Geochemical appraisal of hydrocarbon generative potential of Bulgarian part from the Thrace Basin: I. Linear biomarkers

Bulgarian part from the Thrace Basin is recognized as its NW flank. Hydrocarbon generation for some formations in the Bulgarian part, analogue to Turkish ones with already proven generative abilities, is expected. In order to assess source rocks and to foresee exploration activities in the region, eleven samples from the prospective formations, i.e. seven well samples and four outcrops samples, are evaluated by geochemical proxies. Extractable organic matter (OM), the so-called "free" bitumens, and extracted after demineralization ("bound" bitumens), are characterized. A broad range of biomarkers is considered. In this study linear structures, i.e. n-alkanes, regular isoprenoids, n-alkan-2-ones, n-alcohols, fatty acids, etc. will be considered. Rock Eval data and component analysis determine immature Type III and mixed Type II/III kerogen. n-Alkanes are dominant in all bitumen extracts. Their distributions are complex, depth dependent and give a hint for more mature OM redeposition. The cross-plots of regular isoprenoids in Pr/nC(17) vs. Ph/nC(18) diagram implies immature mixed Type II/III kerogen. Pr/Ph ratios, argue for immature OM deposited in anoxic/sub-oxic environment. The cross-plots of TOC vs. P-aq argue for at least two pools of n-alkanes: (i) short chain homologous from algal/bacterial contribution; (ii) and, mid-chain, from submerged/floating macrophytes wax. Fatty acids, nC(12)-nC(34) with "even" numbered dominance and changes with depth, are identified only in the "bound" bitumens. A similarity in possible origin with n-alkanes is assumed. Dicarboxylic fatty acids, nC(18)-nC(30), maximizing at nC(26), nC(28), nC(30) are recognizable as well.