Variations of bitumen fraction, biomarker, stable carbon isotope and maceral compositions of dispersed organic matter in the Miocene strata (Carpathian Foredeep, Poland) during maturation simulated by hydrous pyrolysis

Abstract

The Carpathian Foredeep is one of the largest Miocene sedimentary basins in Central Europe. Biomarkers, polycyclic aromatic hydrocarbons (PAHs), stable carbon isotope and maceral compositions were determined to evaluate the depositional conditions, source of Miocene dispersed organic matter (DOM) in claystones as well as the effect of thermal maturity on these parameters based on hydrous pyrolysis (HP) experiments at 330 and 355 °C for 72 h. DOM is represented mostly by humic particles of Type III with an admixture of Type IV kerogens; therefore, they only have hydrocarbon gas potential. Alginite, which originates from marine or lacustrine organic material, was recorded in some samples. Vitrinite reflectance increases from 0.27 to 0.44% Rr for Original samples to 1.01–1.14% for samples After HP at 330 °C, and 1.38–1.53% after 355 °C. During HP experiments, the optical and chemical properties and maceral composition change with maturation, and the generated char and liptinite macerals mainly undergo vitrinitization and are transformed into liquid hydrocarbons. The distribution of biomarkers, selected PAHs and sulphur-containing compounds in recovered extractable organic matter (EOM) fraction indicates that (i) the DOM of the Miocene claystones was mainly deposited in an estuarine environment in the shallow Miocene sea, generally under anoxic conditions, (ii) they were accumulated in siliciclastics with marine/lacustrine shales and marls, and (iii) the contribution of terrigenous DOM was usually associated with deltaic environments during the deposition of the Miocene strata. Both geochemical and petrographic parameters change with thermal maturity. Sofer's (1984) genetic graph was constructed based on analyses of the isotopic composition of oils generated from DOM, however, our carbon isotope data reveal this graph is not always useful for genetic characterization of EOM. The observed shift could stem from isotopic fractionation during thermal maturation of EOM.