Budowa geologiczna rejonu Żarnowca

Abstract

GEOLOGICAL STRUCTURE IN THE REGION OF ŻARNOWIECSummaryThe structural element of Żarnowiec, situated in the eastern part of the Łeba elevation (North Poland), was in the last years (1968–1973) an object of intense geophysic-geological studies.These studies have corroborated a possibility of oil and gas occurrences in the Cambrian formations, particularly in the Middle Cambrian ones. These latter, developed in a sandstone-siltstone-clay facies, representing – according to K. Lendzion (1970) – the horizons Paradoxides oelandicus and P. paradoxissimus, have been subdivided into four lithostratigraphic members (Fig. 1): lower clay-siltstone member (176 m), sandstone member (38–63 m), sandstone-siltstone member (12–41 m), and upper c1ay-siltstone member (18–216 m). Marked indications and pulsatory efflux of oil have been noted in the sandstone and sandstone-siltstone members.Oil-bearing horizons are built up of quartzite sandstones, 0,7–12 m in thickness, their porosity amounting to about 10,5%, permeability being equal to 16 millidarcies.They are saturated either with crude oil or with salt brine. It results from the investigations made using a sampler that higher reservoir properties are related to fissurity phenomena which are a basis of better porosity and permeability, as compared with those obtained during laboratory examinations. The oil-bearing strata distinguish themselves by a markedly changing thickness, by the number of beds, and by their distribution in the section, what proves the occurrence of lithological traps here. The common quartzite sandstones are here an epigenetical formation produced due to the recrystallization of siliceous cementing material (W. Rydzewska, 1971).The area of the structural zone of Żarnowiec is characterized by a block-like structure developed, on the regenerated dislocations, in the Early Palaeozoic and Precambrian (Figs. 2 and 6). The principal period of shaping the main elements of the deep geological structure was that after the Ordovician. This was caused by epeirogenic movements that in the area of the platform were a reflection of the phases of both the Caledonian and the Variscan orogenic cycles. No pre-Arenigian structures, known from the eastern area of the Peribaltic syneclise, have so far been discovered.In the reconstruction of the spatial system of the Old-Palaeozoic structural stage the results of reflection seismics measurements (Figs 4 and 5), made to investigate the key Intra-Ordovician horizon, are of the utmost importance. The not too great amplitude of the structures is frequently contained within the boundaries of one phase only. At places, there is observed a convergence of the results of refraction and reflection seismic research works (Fig. 3) carried out to investigate the course of parallel and diagonal dislocation zones in the crystalline basement and in the Old-Palaeozoic sedimentary cover.So far, the disputable problem concerning the age of origin of the two main dislocation systems of meridional and parallel directions – has not been explained univocally. In this case, two antagonistic interpretation ways are possible: 1 – the older system is that of a parallel direction (S. Lisiakiewicz, 1970); 2 – the older system is that of a meridional direction (the present author's opinion).