--> Mesozoic salt tectonics of the Mid-Polish Trough and its control on potential unconventional hydrocarbon system

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Mesozoic salt tectonics of the Mid-Polish Trough and its control on potential unconventional hydrocarbon system


The Permian to Cretaceous Mid-Polish Trough was filled with several kilometers of Permian and Mesozoic sediments, including thick Zechstein salts, and was completely inverted in Late Cretaceous - Paleogene times. The presence of mobile evaporites gave rise to the development of a complex system of salt structures that includes salt pillows, small reactive diapirs, and taller passive and rejuvenated diapirs. These structures and their associated faults impacted Mesozoic depositional thicknesses and facies development and thus unconventional petroleum systems.

Salt movement started in the Early Triassic, triggered at least in part by regional basement faulting during crustal extension. The linkage between suprasalt deformation and presalt structures varied throughout the region. For example, the growth of the Klodawa diapir, located in the central, Kuiavian segment of the Mid-Polish Trough, was directly controlled by an underlying fault. In contrast, the Szamotuly diapir, located in the NW segment of the Trough, is located over the rift shoulder more than 30 km from the rift-bounding faults due to decoupling by the Zechstein salts. Throughout the area, extensional faulting and diapirism continued into the Jurassic, with salt locally breaking through to form passive diapirs. After continued growth in Jurassic to Early Cretaceous times, the diapirs, along with smaller salt structures, were compressionally reactived during regional inversion of the Mid-Polish Trough during the Alpine Orogeny. Again, the suprasalt deformation was in some places linked directly to deeper faulting, as at Klodawa diapir; in others such as Szamotuly diapir, shortening was decoupled.

The continuous growth of salt structures throughout the Mesozoic strongly controlled depositional systems, with the effects differing both spacially and temporally. In cases and times of extensional reactive diapirism (generally during the Triassic), strata thickened in the hanging walls of normal faults above salt structures. In contrast, once passive diapirs formed, usually beginning during the Late Triassic, they formed topographic high. Thus, thinner sedimentary cover characterized by generally shallower facies developed above the diapirs, and greater thickness and deeper facies were concentrated within the intervening synclines. Potential unconventional targets in such synclines include Middle Jurassic Dogger shales (TOC of 1% - 3%) and Upper Jurassic Kimmeridgian shales (TOC up to 4.5%).

Research on the Szamotuly diapir was completed as part of the MNiSW grant No. WND-POIG., work on the Klodawa diapir was partly funded by the NCBiR BlueGas - JuraShale grant.