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POKORSKI, JEDRZEJ, and RYSZARD WAGNER, Polish Geological Institut, Warsaw, Poland

ABSTRACT: Geology of the Oil and Gas Bearing Permian Formation in the Polish Lowlands

Permian rocks occur over more then 80% of the Polish territory but are exposed only in the southern part of Poland, the Swietokrzyskie (Holy Cross), and the Sudeten Mountains. In middle Poland they occur at considerable depth, from 2 to 6 km. The Polish part of the Permian basin constituted the eastern margin of the European basin.

The Early Permian (lower Rotliegendes, classified also as Autunian) was a period of long-lasting intensive volcanic activity.

The Late Permian (upper Rotliegendes, sometimes classified as Saxonian) and Zechstein began with desert deposition which was followed by evaporitic deposition of a shallow epicontinental sea.

The middle Polish trough (MPT) constituted the central part of the Late Permian basin and was the site of the earliest and longest deposition, as well as the most intensive subsidence periodically not compensated by sedimentation.

In areas adjacent to the Precambrian platform (northeast of

MPT) and the Variscian platform (southwest of MPT), the Late Permian thickness is several times smaller, with long depositional gaps being observed in the sequence. In the Zechstein, these gaps are related to shifting of the shoreline (marine transgression and regression phases).

Subsidence rate and syndepositional faulting substantially controlled the paleogeographic pattern of the Permian basin. The final structure of the oil and gas fields was caused by late diagenesis and Upper Cretaceous structural remodeling.

The upper Rotliegendes have the biggest natural gas fields (about 50% of the production in Poland). Reservoir rocks are sandstones (fluvial, eolian, alluvial cones) and sandstones interfingering with conglomerates in tectonically active zones. Source rocks most probably are related to the Upper Carboniferous. The most promising areas for hydrocarbon exploration are the marginal parts of the basin (UPL) and the contact zone between MPT and the adjacent platforms.

In the central part of the basin, despite the predominating fine-clastic sediments (playa), the most promising are sandstone complexes on elevated tectonic blocks.

Zechstein hydrocarbon fields occur in carbonate horizons of the first three cycles (PZ1, PZ2, and PZ3). In some areas, the Zechstein limestones (Cal), developed in shallow facies together with Rotliegendes sandstones, constitute the natural gas reservoir.

Main dolomite (Ca2), oil, gas, and condensate fields are connected with the carbonate platform or its slope. Source rocks for oil occur in the Ca2 basinal facies or in the deeper parts of the platform-type lagoons. Oil migration is short and lateral, from either the basin or lagoon toward the carbonate platform. Gas in Ca2 derived from the sub-Zechstein basement and migrated vertically along fault zones.

The most prospective areas are reservoir horizons of the carbonate platform occurring in the neighborhood of source rocks.

The platy dolomite Ca3 is not very promising. Single, small oil and gas fields are caused by vertical migration from deeper Permian horizons.

AAPG Search and Discovery Article #90990©1993 AAPG International Conference and Exhibition, The Hague, Netherlands, October 17-20, 1993.