BIOMARKER GEOCHEMISTRY OF CRUDE OILS AND NEOGENE BITUMINOUS SHALES IN THE YENIKÖY AREA, EREĞLI‐ULUKIȘLA BASIN, CENTRAL ANATOLIA, TURKEY

• Published in: Journal of petroleum geology Vol. 42; no. 2; pp. 173 - 191
• Main Authors: Kara‐Gülbay, R., Korkmaz, S., Erdoğan, M. S., Kadınkız, G.
• Format: Journal Article
• Language: English
• Published: Oxford Wiley Subscription Services, Inc 01-04-2019
• Subjects: Algae; Alkanes; Anoxia; Biomarkers; Boreholes; Crude oil; Ecological succession; Ereğli‐Ulukıșla Basin; Geochemistry; Hydrocarbons; Kerogen; Miocene; Neogene; Non‐marine shales; oil generation; oil – source rock correlation; organic geochemistry; Organic matter; Pleistocene; Pliocene; Pyrolysis; Ratios; Rocks; Sedimentary rocks; Shale; Shales; thermal maturation; Turkey; Volcanism; Water column; Turkey
• DOI: 10.1111/jpg.12729
• EISSN: 1747-5457
• ISSN: 0141-6421

In the Ereğli‐Ulukıșla 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 Tmax 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.