oa Organic Petrological Characterisation of the Upper Triassic Overmature Black Limestone in the Lesno Brdo Area (Slovenia)

The study analysed Carnian black limestones composed of micritic and biomicritic limestone interbedded with clay stone and marl stone, deposited in a shallow coastal lagoon with terrestrial input. A total of 38 samples were collected from an 11 meters section and TOC analysis, Rock_eval pyrolysis and vitrinite reflectance were conducted to assess organic matter content, type, quality and thermal maturity. TOC content varied from 0.03 to 3.68 wt. %, averaging 0.70 wt. %, with one sample (DRG-28) reaching 15.59 wt. %. Rock-Eval results showed extremely low S1 and S2 values, indicating poor hydrocarbon generation potential. Vitrinite reflectance ranged from 2.71±0.19 %Ro to 3.89±0.17 %Ro, confirming that the organic matter is overmature. Dominant vitrinite macerates and dark, thermally altered amorphous organic matter suggest a primarily terrestrial origin. Despite the sufficient quantity and type of organic matter, the Carnian black limestones do not have the potential for hydrocarbon generation because they have reached an overmature stage of thermal maturity. therefore, the limestones represents poor source rocks.