Organic petrological characterisation and facies interpretation of the Upper Triassic (Carnian – Julian) black limestone in the Lesno Brdo area (External Dinarides, Central Slovenia)

Abstract

Upper Triassic (Carnian - Julian) bedded dark limestone in the Lesno Brdo area, more precisely in the Drenov Grič quarry (Central Slovenia), was investigated in this study to describe its carbonate facies, its geochemical characteristics of the inorganic and organic matter, and to estimate its hydrocarbons generative potential. The site was chosen as a well referenced location of previous lithological and paleontological studies with the aim to fill a gap in the up-to-date knowledge on organic petrology of the investigated rocks termed as the Julian “Carbonate Member” within the Lesno Brdo stratigraphic sequence. The study represents the first multi-analytical organic petrological investigation of Mesozoic carbonate rocks in Slovenia. In the Drenov Grič (DRG) section, 7 microfacies were defined. A total of 38 DRG samples of micritic limestones, marlstones, and a coaly sample were collected from the section. The average total organic carbon (TOC) content of the samples is 0.70 wt% with one outstanding sample DRG-28 with a TOC content of 15.56 wt.%. According to the TOC content, DRG samples are considered as a fair to good source rock for hydrocarbons generation. In addition, the samples with TOC >0.5 wt.% were characterised by the Rock-Eval S1, S2 and S3 peaks, hydrogen and oxygen indices (HI, OI), Tmax and production index (PI) values. According to these data, typical “black” Julian carbonate beds from the Drenov Grič locality are poor source rocks. Based on pyrolysis data (HI versus OI), the type of the organic matter could not be determined, so a micropetrographic examination was performed. . The predominant and the only clearly visible maceral in the DRG samples is vitrinite within the amorphous organic matter. Given the low S2 peaks, thermal maturity was determined by vitrinite reflectance. Vitrinite reflectance results (between 2.71 ± 0.19 %Ro and 3.89 ± 0.17 %Ro) indicate that the organic matter in the DRG samples is in a thermally mature to overmature stage, which is also consistent with the Thermal Alteration Index (TAI) (4- - 4). All 38 samples were also analysed on main and trace elements, focusing on the redox sensitive trace elements (Ni, Mo, Th, U, V and Zn) to supplementary interpret depositional conditions. Overall, the “black” carbonate rocks of the Drenov Grič section show a negligible potential for the generation of hydrocarbons. Their original dispersed organic matter was thermally strongly transformed. Therefore, the investigated carbonate rocks almost completely exhausted their ability to generate a considerable quantity of hydrocarbons. They can therefore be classified as “spent source rocks”, which could conditionally generate some dry gas.