Rozważania o ewolucji i migracji bituminów na Niżu Polskim

Abstract

ON EVOLUTION AND MIGRATION OF BITUMENS IN THE POLISH LOWLAND AREA

Summary

Geochemical examinations of bitumens, made in the years 1965-1970 in the Geological Institute, and analysis of geological development of the Polish Lowland area, particularly of the Kujawy region, and investigations of geological conditions of evolution and migration of bitumena allow as to assume that three generations of bitumens may be distinguished within the area of Poland: older bitumens that come from the Old-Palaeozoic complexes and from the older complexes, and younger bitumens for which Late Palaeozoic and Upper Jurassic formations may be thought to be mother rocks. Due to a differentiated character of the hydrocarbons of isoprene type, two generations A and B may be distinguished among the younger oils.
The older generation of bitumens, for which both Silurian and older complexes are mother rocks, matured successively up to the end of Permian. Till that time vertical migration took place along tectonic discontinuities, and lateral migration - through porous complexes. Intense impulse for migration of hydrocarbons was related to the intensified tectonic movements at the end of the Middle Triassic, particularly between the Keuper and Rhaetic. At that time disjunctive tectonics increased the migration paths through the Young Palaeozoic and Triassic complexes. The younger generation of biltumens, for which the Upper Palaeozoic complexes are mother rocks, matured successively up to the end of the Mesozoic. During certain time interval, the complexes of the salinary Zechstein played a part of an isolating screen in relation to the migrating Palaeozoic hydrocarbons. The migration of the hydrocarbons of the older and younger generations may have proceded vertically along the zones of tectonic discontinuities within the Permian and older complexes, and lateraly - through the porous complexes. The next strong impulse for hydrocarbon migration was related to the intense tectonic movements between the Cretaceous and Tertiary. Disjunctive tectonics rejuvenated the migration paths within the Palaeozoic and Triassic complexes, and activated within the Jurassic and Cretaceous complexes.
Salt dynamics, increasing with the time, and responsible for the expansion of salts and for their penetration into tectonic fissures, caused that the migration of hydrocarbons was little by little stopped through the dislocation system within the central area of the Zechstein-Mesozoic sedimentation. Due to disjunctive tectonics, a collateral contact of the Devnian and Carboniferous deposits with the Permian and Triassic deposits may have taken place too. If so, the hydrocarbons may have laterally migrated through the porous Permian and Triassic complexes and moved vertically and horizontally. Such conditions of migration may be expected to occur at the tectonically disturbed walls of salt plugs.
Beyond the central area of the most intense subsidence and sedimentation during the Zechstein-Mesozoic time, the kinematic role of salt masses that fill in the tectonic fractures may have been of lesser importance, thus the migration paths may have been easier.
Bitumens of the younger generation, for which the Upper Jurassic deposits may be mother rocks seem to be, according to the present-day knowledge, of lesser significance. In the tectonic and structural zones, related to deeply seated dislocations, bitumens of both younger and older generations may be expected to occur even in the deposits of the younger Mesozoic time.
The best perspectives in search for hydrocarbon deposits within the Polish Lowland area seem to be related to the zones of long-lived and deep-seated dislocations. Particular attention should be paid to the long-lived and deep-seated faults and reversed flexures, which at their hanging limbs may produce favourable conditions for both vertical and lateral migrations, and for accumulation of the younger and older gnerations of bitumens. Hydrocarbons may have accumulated in structural, lithological and stratigraphical traps here. The outer zones of the piercing salt structures create the best possibilities for the existence of reversed faults. The positive results in search for bitumens may also be related to dislocations that were not so distinctly fixed with the process of displacement of salt masses, but were responsible for a considerable differentiation in the rate of sedimentary process on both sides. Dislocations of this type are deep and in the Palaeozoic formations they are of considerable amplitudes which make possible the migration of bitumens of the older and younger generations into the Mesozoic formations.
Structural arrangement and frequently deep-seated bottom of the Zechstein formation force to apply deep geophysical surveys and deep drillings. Comprehensive stratigraphic, petrographic and sedimentary studies are necessary to reconstruct dynamic evolution systems in the palaeofacial and palaeolithological aspect, from the viewpoint of changes in the porosity of rock complex. Necessary are also hydrogeological studies to present the dynamic evolution systems with respect to chemical nature and dynamics of water. Geochemical and tectonic-structural research is indispensible also to reconstruct the dynamic evolution systems. The results of these studies should be synthesized into adequate evolution-palaeogeographical conceptions that taking into account an parameters variable in time, allow us to select the most promising areas.