Zagadnienie genezy i migracji gazu ziemnego w miocenie SE części Przedgórza Karpat
Abstract
ON THE ORIGIN AND MIGRATION OF NATURAL GAS IN THE MIOCENE OF SE FOREFIELD OF THE CARPATHIANS
Summary
Miocene deposits of SE Poland were studied using complex geochemical methods involving all available techniques of organic and isotopic geochemistry. The studies made it possible a more precise reconstruction of the gases, occurring in these deposits. A high content of isotopically-light methane in these gases (values δ13C ranging from -71‰ to 61,6‰) almost equivocally indicates that methane originated at low temperatures. There is no greater differentiation of the values δ13C in the vertical so it may be assumed that the conditions of origin of these gases were similar in all the gas-bearing horizons. Small spatial variability in isotope features of methane indicates that the zone of generation of methane was situated relatively close to the zone of its actual accumulation. It follows that this gas is syngenetic with Miocene deposits and that distances between zones of supply and accumulation are not-large. Trends of changes in isotopic characteristics of methane suggest an existence of a general direction of migration of this gas from south-east to north-west with some trend to migration from deep zone of the Miocene basin towards the boundary of overthrust of the Carpathian Flysch. Similar but somewhat less clear trends are marked in degree of isomerization of butane. Isotopic characteristics of nitrogen usually present in these gases in trace amounts display a clear differentiation depending on individual gas deposit. Differences in values δ15N between deposits approach 20‰. The data concerning isotopic characteristics of nitrogen are, however, too scarce for drawing any more detailed conclusions. The data on isotopic characteristics of carbon present in bitumens, although still scare, supplied some new data on geochemistry of sedimentary environment of the Miocene. A marked enrichment of bitumens from the Lower Tortonian in heavier carbon isotope 13C indicates that the organic matter present in these deposits is derived from another environment than the organic matter of the Sarmatian and Upper Tortonian. It may be assumed that this differentiation reflects marine origin of parent material of Lower Tortonian bitumens (sapropel) and terrestrial (lignite-humus) origin of organic matter of the Sartmatian and Upper Tortonian. This image based on the data concerning carbon isotopes is fully supported by the results of geochemical studies on dispersed organic matter: Detailed characteristics of n-alkanes seems to confirm that temperatures of generation of hydrocarbons were low despite of the fact that Miocene deposits were subsided to great depths (4000 m). This is indicated by a high value of maturity index (CPI) of n-alkanes. At the same time quantitative predominance of heavier n-alkanes (C27-C31) on lighter ones (C17-C21) and their distribution indicate the lignite-humus type of parent matter of bitumens present in younger Miocene strata. The studies have shown high values of degree of thermal involvement of kerogen (CR/CT equal about 0.7 which may indicate action of high temperatures of the organic matter. Taking into account the type or organic matter present in younger Miocene strata (humus-lignite matter of terrestrial origin they also confirm relatively low temperatures acting on organic matter of the Miocene in that region. Thus, also the CR/CT data also confirm the thesis on low-temperature origin of methane from the Miocene deposits.
Summary
Miocene deposits of SE Poland were studied using complex geochemical methods involving all available techniques of organic and isotopic geochemistry. The studies made it possible a more precise reconstruction of the gases, occurring in these deposits. A high content of isotopically-light methane in these gases (values δ13C ranging from -71‰ to 61,6‰) almost equivocally indicates that methane originated at low temperatures. There is no greater differentiation of the values δ13C in the vertical so it may be assumed that the conditions of origin of these gases were similar in all the gas-bearing horizons. Small spatial variability in isotope features of methane indicates that the zone of generation of methane was situated relatively close to the zone of its actual accumulation. It follows that this gas is syngenetic with Miocene deposits and that distances between zones of supply and accumulation are not-large. Trends of changes in isotopic characteristics of methane suggest an existence of a general direction of migration of this gas from south-east to north-west with some trend to migration from deep zone of the Miocene basin towards the boundary of overthrust of the Carpathian Flysch. Similar but somewhat less clear trends are marked in degree of isomerization of butane. Isotopic characteristics of nitrogen usually present in these gases in trace amounts display a clear differentiation depending on individual gas deposit. Differences in values δ15N between deposits approach 20‰. The data concerning isotopic characteristics of nitrogen are, however, too scarce for drawing any more detailed conclusions. The data on isotopic characteristics of carbon present in bitumens, although still scare, supplied some new data on geochemistry of sedimentary environment of the Miocene. A marked enrichment of bitumens from the Lower Tortonian in heavier carbon isotope 13C indicates that the organic matter present in these deposits is derived from another environment than the organic matter of the Sarmatian and Upper Tortonian. It may be assumed that this differentiation reflects marine origin of parent material of Lower Tortonian bitumens (sapropel) and terrestrial (lignite-humus) origin of organic matter of the Sartmatian and Upper Tortonian. This image based on the data concerning carbon isotopes is fully supported by the results of geochemical studies on dispersed organic matter: Detailed characteristics of n-alkanes seems to confirm that temperatures of generation of hydrocarbons were low despite of the fact that Miocene deposits were subsided to great depths (4000 m). This is indicated by a high value of maturity index (CPI) of n-alkanes. At the same time quantitative predominance of heavier n-alkanes (C27-C31) on lighter ones (C17-C21) and their distribution indicate the lignite-humus type of parent matter of bitumens present in younger Miocene strata. The studies have shown high values of degree of thermal involvement of kerogen (CR/CT equal about 0.7 which may indicate action of high temperatures of the organic matter. Taking into account the type or organic matter present in younger Miocene strata (humus-lignite matter of terrestrial origin they also confirm relatively low temperatures acting on organic matter of the Miocene in that region. Thus, also the CR/CT data also confirm the thesis on low-temperature origin of methane from the Miocene deposits.