O pionowej inwersji hydro geochemicznej w basenach osadowych

Abstract

Dokonano analizy dotychczasowych poglądów na genezę pionowej inwersji hydrogeochemicznej, zarówno pod kątem spadku mineralizacji, jak i zmian składu chemicznego wód ze wzrostem głębokości. Wydzielono pełną i częściową inwersję hydrogeochemiczną oraz przedstawiono podstawowe czynniki sprzyjające jej powstawaniu. Głębokość występowania i wyrazistość tego zjawiska zwiększa się stopniowo od obszarów objętych orogenezą alpejską do rejonów starszych fałdowań. W wielu basenach sedymentcyjnych na świecie obserwuje się wyraźną zależność między inwersją. pionowej strefowości hydrogeochemicznej a obecnością złóż węglowodorów, co pozwala na uznanie zjawiska inwersji za regionalne, hydrogeologiczne kryterium poszukiwawcze. Omówiono charakter pionowej strefowości hydrogeochemicznej w kilku rejonach wydzielanych prowincji hydrogeologicznych w Polsce.ON VERTICAL HYDROGEOCHEMICAL INVERSION IN SEDIMENTARY BASINS Frequent departures from a classis hydrogeochemical profile have been recently revealed in result of increasing depth of penetration through a sedimentary mantle covering sedimentary basins. The phenomenon, presenting particular regularities of its own, has been designated as a vertical inversion of hydrogeochemical zonation. With respect 10 its nature the two types of the inversion have been distinguished: a complete one consisting in distinct decrease in both the water mineralization and the change in its chemical composition from CI - Ca type to HcO3 - Na one; another is of partial nature, marked out with decrease in water mineralization only. Occurrence of the complete inversion is mostly limited to young sedimentary basins, intermontane depressions and foredeeps within areas of Alpine orogenies, and areas of young platforms as well. Burried zones of old platforms, tectonically most active can be respectively pointed out as the type areas for occurrence of the partial inversions (Fig. 1). There are many hypotheses capable of explanation the origin of hypogenic alkaline waters responsible for formation of the complete hydrogeochemical inversion. Noteworthy are hypotheses: of percolation of water to deep parts of the basins, of base exchange of calcium in fresh waters leaching the calcareous rocks and sodium ion adsorbed in water-bearing rocks, of squeezing out a porous water from clay formation, of dehydration of montmorillonite during processes of epigenetic alterations, of distillation and condensation of water under deep subsurface conditions existing in oil-and gas-bearing fields. and of exhalation of vaporous and gaseous fluids composed of water, CH4, CO2, and H2. Carbon dioxide plays important part in process of generation of hypogenic alkaline waters. It is impossible to negate position of CO2 originating in result of decay of scattered organic substance; it is worth noting. however, that under high temperatures and high pressure the paramount importance is attributed to the hypogenic carbon dioxide. It can evolve from crystalline rocks of basement or originate in particular formations of sedimentary cover from thermo-metamorphic alterations of various minerals. The following factors affect the formation of vertical hydrogeochemicai inversion: -          tectonic regime within sedimentary basin, -          -geothermal regime within sedimentary basin, -          -geologic time. It seems worth stressing that there is a close relation of areas with zonal and local occurrences of hydrogeochemical inversions and hydrocarbon accumulations. The relation has been revealed in numerous oil and gas fie.lds in the Soviet Union. This makes it possible to conclude a common conditions governing the origin of both types of fluids; furthermore, a common vertical migration during periods tectonically most active seems to be most likely contributed to the relation mentioned above. Rate of diversification of hydrogeochemical environment can be taken in areas of the hydrogeochemical inversions as an indicator of oil and gas potential. The index is expressed by set of particular factors (Tab. 1) and a coefficient of contrast as well. The latter is determined by a ratio of content of any element or chemical compound in waters existing within oil and gas structure and the value of regional hydrogeochemical background. Nature of the vertical hydrogeochemical inversions existing in some selected ground-water provinces in Poland has been briefly discussed. A normal vertical hydrogeochemical zonation is in principle the main feature of the Mesozoic complex within the East-and the Central-European Platform province. In underlying Zechstein water-bearing horizons existing in most part of Poland's territory. an impressive increase in total salinity is observed (which governs an abnormal distribution of mineralization along their vertical hydrogeochemical profiles), while decrease in mineralization is strongly marked within the Rotliegendes and older Paleozoic formations. The phenomenon undoubtly results from leaching salt series or eventually from participation of post-crystallization residual lyes and inflow of these high-saline fluids to calcareous Zechstein aquifers. Water mineralization in the Flysch Carpathians (a folded Carpathian socle in the Alpine province) presents quite different distribution, without its depth consequence. It is most likely that the case comes from essential differences in geologic structures of the folded and the platform areas (of the Polish Lowland).