The Black Sea - especially deep-water plays - remains underexplored although recent successes have stimulated an upturn in interest (Simmons et al., 2018). In the Western Black Sea (WBS) discoveries of biogenic gas have occurred (e.g. "Domino" offshore Romania) but have yet to be made for thermogenic petroleum systems. Nevertheless, several thermogenic plays associated with both Oligocene sources and reservoirs are proposed (with some proven) in other areas of the Black Sea (Tari & Simmons, 2018). Rees et al. (2017) has also demonstrated several sediment-source pathways into the WBS during the Oligo-Miocene for reservoir sands shed from various paleo-river systems along the western WBS coast, one of which includes a route that passes into the Burgas Basin.

Simplified regional palaeogeographic setting of the Early Oligocene of Eastern Paratethys, modified and synthesized after various sources notably Popov et al. (2004).

The Burgas Basin lies along the SW coast of the WBS adjacent to the Thrace platform, with sediment sourced from the Strandja and Srednorgie massifs to the west. Studies of latest Eocene and Early Oligocene sediments in the coastal area between Kiyikoy and Karaburn on the SW margin of the Black Sea (mainly Ihsaniye Formation) yielded extremely rich and diverse microfaunas, nannofloras and palynofloras from various facies including "beach", reefal, shelf and bathyal settings, some with evidence for reduced oxygen levels.

Selection of foraminifera, nannofossils and palynomorphs observed at Karaburun.

Sometimes, complicated field relationships between strata make biostratigraphic interpretations challenging but the bulk of these sediments are dated as Early Oligocene (O1 and O2 planktonic foram zones, NP21, NP22 and NP23 nannofossil zones and D13 and D14a dinocyst zones - Simmons et al., in press.).

The studied section of the İhsaniye Formation at Karaburun.

Regionally there are comparisons to be made between the Oligo-Miocene succession in the Black Sea and that of the world-class Maykop source rock suite of the greater Caspian region. However, paleontological data shows significant differences, particularly in estimation of changes in sea water salinity, which in turn has implications for the timing and nature of how Eastern Paratethys became isolated from the world's oceans during the Late Paleogene.

Biostratigraphic evidence from Karaburun and other nearby localities suggest a major and sudden sea-level fall in the very latest Priabonian (Late Eocene), perhaps by as much as 100 metres, but with also very rapid transgression shortly afterwards. The whole cycle may have taken as little as 2 or 3 hundred thousand years to complete.

This leads on to how paleo-drainage patterns may have changed and which patterns are considerably different from those prevailing today. Knowledge of these patterns is crucial to determining source-to-sink routes and the nature of the hinterland being eroded, which may have significant impact on potential reservoir presence and quality.

Full results are described in the forthcoming paper which is expected to be published later this year:

M.D. Simmons, M.D. Bidgood, P.G. Connell, S. Ćorić, A.I. Okay, D. Shaw, E. Tulan, J. Mayer & G.C. Tari. Biostratigraphy and palaeoenvironments of the Oligocene succession (Ihsaniye Formation) at Karaburun (NW Turkey) (in press). Turkish Journal of Earth Sciences.