Warunki formowania się złóż gazu ziemnego w Wielkopolsce

Abstract

Przedstawiono rozmieszczenie i charakterystykę: złóż gazu ziemnego w stropowej części osadów czerwonego spągowca w Wielkopolsce. Wskazano również na związek występowania złóż węglowodorów z wykształceniem facjalnym skal zbiornikowych oraz paleomorfologią stropu czerwonego spągowca. Na podstawie analizy tektonicznej zwrócono uwagę na przyczyny zróżnicowania występowania złóż gazu w zależności od kąta regionalnego nachylenia skal permskich.

CONDITIONS OF ORIGIN OF GAS DEPOSITS IN THE WIELKOPOLSKA REGION In the Permian paleogeographic image, the Wielkopolska region appears situated between the Wolsztyn Swell and central part of the Permian basin, almost completely filled with clay sediments (Fig. 1). Distribution of gas deposits hitherto found in (his area is irregular but they appear mainly concentrated in an area south-west of Poznan. In eastern Wielkopolska, there have been fund a few small deposits, mainly in the Poznan -Kalisz dislocation zone (Fig. 2). The character of these deposits is found to be related to their tectonic setting (Fig. 3A) and facies conditions in top parts of the Rotliegendes (Figs. 3B and C). The deposits are markedly varying in type but they arc gene-rally small in size, i.e. in area and effective thickness (Table 1). In the majority of cases collector is built of Rotliegendes sandstones with mean porosity of 15 - 20% and permeability varying from 20 to 200 mD. The recorded gas mainly contains hydrocarbons (82% at the average) and subordinate amounts of nitrogen (c. 13%), hydrogen, carbon dioxide and helium (c. 0.1%).

In analysing character of deposit traps it is necessary to take into account paleomorphology of top surface of the Rotliegendes, the present shape of which appears related to conditions of sedimentation of clastic Rotliegendes rocks and subsequent tectonic history. Coarse-clastic sediments from margins of the Wolsztyn Swell and Lw6wek Elevation (Figs. 2 and 4) may be characterized as sediments of alluvial fans. At far margins of the fans. conglomerates often interfinge with sandstones. Further northwards there occur pure sandstones only, and interfinging of the latter and sediments of clay facies is found not closer than north-western part of Wielkopolska. The above mentioned sandstones represent sediments of foreland of alluvial fans, corresponding to an environment of sedimentary plain of alluvial braided rivers developing under arid climate conditions. Wind was playing important role in modelling the plains because of formation of dunes and dune fields.

Analysis of distribution of the Copper-bearing shale shown marked differentiation in morphology of top surface of the Rotligendes at the beginning of the Zechstein. The shale is assumed to originate below waving base. The presence of dune fields was favorable for marked differentiati0I?-of sea bottom during Zechstein transgression and rapid development of the latter resulted in rather limited destruction of original sea floor morphology. Direct evidence in support for this hypothesis was obtained in the course of studies in the Lubin copper mine. where elevations from the top of the Weissliegendes were found to be partly destroyed dunes with mean inclination of slope equal 1.5°.

The present distribution of gas deposits was influenced by both lithology and morphology of top part of the Rotliegendes and also in significant way by activity of the Poznań-Oleśnica tectonic zone as mean regional dip of this top surface is two times greater in eastern Wielkopolska than in the western (Fig. 5). Deviation of structural forms of the eolian type from original level by angle greater than mean inclination of surface of elevation leads to opening of a trap and further migration of gas. Such phenomenon presumably took place in eastern Wielkopolska.

The above regularities in distribution of gas deposits appear closely related to paleogeographic and paleofacies development of the Rotliegendes in Wielkopolska and their knowledge should open new perspectives of gas discovery at the present stage of search.