Hydrocarbon potential of the Zechstein Main Dolomite in the western part of the Wielkopolska platform, SW Poland: New sedimentological and geochemical data
Introduction
The Southern Permian Basin (SPB) is a huge basin that extends in the entire area between Lithuania and Western Belarus in the east and the onshore part of the United Kingdom in the west (Fig. 1A; van Wees et al., 2000, Geluk, 2007). The study area comprises the Polish southwestern part of the SPB (Fig. 1A), and is located within the Fore-Sudetic Monocline that belongs to the least researched regions as regards facies and microfacies of the Zechstein Main Dolomite (Ca2) strata and their petroleum characteristics. The Zechstein as a whole is attributed to the Late Permian, although the lack of index fossils makes precise dating difficult (Słowakiewicz et al., 2009, Peryt et al., 2012). It is certain that the PZ1 cyclothem rocks reveal early (but not the earliest) Lopingian age (Fig. 2; Korte et al., 2005, Sørensen et al., 2007, Słowakiewicz et al., 2009, Peryt et al., 2010a).
At the same time, in this region a number of hydrocarbon shows and accumulations have been found, a considerable part of which are of great economic importance (Fig. 1B). The previous views on the development and sedimentary evolution of the Polish part of the Ca2 basin were presented with relation to the Gorzów Platform (Jaworowski and Mikołajewski, 2007, Słowakiewicz and Mikołajewski, 2009, Słowakiewicz and Mikołajewski, 2011) and Grotów Peninsula (Kotarba and Wagner, 2007, Słowakiewicz and Mikołajewski, 2009) (Fig. 1B). As a result of the occurrence of the greatest Polish oil and gas fields (Czekański et al., 2010), these areas were well researched considering facies and they became model areas for other Ca2 occurrence in Poland. In case of the Fore-Sudetic Monocline, microfacies studies were sporadically published based on few wells in the second half of the 20th century (Peryt, 1978). Since that time, information on geochemistry, microfacies and sedimentary facies, derived from wells drilled in the 1980s, 1990s as from more recent ones, has not been published and verified, and most of drill cores were cast off. When analysing unpublished materials (well documentation and microscope thin-sections), it can be clearly seen that the facies subdivision and paleogeographic maps were done based on very simplified criteria, i.e. subdivision of rocks into mud-supported facies that represent low-energy environments of basin plain or inner platform flat and grain-supported facies represents high-energy environments of outer oolithic barrier and a number of inner oolithic barriers of the platform. However, in some cases we observed that even this simplified criterion was not used either and the way of attributing the areas to particular paleogeographic zones is not clear.
The previous views on the development and paleogeography of the Ca2 strata in the Fore-Sudetic Monocline were presented on a paleogeographic map (Fig. 1B; Kotarba and Wagner, 2007, Słowakiewicz and Mikołajewski, 2011). As early as during preliminary analysis of this map a number of doubts arouse as regards the position and extent of particular paleogeographic zones, which resulted in update. However this updated map is unavailable. Therefore throughout this paper we refer to the most recent published map (Kotarba and Wagner, 2007, Słowakiewicz and Mikołajewski, 2011). Macroscopic examination of cores and microscopic observations confirmed a number of discrepancies between the observations and the existing view on the facies and paleogeography differentiation of the Ca2 basin in the Fore-Sudetic Monocline. The above presented doubts have been confirmed by sedimentological studies of the Zechstein Main Dolomite rocks. Also proposed changes in the paleogeographic setting have been presented.
Geochemical studies and modelling of hydrocarbon generation and expulsion processes considerably improved our knowledge of the value of the Zechstein Main Dolomite in terms of petroleum prospects, and in the corrected paleogeographic setting they enabled new characterization of the distinguished zones: basin plain, carbonate platform slope and carbonate platform with the oolitic barrier. Similarly, facies identification based on the new methodology allowed to verify views on the source potential of individual facies types of the Zechstein Main Dolomite. Particularly important changes in this characterization concern the Kargowa – Babimost Bay, previously located in the zone of the basin plain and now in the zone of the carbonate platform slope. In the latter, a few hydrocarbon accumulations occur, namely the Kargowa, Babimost and Wilcze deposits. In this zone, the Zechstein Main Dolomite rocks reveal significant source potential and are characterized by significant undiscovered petroleum potential. Also other zones of the analysed area of the Fore-Sudetic Monocline demonstrate essential changes in the characteristics of the Zechstein Main Dolomite source potential and possibilities of hydrocarbon generation.
Section snippets
Geological background
During the Late Permian the SPB extended over an area of approximately 600 000 km2 (van Wees et al., 2000, García-Veigas et al., 2011). The Polish part of the Zechstein Basin, with thickness of the strata exceeding 1500 m (Wagner, 1994, Peryt et al., 2010b), was situated in the eastern part of the SPB (Fig. 1A). Tectonic and paleogeographic factors caused a specific distribution of facies and thickness of the Polish Zechstein (Wagner et al., 1978, Wagner, 1994). Successive transgressions of the
Samples and methods
The study applied classical methods of sedimentological analysis to unpublished data from several tens of wells located in the northern part of the Fore-Sudetic Monocline. 372 core samples from 47 wells were collected from which thin-sections and polished slabs were prepared. Moreover, archival thin-sections belonging to the Polish Oil and Gas Company (POGC) and data from geological documentation of wells were utilized. In total, 917 thin-sections (from the AGH-UST and POGC collections) were
Paleogeography and facies architecture versus new facies and microfacies data
For the Zechstein Main Dolomite basin in Poland, the following paleogeographic and facies zones are distinguished by the previous authors: basin (deep and shallow basin including bays), ramp, platform slope, toe-of-slope apron, and carbonate platform (outer and inner barriers, restricted lagoons, tidal flat, channel, and supratidal sabkha; e.g. Peryt, 1986, Wagner, 1994, Jaworowski and Mikołajewski, 2007, Czekański et al., 2010, Słowakiewicz and Mikołajewski, 2011). Sedimentation of the
Quantity of organic matter
Data from the Rock-Eval pyrolysis were analysed to acquire information on quantity and quality of organic matter. The study of its geochemical characteristics was performed on core samples taking into account the paleogeographical position of the wells. General paleogeography of the Zechstein Main Dolomite Basin was adopted after Kotarba and Wagner (2007), despite the fact that the achieved results of microfacies analysis showed some departure from it.
The following geochemical characterization
Conclusions
Our studies revealed a number of discrepancies between the so far accepted paleogeographic and facies model accepted so far and the conclusions presented in this work. The previous model, which assumed the existence of a shallow-water rimmed platform bounding narrow basinal zones does not fully confirm with the results obtained most recently as regards paleogeography and facies. This implies the need of verification of the previous views. Evaluation of deposition of the Zechstein Main Dolomite
Acknowledgements
Financial support of this work arrived from the statutory fund of the AGH University of Science and Technology No. 11.11.140.175, and grant of the Polish Ministry of Science and Higher Education Grant No. N N307 460638.Dr. A. Kowalski and Mr. H. Zych helped with the analytical work, Special thanks go to Prof. Dr O. Weidlich and anonymous Reviewer for constructive comments and critical suggestions, and Prof. M. Wendorff for language review of the manuscript.
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