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

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.

Biostratigraphy and paleoenvironments of the Oligocene succession(İhsaniye Formation) at Karaburun (NW Turkey)

“…The oldest approximately 20 m of the İhsaniye Formation (Biozone NP21, Subzone CNO1) is relatively enriched in organic matter (Tulan et al, 2020) and contains significant terrestrial palynomorph input and foraminiferal indicators of low oxygen bottom waters. Above this, the microfossils from the İhsaniye Formation reflect an increasingly transgressive character for approximately 35 m (Biozones NP22 -NP23, Subzone CNO3), with the footwall side of the fault transgressed by sediments that can be assigned to Biozone NP23.…”

“…Biozones NP21/O1) might have experienced bottom water dysoxia and/or oligotrophy. The highest organic carbon content occurs within this lower part of the section (Tulan et al, 2020) indicating bottom-water conditions conducive to the preservation of organic matter. Above this, the succession is transgressive in nature between samples 20 and 55, based on planktonic foraminifera abundance and the presence of dominant fine grained lithologies, with a maximum flooding surface being present at around sample 55.…”

“…While marls with variable organic content (Tulan et al, 2020) and calcareous siltstones and occasional sandstones (of debris flows origin) were being deposited during Biozone NP21 and NP22 times in the hanging wall of the fault, no deposition was taking place on the footwall. There, deposition was initiated with the coarse pebbly sandstones, passing up into marls, both of which onlap the Soğucak Formation.…”

“…Full details of the organic geochemistry of the İhsaniye Formation at Karaburun are presented separately (Tulan et al, 2020). In summary, it is the oldest İhsaniye Formation that is relatively, the most organic-enriched.…”

“…Sandstones within the Maykop Suite succession also have potential as hydrocarbon reservoirs (Tari et al, 2009(Tari et al, , 2011Rees et al, 2018;Simmons et al, 2018;Tari and Simmons, 2018). Consequently, the stratigraphic and paleoenvironmental description of the İhsaniye Formation which is a lateral equivalent of the Maykop Suite enables an improved understanding of the regional depositional framework of Oligocene sediments in Paratethys with an impact for predicting both source rocks (Tulan et al, 2020) and reservoirs (Rees et al, 2018) of this age in the Black Sea depocenter. Note that the İhsaniye Formation at Karaburun is mostly marly (as opposed to purely siliciclastic) and contains a fully open marine diverse fauna with no evidence for restriction and only limited evidence for bottom-water anoxia within its lower part.…”