Geodynamiczne i geotermiczne przesłanki ropo-gazonośności południowego Bałtyku

Abstract

Na podstawie analizy zmienności paleostrukturalnego położenia spągowej powierzchni osadów kambru środkowego syneklizy bałtyckiej, kształtowanej subsydencją basenu sedymentacyjnego w ordowiku - dewonie, zanalizowano warunki paleogeotermiczne niezbędne dla uruchomienia potencjału generacyjnego skał macierzystych kambru. Dokonano porównania paleotemperatur wynikających ze współczynnika refleksyjności (Ro) i analizy paleogeodynamicznej, stwierdzając znaczne rozbieżności wyników dla strefy Słupska, Bornholmu i zachodniej części polskiego sektora Bałtyku. Określono czas migracji i akumulacji węglowodorów w złoża ropy naftowej i gazu ziemnego oraz destrukcyjny wpływ inwersji waryscyjskiej na akumulacje wschodniej części syneklizy.

Geodynamical and geothermic conditions of oil and gas bearing in the Southern Baltic

The paleogeodynamical analysis has been used to reconstruction processes of generation, migration and accumulation of hydrocarbons into oil and gas deposits in the Lower Palaeozoic structural complex in onshore and offshore parts of the Baltic Syneclise. The map of the present geothermic gradient distribution (Fig. 2) after S. Depowski and J. Majorowicz (1979) - has been used to reconstruction the estimated magnitude of geothermic palaeogradient in Lower Palaeozoic. The paleogradient higher by 5-l0°C (in comparison to the present) has been assumed in accordance to S. Majorowicz (1977, 1978a, b, 1982). Subsidence compensated by sedimentation in the Lower Palaeozoic basin when Ordovician – Devonian has been reconstructed with the palaeogeodynamic analysis. Alternation of a palaeostructural position of the Middle Cambrian bottom surface in the end of Ordovician (Fig. 4), Lower Silurian (Fig. 5), Silurian (Fig. 6), and Devonian (Fig. 7) has been presented. Duration of hydrocarbon generation in particular zones of the Baltic Syneclise has been determined on the basis of dependency of oil and gas generation on palaeotemperature (Fig. 1), according to the correlation diagram after N. Wassojewicz et al. (1969). Generation of oil from source rocks of the Lower and Middle Cambrian took place at the earliest at Wenlockian in the south-western, near-margin part of syneclise after the posits sank into the depth of 2000 m (Fig. 5). Considerable, subsidence compensated by the Upper Silurian sedimentation led to sinking the Cambrian deposits in the whole western and south-western parts of the Baltic Syneclise up to the Vistula line into the depth exceeded 2000 m, which allowed increasing the oil generation area. The same time the Cambrian deposits (mainly Lower and partly Middle Cambrian) begun to generate methane gases (Fig. 6). In the Klaipeda region where the geothermic palaeogradient was anomalous high, oil generation begun earlier, during sinking the Cambrian deposits into the depth of 1200-1300 m (Fig. 2).

At the end of subsidence of the Devonian basin, the majority of the Cambrian deposits exceeded the palaeothermic threshold t° 60°C almost in the whole area of the Southern Baltic (Fig. 7). In the near marginal part of the platform, the methane gas generation phase was ended (Fig. 8) after the Cambrian deposits had reached to exceeding 250°C. Comparing the palaeogeothermic data from the palaeostructural analysis with results from investigation of the reflection coefficient (Ro) from Bornholm, Słupsk and Gdańsk regions, divergences have been stated, indicated existence in the geological past the local overheating areas connected perhaps with deep frachures (Fig. 3, 8, 9). The hydrocarbon migration begun simultaneously with the generation process (Wenlockian); a main phase of migration took place at the end of the Caledonian reconstruction of the area in Siegenian and continued also during the Variscan inversion. This inversion, especially the Bretonian and subsequent movements shaping the Baltic Syneclise led to destruction of deposits located on its south-eastern slope. Moreover, the results show that in the eastern Polish part of the syneclise there were no palaeogeothermic conditions for formation of hydrocarbon deposits. Perspectives of this region does not exceed the Pasłęk meridian.