Regionalne badania Instytutu Geologicznego na Niżu Polskim (1969-1978)

Andrzej Witkowski

Abstract


Przedstawiono postęp w rozpoznaniu wgłębnej budowy geologicznej Niżu Polskiego za pomocą badań regionalnych prowadzonych przez Instytut Geologiczny w latach 1989-1918 dla określenia perspektyw poszukiwania złóż węglowodorów. Na podstawie nowych danych z ponad 100 otworów badawczych, z których 9 przekroczyło głębokość 5000 m, podano charakterystykę zasadniczych kompleksów strukturaInych na platformie prekambryjskiej i paleozoicznej. Na platformie prekambryjskiej określono głębokość występowania podłoża krystalicznego w syneklizie bałtyckiej zachodniej i uściślono jego występowanie w zapadlisku podlaskim, badając głównie utwory staropaleozoicznego kompleksu strukturalnego. W niecce warszawskiej i obszarze lubelskim badano głównie utwory dewonu i karbonu, uzyskując nowe dane o rozwoju miąższości, facji i zasięgach poszczególnych pięter.

W obrębie platformy paleozoicznej badano utwory podłoża permu na monoklinie przedsudeckiej, w niecce pomorskiej i na wale pomorskim oraz osady permu i mezozoiku, a zwłaszcza czerwonego spągowca jako najbardziej perspektywicznego dla poszukiwania złóż węglowodorów. Uzyskane dane umożliwiły uściślenie dalszych prac poszukiwawczych. Badania cechsztyńsko-mezozoicznego kompleksu skalnego stanowiły zagadnienia drugoplanowe. Przedstawiono kierunki dalszych prac regionalnych, wynikające z dotychczasowego stanu rozpoznania i pierwszoplanowych zadań związanych z uściśleniem perspektyw poszukiwania ropy naftowej i gazu ziemnego.

REGIONAL STUDIES CARRIED OUT IN THE POLISH LOWLAND BY THE GEOLOGICAL INSTITUTE IN 1969-1978

Regional studies carried out by the Geological Institute in the last decade were mainly aimed at establishing prerspectives of occurrence of hydrocarbon accumulations. By the end of the sixties, there began elaboration of regional geological-structural monographs illustrated with maps in the scales 1: 200 000 and 1: 500 000.

The Polish Lowland stretches on the boundary of the East-European Precambrian and the Central and West-European Paleozoic platforms which meet in the Teisseyre-Tornquist zone along the line Koszalin-Toruń-Ruda Lubycka (Fig. 1). Several structural-tectonic units were differentiated here. The subdivision proposed by J. Sokołowski (1968), with reference to that of W. Pożaryski (1956), is used in the current research of the Geological Institute (Fig. 1).

In the area of the Precambrian Platform, the top of crystalline basement is situated at depths ranging from some hundred meters in the east to 6000 m in the west. Sedimentary cover comprises three structural complexes: Early Paleozoic, Devonian-Carboniferous (Variscan) and Zechstein-Mesozoic (Figs. 2, 3). In the Paleozoic Platform, strongly folded Early Paleozoic structural complex is known from the Pomeranian Basin and south-western Lublin region. Variscan structural complex comprises folded Culm in the Fore-Sudetic Monocline and platform Devonian-Carboniferous deposits in the Pomerania and Warsaw Basin. It is overlain by the Rotliegendes over 1000 m thick and, in some places (western Poland), with lava covers. Zechstein and Mesozoic rocks form a single structural stage sometimes over 7000 m thick. The phenomena of salt tectonics, bearing a decisive influence on the development of younger sedimentary cover, are strongly developed in central part of the Platform.

Table 1 shows increase in intensity of drillings and seismic surveys in the Polish Lowland in the years 1969-1978, and Figures 4 and 8 - comparison of the degree of knowledge at the example of the Lublin region. In that time interval there were made over a hundred of deep drillings: 9 over 5000 m deep and the remaining ones over 4000 m deep (Figs. 1, Tab. 2). The main research problems included:

1. The studies of Early Paleozoic structural complex in the western Baltic Syneclise, Podlasie Depression and Lublin region.

2. The studies of Variscan structural complex in the Lublin region, southern part of the Warsaw Basin, Fore-Sudetic Monocline and western Pomerania.

3. The studies of the Permian, mainly Rotliegendes and carbonate Zechstein horizons.

4. The studies of the Mesozoic, especially Lower Triassic, Middle Jurassic, Oxfordian and Lower Cretaceous in zones of regional variability.

Sixty one drillings (146 000 m long) were made in the area of the P re c a m b r i a n  P I a t f o r m. The studies comprised western part of the Baltic Syneclise (Fig. 5, Tab. 3). The sedimentary complex begins there with clastic Vendian Lower Cambrian rooks (Żarnowiec series), passing upwards into Lower and Middle Cambrian sandstones and siltstones some hundred meters thick (Starszy paleozoik zachodniej części syneklizy perybałtyckiej, 1976). The Upper Cambrian is very thin. The Ordovician is represented by marly limestones and claystones, and the Silurian - by siltstones and clays tones up to 3000 m thick. Carboniferous and Devonian rocks are missing in result of Pre-Zechstein erosion. The Zechstein-Mesozoic complex was primarily shaped by Laramie movements and it ranges from 600 to, 2900 m in thickness.

Three drillings made in the P o d l a s i e  D e p r e s s I o n (Fig. 6, Tab. 4) were aimed at reconstruction of facies of the Cambrian and checking the possibilities of occurrence of gas with reference to traces of gas recorded in the Tłuszcz IG 1 borehole. The Cambrian appeared to be highy varying in facies. Ore-bearing ferruginous quartzite series, an equivalent of the Krivoi Rog series, was recorded in the crystalline basement for the first time in Poland.

The studies carried out in the W a r s a w B a s i n, especially its south-eastern part, were aimed at exploration of the Permian substratum (Fig. 7). The Westphalian was found to rest directly on the Upper Silurian. In the Mszczonów IG 2, it begins with conglomeratic series which may represent Variscan foredeep which is still not evidenced in Poland. The Carboniferous section comprises sandstone horizons with porosity up to 16.5% and permeability up to 690 mdcy, which gave gas-bearing brines. The Lower Permian is usually lacking whereas the Zechstein profile is almost complete (Tab. 5). The area was affected by epeirogenic movements of the Bretonian and Asturian phases and its structure was formed after the Westphalian D according to A. M. Żelichowski.

The sedimentary caver of the L u b I i n  r e g ion was studied by 43 drillings (Fig. 8, Table 6). The tectonic regionalization of this area includes differentiation of the Łuków-Hrubieszów elevation, Lublin trough, and Rardom-Kraśnik elevation (A. M. Żelichowski, 1972). Vendian rocks were found in southern part of this area. The Lower Paleozoic is developed similarly as in the Baltic Syneclise arid Pordlasie Depression. Traces of hydrocarbons were found in the Cambrian. Gas occurring in the brines shows anomalous (up to 4.57%) content of helium (Parczaw IG 10 borehole). The Devonian was subdivided into 7 lithostratigraphic formations (L. Miłaczewski, 1975). Its profile displays gaps and erosional surfaces reflecting the activity of Early Variscan movements (Fig. 9). Traces of hydrocarbons are fairly common here. Givetian and Frasnian rooks characterized by the best reservoir properties are considered as the most perspective. The Carboniferous was subdivided into 5 lithostratigraphic formations, (A. M. Żelichowski, 1979). Namurian and  Westphalian rooks with porosity up to 25%, and permeability equal 6750 mdcy are regarded as the most perspective for search for gas. Outflow of gas-bearing brines was recorded in several places. The main geological structure of this area originated after the Westphallian.

In the area of , the P a l e o z o i c  P I a t f o r m, were made 40 boreholes (123 000 m long - Tab. 2). Twenty one of these boreholes, made in the For e-S u d e t i c  M o n o c l i n e recorded, among other things, folded Carboniferous rocks developed in the Culm fades as well as those of bipartite Lower Permian (Autunian, comprising mainly volcanogenic rocks and conglomerates, and Saxonian - sandstones and siltstones). The extent and thickness and facies variability of the Zechstein (Fig. 11) and Rotliegendes (Tab. 7) were reconstructed. The rocks most perspective for search for hydrocarbons include the Saxonian (with porosity up to 22% and permeability up to 900 mdcy) and the Main Dolomite of the Zechstein, in which oil accumulation was found (Buk IG 1 borehole). The Variscan Platform has been consolidated at the end of the Sudetic phase or in the Erzgebirge and Asturian phases. The Zechstein-Mesozoic complex was shaped by the Cimmerian and Laramie movements.

Eleven boreholes made in the P o m e r a n i a n  S w e l l and B a s i n casted some light on regional features of block structure of Variscan structural complex (Figs. 12, 13) and evidenced the presence of Devonian and, in some areas, Carboniferous rocks. Frasnian and Givetian rocks are most perspective for search for hydrocarbons and some traces of oil have beena1ready found. Permo-Mesozoic rocks are best known in the Pomeranian Basin (Perm i mezozoik niecki pomorsikiej, 1976). The most perspective are here the zone of wedging out of the Rotliegendes and Main Dolomite (where deposit pressure is very high) and some parts of the Oxfordian.

The greatest thickness of the Rotliegendes was recorded in the Szubin IG 1 borehole (Fig. 2) where also anomalous deposit pressure of brine, with 1.99 gradient, was found. The present structural pattern of the Permian substratum originated in result of Late Variscan movements (R. Dadlez, 1978) and that of the Zechstedn-Mesozoic complex - mainly due to the Laramie ones.

Eight boreholes made in the Kuiavian Swell and the Mogilno-Łódź Basin (Fig. 14) were primarly aimed at exploration of Mesozoic and, partly, Zechstein :rocks. The regional thickness and fades distribution was reconstructed. The main structural pattern (Budowa geologiczna wschodniej części niecki mogileńsko-łódzkiej - strefa Gapło-Ponetów-Pabianice, 1977) was determined mainly by salt tectonic phenomena. The Jurassic and Lower Cretaceous comprise sandy series with good reservoir properties and large output of brines and brackish water. Subordinate occurrences of gas were found in the Upper Triassic and Oxfordian.

The Zechstein rocks are underlain by conglomeratic series and folded Carboniferous rocks developed in the Culm facies (Fig. 5), the same as those of the Variscides from the Fore-Sudetic Monocline. The studies of the Polish Lowland, planned for the near future, are aimed at exploration of:

a - Lower Permian and its substratum in central and eastern parts of the basin;

b - Variscan strulctural complex in some parts of the Lublin region and western Pomerania;

c - contact zone of the Paleozoic and Precambrian platforms;

d - Mesozoic in variability zones related to salt tectonics.


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