Source rock potential and depositional environment of the Lower Oligocene İhsaniye Formation in NW Turkey (Thrace, Karaburun)

Authors: EMILIA TULAN, REINHARD F. SACHSENHOFER, GABOR TARI, RACHEL FLECKER, VANESSA FAIRBANK, MAGDALENA PUPP, RYAN B. ICKERT

Abstract: The Lower Oligocene succession of the İhsaniye Formation, exposed at Karaburun on the Turkish (Thrace) coast of the Black Sea, provides insights into potential hydrocarbon source rock extent in the southwestern part of the Black Sea Basin. Presently, no detailed study of the İhsaniye Formation exists with regards to organic matter content and type. We analyzed 78 fine-grained samples from a c. 70-m-thick section near Karaburun. Relatively high total organic carbon (TOC) contents (average 1.45 wt.%) are present in the lower part of the İhsaniye Formation; the hydrogen index values reach a maximum of 252 mg HC/g TOC, indicating the presence of type III-II kerogen, and the organic matter is thermally immature (T_max 418 °C). The presence of foraminifera and TOC/S ratios below 2.8 confirm a fully marine environment. Elevated TOC contents and very low pristane/phytane ratios indicate an oxygen-depleted environment during deposition of the lower part of the İhsaniye Formation. The quartz/clay minerals ratio suggests a deepening trend in the lower part of the succession and a shallowing trend in the upper part. Peculiarly, the low salinity "Solenovian Event" (in nannofossil zone NP23) is not developed although Sr isotope ratios indicate that the basin was only connected to the Mediterranean for short periods during deposition of the İhsaniye Formation.

Keywords: Paratethys, West Black Sea Basin, Lower Oligocene, hydrocarbon source rock, strontium isotopes

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