In this study, optical microscopic analyses were applied to evaluate the thermal maturity, characteristics of solid bitumen, and other organic matter finely dispersed in Oligocene shales of the Menilite Formation in the Iwonicz-Zdrój–Rudawka Rymanowska Fold (IRF) and Bóbrka–Rogi Fold (BRF) of the Central Carpathian Synclinorium of the Silesian Nappe, Outer Carpathians, Poland. The investigation was carried out at two-unit depths of the shallow and deeper D-1 sections (430 m – IRF and 4,300 m – BRF) and outcrop samples (BRF). The mean random huminite reflectance values indicate immature conditions with respect to hydrocarbon generation in samples from the D-1 shallow section (VRo≈0.40%) and in the outcrop samples (VRo=0.36%). The degree of thermal maturity of the organic matter from a depth of about 4,300 m – BRF based on random vitrinite (VRo≈0.80%) and solid bitumen (BRo≈0.65%) reflectance measurements is associated with the “oil window” for petroleum generation. The organic components dispersed in the examined Menilite Formation samples are typical for hydrocarbon-prone organic matter, suggesting the dominant kerogen type II. The potential precursor maceral for solid bitumen occurring in the examined samples from the deeper D-1 sections is largely the alginite maceral.

Humic type III kerogen occurs in all three Miocene analysed clayey complexes (Lower Badenian, Upper Badenian and Lower Sarmatian) of mainly deltaic sediments in the Carpathian Foredeep. Organic matter was supplied by rivers from surrounding land or islands to the shallow Miocene sea. Organic matter on the north from the Carpathian Overthrust is immature. It generated microbial hydrocarbons, mainly methane. Hydrous pyrolysis (HP) simulation and stable carbon isotope composition in CH4, C2H6, and stable hydrogen isotope composition in CH4 of HP gases and vitrinite reflectance analyses of organic matter after HP revealed that type III kerogen in Miocene strata beneath the Outer Carpathian nappes was under the influence of thermogenic processes (“oil window”). Highest thermogenic gas yields revealed kerogen from Lower Badenian strata of western sector of the Carpathian Foredeep.