Depositional conditions, wildfires, maturity, and hydrocarbon potential evaluation of Central Carpathian Paleogene Basin based on integrative approach from Orava Basin

Abstract

Central Carpathian Paleogene Basin (CCPB, Central Western Carpathians) comprises mainly Oligocene clastic autochthonous age-equivalents of the widely known Menilite shale formation from the Outer Carpathians. However, little is known about the paleoenvironment and its subsequent changes during the basin's evolution. Furthermore, the available hydrocarbon potential data are based on anachronous methods and are not investigated on the sub-basin level. Gas chromatography-mass spectrometry (GC–MS) analyses supported by Rock-Eval data along with petrographic measurements enabled us to identify and document the paleoenvironmental evolution of the Orava sub-basin (NW remnant of CCPB). Thermal maturity based on vitrinite reflectance, 22S/(22S + 22R) homohopane ratio and 20S/(20S + 20R) sterane ratio increases from N to S and from Upper to Lower Oligocene. In the least mature samples ββ-hopanes, hopenes, and oleanenes are present, whereas in the most mature deposits less thermally stable compounds dissapeared. This maturation trend is shown also by the Rock-Eval data. Terrestrial organic matter input is documented by the predominance of III- and II/III-type of kerogen and the occurrence of several biomarkers, such as 3,3,7-trimethyl-1,2,3,4-tetrahydrochrysene, cadalene, retene, and perylene. The significant contribution of polycyclic aromatic hydrocarbons (PAHs) may be linked with wildfire-related land degradation and following runoff to the basin. Based on the measured fusinite reflectance values the wildfire types could range from hotter crown fires to colder surface fires. Depositional conditions in Lower Oligocene units are characterized by intermittent euxinia, as derived from small (<5 μm) pyrite framboid diameters and the presence of isorenieratane. Subsequently, a change of conditions to oxic/dysoxic in younger units is observed, and the input of terrestrial organic matter increased.