Podstawy geologiczne metodyki badań utworów mezozoicznych na Niżu Polskim pod kątem poszukiwań bituminów

Ryszard Dadlez, Sylwester Marek



Exploration of bitumens in Mesozoic deposits from the Polish Lowlands was deeply influenced by a tendency to penetrate a few traditional reservoir horizons of the Triassic and Jurassic in top parts of local brachyanticlines, seismically recorded mainly in Jurassic structural surfaces. Numerous examples from well-known oil- and natural-gas-bearing platform basins in the world show that such a method is oversimplified and that distribution of oil and natural gas accumulations may depend on other geological conditions, deceptively appearing as less important or less promising. Increasing knowledge of Mesozoic deposits from the Polish Lowlands makes possible preliminary characterization of such geological settings. Paleogeographic analysis indicates importance of zones, in which sandstone or carbonate facies interfinge with clay facies, zones of transgressive overlying of beds in trensgressive sequences, as well as importance of areas of potential development of source rocks: analysis also shows the significance of the relation of isolating complexes to reservoir complexes. Examples of regionalization, based on such criteria, are given for reservoir horizons of the Middle Buntsandstein, Lower Bajocian, Middle Kuiavian, Lower and Middle Bathonian, Oxfordian and Middle Valanginian. Other, potential reservoir horizons, previously usually underestimated, are enlisted: Lower Buntsandstein, Upper Buntsandstein, uppermost Lias, Upper Kimmeridgian, Portlandian, and Lower Hauterivian. Presently, tectonic-structural analysis makes it possible to delineate areas of maximum subsidence within basins of particular systems. Therefore, variability of subsidences in space and time, which influenced both formation and migration of bitumens, may be traced by means of paleotectonic and paleostructural reconstructions. Moreover, such analysis shows that the majority of local structures and faults paid to buried structures, which could facilitated hydrocarbon accumulation for a long time, as well as to fault zones, dying out and passing into continuous deformations upwards. Complex geological analysis, taking into account the dynamic nature of paleogeographic-structural development of the basins, together with geophysical, geochemical and hydrogeological data, will make it possible to optimalize effors to find hydrocarbon accumulations in Mesozoic formations.

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