BIOMARKER GEOCHEMISTRY OF CRUDE OILS AND NEOGENE BITUMINOUS SHALES IN THE YENIKÖY AREA, EREGLI-ULUKIŞLA BASIN, CENTRAL ANATOLIA, TURKEY

R. Kara-Gülbay^a,*, S. Korkmaz^a, M. S. Erdoğan^a and G. Kadınkız^b

^a Department of Geological Engineering, Karadeniz Technical University, Trabzon, Turkey.

^b Department of Mineral Research and Exploration, MTA, Ankara, Turkey.

^* Corresponding author, email: kara@ktu.edu.tr

In the Ereğli-Ulukışla Basin, southern Turkey, crude oil shows have been observed in the subsurface in the shale-dominated non-marine Upper Miocene – Pliocene succession. Based on analyses of samples from four boreholes, the shales’ organic matter content, thermal maturity and depositional characteristics are discussed in this study. Geochemical correlations are established between shale extracts and a crude oil sampled from the shale succession. The shales have moderate to high hydrogen index (HI) and very low oxygen index (OI) values. Pyrolysis data show that the shales contain both Types I and II kerogen, and n-alkane and biomarker distributions indicate that organic matter is dominated by algal material. Very high C₂₆/C₂₅ and C₂₄/C₂₃, and low C₂₂/C₂₁ tricyclic terpane ratios and C₃₁ R/C₃₀ hopane, C₂₉/(C₂₈+C₂₉) MA and DBT/P ratios in shale extracts indicate that deposition occurred in a lacustrine setting. High gammacerane and C₃₅ homohopane concentrations and low diasterane/sterane ratios with a very low Pr/Ph ratio suggest that both the shales and the source rocks for the oil were deposited in a highly anoxic environment in which the water column may have been thermally stratified.
Although the shales analysed have very low Tmax values, the production index is quite high which suggests that the shales are early-mature to mature. Biomarker ratios including C₃₂ 22S/(22R+22S) homohopanes, C₂₉ 20S/(20R+20S) and ββ(ββ+αα) steranes, moretane/hopane, TA(I)/TA(I+II) and MPI-3 all suggest that the shales are within the oil window. Heavy components of free hydrocarbons (S₁) within the shales may have been recorded as part of the Rock-Eval S₂ peak resulting in the low T_max values.
The oil and shale extracts analysed are similar according to their sterane and triterpane distributions, suggesting that the oil was generated by the shales. However burial depths of the Upper Miocene – Pliocene shale succession are not sufficient for thermal maturation to have occurred. It is inferred that intense volcanism during the Pliocene – Pleistocene may have played an important role in local maturation of the shale succession.

Key words: Non-marine shales, crude oil, Neogene, Ereğli-Ulukışla Basin, Turkey, biomarkers, oil generation, oil – source rock correlation, organic geochemistry, thermal maturation.

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