Paleogeotermika centralnego i południowo-wschodniego Niżu Polskiego i jej wpływ na generowanie i zachowanie węglowodorów

Abstract

Paleogradienty geotermiczne, określone na podstawie refleksyjności witrynitu oraz krzywych subsydencji i erozji osadów macierzystych dla węglowodorów, są ogólnie wyższe od gradientów temperatury obserwowanych współcześnie. Szczególnie drastycznie jest to widoczne na Lubelszczyźnie oraz w strefie brzeżnej monokliny przedsudeckiej. Wysokie paleogradienty geotermiczne, spowodowane głównie zjawiskami termicznymi orogenezy waryscyjskiej (przegrzanie waryscyjskie i powaryscyjskie), mogły mieć istotne znaczenie dla generowania węglowodorów.

THE PALEOGEOTHERMICS OF CENTRAL AND SOUTH-EASTERN POLISH LOWLANDS AND ITS INFLUENCE ON GENERATION AND PRESERVATION OF HYDROCARBONS

Paleogeothermic conditions are reconstructed in the central and south-eastem Polish Lowlands and compared with the present geothermic field. The values of geothermic paleogradients estimated on the basis of data on vitrinite reflectivity and curves of subsidence and erosion of rocks (Figs. 2 -4), including parent rocks for hydrocarbons, were found to be generally higher than recently measured temperature gradients.

The analysis of paleotectonic development of rock complexes and magmatic events, carried out taking into account geological setting of the area of Poland, gave further support to essential influence of the type of organic matter, temperature, and effective time of heating on generation of liquid hydro-carbons in parent rock series. Generation of hydrocarbons proceeds at temperatures ranging from 60 to 150°C, with a peak at temperatures from 120 to 130°C. This is expressed by the value R = 0.5 - 1.3% for the former temperature range and R = 1% for the latter. In the Polish Lowlands, the time of effective heating is estimated at 50-100 m. y.

Pressure (and, therefore. indirectly large depths) is of secondary importance for generation of hydrocarbons. However, it should be remembered that too high pressure results in delay in hydrocarbon generation.

At temperatures over 150^oC. liquid hydrocarbons are transformed into light oils, condensates and wet gases, and at temperatures over 180°C they pass into methane. In the former case the index R exceeds 1.3% and in the latter it increases to 2% or even more.

Convection processes phase changes in the upper mantle and lower part of the crust, and decay of radioactive elements are the sources a heat which activate hydrocarbon generation processes through the action of igneous. metamorphic and tectonic ones. The heat also spreads in the crust in the form of overheating fronts. In the platform areas, effective heat of overheating fronts is manifested by "shallow" volcanic events and so-called “cool” plutonism. The heat was mainly migrating along deep crustal fracture zones and by normal rock conductivity. In some cases, remnant heat from previous geological epochs could be of special importance for hydrocarbon generation.

In the studied area, Variscan overheating resulted in generation of hydrocarbons in Paleozoic rock complexes and, as a remnant heat, in increase of geothermic gradient in the Mesozoic time and, therefore, also in activation of that process.

The analysis of paleotemperature gradients, the present thermic gradient Grad T and the degree of metamorphism of organic matter in Paleozoic and Mesozoic rocks in the studied area shows that:

a - the bulk of Paleozoic rocks of south-eastem Lublin region (Lublin Trough and Łukow-Hrubieszów Uplift) and marginal part of the Fore-Sudetic Monocline and its direct foreland passed the stage of transformation of liquid hydrocarbons into the gaseous in their geological evolution so the hydrocarbon potential is there mainly connected with Permian rocks;

b - at least a part of Paleozoic and Lower Mesozoic rocks of the Plock Basin and north-western Lublin region are nowadays at the stage of generation of mainly liquid hydrocarbons;

c - Paleozoic rocks of the Kujawy - Gielniów Swell and adjoining zones passed through the stage of destruction of liquid hydrocarbons in their geological evolution whereas Mesozoic, especially Jurassic and Lower Cretaceous rocks entered the stage of generation of liquid hydrocarbons.