Geochemical appraisal of hydrocarbon generative potential of Bulgarian part from the Thrace Basin: II. Cyclic biomarkers

Assemblages of cyclic components in bitumens help to verify some assumption based of linear structures interpretation and supply relevant information for the generative potential of the region and palaeoenvironment of deposition. Hopanes and steranes determine immature Type III to mixed Type II/III kerogen for the Bulgarian part of the Thrace Basin. The view for Type III kerogen is additionally supported by the signatures for triterpenoids (TTs) abundance. In bitumen fractions monounsaturated, aromatized, C-3 functionalised or partly destructed TTs are identified. All these components unequivocally attest angiosperm taxa contribution to the palaeoenvironment. Consistent with the composition of the products from TTs destruction or aromatization it is assumed that oleanane type TTs strongly prevail ursane/lupane structures. Saturated diterpenoids were practically absent in bitumen extracts. Cross-plots of Pr/Ph vs. C-29/C-27 regular steranes attest deposition in anoxic/sub-oxic environment. Positions of samples in the diagram confirm the assumption for mixed type of OM (algae/bacteria and from land plants). Plots of steranes in tertiary diagram depict OM formation in open water environment (bay or estuarine) in lacustrine-fluvial/deltaic environments. A decrease in MPI-3 values with depth parallel to T-max changes was observed. It might be a hint for redeposition of more mature OM. This surmise was not supported by the changes in biomarker parameters, i.e. T-s/(T-s+T-m), hopane ratio, homohopane index, sterane C-29 beta beta/(alpha alpha+beta beta) and C29 20S/(20S+20R) ratios and positions in Pr/nC(17)vs. Ph/nC(18) diagram, all attesting maturity increase with depth. More samples and additional study are needed to verify this assumption. Rock Eval data and cyclic biomarker assemblages give us a confidence to denote that samples from the shale interval of the shale-marl formation in the Bulgarian territories and analogues to the Yenimuhacir group in the Turkish part of the Thrace Basin, can be regarded as "gas-prone" source rocks capable to generate biogenic gas. Although based on limited number of samples, some of them with low total organic carbon content, the current study is informative for a region where geochemical data are practically absent.