Evidence of solid bitumen and hydrocarbons in low thermal maturity Menilite shales from the Polish part of the Outer Carpathians

This study presents the results of microscopic and geochemical analyses of the Menilite Formation from the Polish part of the Outer Carpathians. Twelve Oligocene shales from the Central Carpathian Synclinorium of the Silesian Nappe were examined to evaluate the petrological characterization of the solid bitumen and other organic matter (OM), identify the hydrocarbon evidence in the analyzed samples, and to understand the hydrocarbon generation in low thermal maturity rocks. The OM dispersed in the analyzed Menilite Formation is mainly represented by alginite and bituminite-rich, oil-prone kerogen. The reflectance measurements of vitrinite (VR_o %), solid bitumen (BR_o %), and the values of the maturity biomarker parameter indicator revealed that the analyzed samples contain immature to early mature organic matter in terms of hydrocarbon generation. Three types of solid bitumen (SB) were noticed: dark brown, almost black in reflected white light with a scratchy or granular surface bitumen (GR), as well as two types of homogenous SB. One reflected shades of brown-orange when observed under UV light, while the other was a non-fluorescent, lighter gray in reflected white light. Solid bitumen appears in samples where the reflectance of vitrinite is equal to at least 0.43 % VR_o and has the ability to reflect light from 0.26 to 0.32 % BR_o. A strong correlation was found between lighter homogenous solid bitumen reflectance with vitrinite reflectance, CPI 25–31, and dinosterane distribution. The lack of a broad interrelation between geochemical parameters and vitrinite reflectance values was linked with different kerogen mixtures within the analyzed strata. Higher content of bacterial Type II kerogen prompted the generation of early-mature hydrocarbons, while higher input of Type I and/or III kerogen leads to a decrease in the scale of the phenomenon of early-mature hydrocarbons. In Menilite shales, the hydrocarbons are generated in the forms of oil droplets and oil exudates.