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Salt Tectonics and Mesozoic 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 also thick Zechstein (U. Permian) salts, and was completely inverted in Late Cretaceous-Paleogene. The presence of thick salts gave rise to the development of a complex system of salt structures. Salt pillows started to form in early Triassic. 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 have been strongly compressionally reactived during inversion of the MPT. 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 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. Middle Jurassic Dogger shales are 42m to 154m thick, with TOC's in the 1% - 3% range. Upper Jurassic Kimmeridgian shale is between 72m and 123m thick, with TOC's up to 4.5%. 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. Reprocessed legacy and newly acquired 2D 3C data 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. Apart from the unconventional exploration targets also several potential structural traps have been identified that could be associated with accumulation of conventional hydrocarbons.