Organic geochemical study of the upper layer of Aleksinac oil shale in the Dubrava block, Serbia

A detailed evaluation of geochemical properties of oil shale samples,from the outcrops of the Lower Miocene upper layer in the Dubrava area, Aleksinac basin, Serbia, was performed. For that purpose X-ray diffraction (XRD) analysis, Rock Eval pyrolysis, gas chromatography-mass spectrometry (GC-MS) analysis of biomarkers and conventional pyrolysis in an autoclave were used. Most of the samples have similar mineral compositions with predominance of clay and feldspar minerals. Three samples are characterised by an elevated content of carbonates, and among them one sample has a notable prevalence of this mineral group. This sample also demonstrated certain differences in biomarker distribution. In most samples organic matter (OM) consists predominantly of type I and II kerogens, showing high oil generative potential, whereas three samples, which contain type II kerogen with a certain input of type III kerogen, demonstrated potential to produce both, oil and gas. The OM of all samples is immature and corresponds to the vitrinite reflectance of ca. 0.40%. Biomarker patterns along with Rock-Eval data indicated a strong contribution of aquatic organisms such as green and brown algae and bacteria with some influence of higher plants OM. The organic matter was deposited in a reducing lacustrine alkaline brackish to freshwater environment under warm climate conditions. Preservation of OM was governed by stratification of the water column rather than its height. Tectonic movements that caused the regional tilting of an investigated area and supported minor marine ingression and influx of fresh water played an important role in formation of the sediments. Conventional pyrolytic experiments confirmed that these sediments at the catagenetic stage could be a significant source of liquid hydrocarbons