian carbonates (35m–116m thick) that lie directly above the organic-rich Kimmeridgian shale. Reprocessed legacy and newly acquired high-resolution 2D data seismic data, calibrated by deep wells, allows better constraints on the timing of salt-structure growth and, as a consequence, the evolution of the source rock. Cross-section construction and restoration suggest that basement faulting beneath the Klodawa diapir exerted significant control on the evolution of the Mesozoic petroleum system; thin-skinned supra-salt syn- and post-depositional faulting also played important though more local roles. The results of seismic inversion, seismic attribute analysis and seismic stratigraphic modelling provide information on lateral facies and thickness variations and on the inferred TOC changes within the prospective target zones.