An extraordinary underexplored basin
New data reveals the potential for a new super basin – an accessible oil play lying within the world-class Maykop oil source.
We all sense the verisimilitude of PK Dickie’s warning against looking for new oil in old places with old ideas, which gives us two clear courses of action: either we can bring new ideas to old basins, or we can bring tried and tested ideas to new basins. Although truly new exploration plays are as rare as hen’s teeth (mostly old ideas in new basins), new ideas stream out of new seismic data as we learn how much of “what we know for sure that just ain’t so” (The Big Short, 2015). Whilst exploration geology has remained sitting on the sofa watching episodes of Friends on endless repeat, the advances in seismic imaging over the last 15 years, removing noise and multiples, are astonishing. So modern, long streamer, deghosted seismic data provides explorers worldwide with a practical idea-engine for old basins.
Maykop proven source
Yet how underexplored can a basin be in 2019? In the Turkish sector of the East Black Sea Basin, a basin half the size of the Gulf of Mexico, there are only three exploration wells. Period. In this Early Cretaceous foredeep basin trapped between the Pontide thrust belt to the south and the Caucasus to the north, the basin fill is Late Cretaceous to Tertiary in age. Yet the first two wells barely escaped the Pliocene, drilling carbonate build-ups that proved not to be build-ups (Sinop-1) and a Pliocene sequence dominated by immature sands sourced from the Rioni Delta offshore Georgia to the east (Hopa-1). The third well (Surmene-1, 1RE) reached only to the Middle Miocene, drilling off-structure at target depth. It is indeed rare to find a large deep basin with 8–10 km of sediment fill that is so underexplored, but the astonishing fact of the East Black Sea Basin is that it has 1–2 km of world-class Maykop source rock (2–5% TOC), sitting in the oil window and generating oil today over the entire basin. This is unparalleled in frontier basins on planet Earth.
The Miocene Maykop Formation makes the East Black Sea Basin singularly different as, deposited across the extensive central Paratethys, the Maykop is the source for successful drilling in offset basins such as the southern Caspian Sea and western Black Sea (Tari and Simmons, 2018), and is also well studied in outcrop around the basin in the collisional Pontides/ Caucasus. Petroleum system modelling of the Maykop in the East Black Sea demonstrates that the Maykop is currently in the oil window under geotherm scenarios controlled by available well and seismic data (Minshull, 2010), consistent with prolific repeating oil slicks visible via optical and radar satellite data, which have been sampled and found to have been generated from a thermogenic source. The two wells drilled in the east reportedly penetrated oil-bearing and Maykop-age (Mid to Early Miocene) Pliocene sands. Seismic correlation shows that these sediments are present across the whole basin, displaying a Type 4 AVA anomaly – a characteristic of identification of source rocks on seismic (Loseth et al., 2011).
Crucially, the Maykop displays seismic velocities that indicate that this interval, where the Maykop basin-floor sands are observed, is generating hydrocarbons. In a normally pressured system velocities extracted from seismic data will show a progressive increase with depth as lithologies compress due to loading. The exceptions to this are if there is a change in lithology type such as, for example, a carbonate shelf sitting above a shale, or if a unit is under-compacted due to either a failure to de-water or the in-situ generation of hydrocarbons. It is the latter expression we interpret from the velocities extracted from the new seismic in the eastern Black Sea, where a strong velocity reversal (decrease in velocity with increased depth) is clear at Maykop depth. A nuanced finesse of this phenomena is seen on the figure below left around the clear fluid escape feature (which leads incidentally to a recurring oil slick location observed on satellite data), showing hydrocarbon maturation-generated overpressure maybe locally decreased due to fluid migration.
Exploration imminent?
So at this point, one might reasonably ask why, with these excellent indicators of a hydrocarbon system, is the East Black Sea Basin not explored more? Firstly, water depths in the main basin exceed 1,000m, so the area has had to wait until this century before exploration began. In addition, the first three wells were drilled on plays that did not work out, and whilst subsequently the Polshkov-1 well drilled offshore Bulgaria has proven the ‘oil in Maykop sands’ play in the Western Black Sea, this has simply not been attempted yet in the east. This then takes us to today, where the first densely shot multi-client data is available to chase Maykop sands in the eastern Black Sea, as well as new plays.
However, there is arguably one last test to pass for the East Black Sea Basin – that of sand quality. The Hopa-1 and Surmene-1 wells found tight, oil-bearing sands in the Pliocene and Miocene, and this poor reservoir quality is ascribed to compositionally immature sands rich in volcanoclastic fragments. These volcanoclastic lithic fragments are provenanced from outcrops in the fetch of the Rioni delta to the east (the source of the Pliocene sands); however, sand sources to the west are likely to be more quartz-rich (Vincent et al., 2013). As their distribution shows, the Maykop and pre-Maykop sequences were deposited before the collision of the Pontides and Caucasus, i.e. before the Rioni delta formed. Therefore, sands within and below the Maykop are likely to have been derived from river systems to the north-west in Russia and/or from a palaeo-Don/Volga (Kuban Fan) system. The new de-ghosted long streamer seismic data, designed to bring out sedimentological detail in the Maykop, has been mapped to show that the Intra-Maykop sand systems are indeed coming in from the north. These basin floor sands are onlapping the southern margin or draping four-way structures in the basin centre. The Shatsky Ridge to the north did not provide a barrier to sand entry as it, like the Rioni, only formed in the late Miocene–Pliocene, developing its dip-to-the-north geometry through Caucasus–Pontides compression.
Another new revelation from the 2018 dataset is the imaging of the Pontides fold and thrust belt at the edge of the foredeep basin. Here, large four-way closed structures are mapped in contact with the thick Maykop source sequence, and in places onlapped by basin floor fans that could provide charge focus systems. Although unexplored to date, the Pontides sedimentary sections are thick, folded, well bedded units likely to be composed of both carbonates and clastics, and it is this style of play that has many giant oil field analogues in foredeep collisional basins around the world.
New potential revealed
As has been described, this new long streamer de-ghosted 2D data is re-imaging the Turkish eastern Black Sea to reveal an extraordinary underexplored basin. Here, quartz-rich sands derived from Russia are ponding against the southern compressional margin of the Black Sea foredeep basin, or are draped over mid basin four-way structures. Previously inaccessible to drilling and un-imaged on seismic, these new data reveal the potential for a new super basin that is unique on planet Earth – an accessible oil play lying within the world-class Maykop oil source.
