Black Sea oil and gas: what the USGS thinks is still undiscovered

In 2023 the U.S. Geological Survey estimated how much oil and gas is still undiscovered under the Black Sea and its margins. The estimates cover conventional resources only — oil and gas pooled in porous rock beneath a seal. They count only what could be recovered with today's technology, and say nothing about cost or whether drilling would pay.

F95 is the amount with a 95 percent chance of being there or more; F5 has a 5 percent chance. The spread is wide mostly because so little is known about the deep basins.

Gas dominates. Most of the source rock under the two deep basins is now hot enough to make gas rather than oil.

How the basin formed

The Black Sea is the back-arc of an old subduction zone. From the Triassic onward the Neotethys ocean floor slid north beneath the southern edge of Eurasia. In the Late Jurassic the sinking slab began to roll back, and in the Early Cretaceous that rollback pulled the crust behind the Pontides volcanic arc apart.

• The western basin and the Karkinit Trough opened as the Western Pontides split away southeast from the Moesian Platform.
• The eastern basin opened as the Andrusov and Arkhangelsky ridges rifted away from the Shatsky Ridge and rotated clockwise.

Rifting broke up the carbonate platforms that had lined the old margins. By the Late Cretaceous the crust in parts of each basin had thinned so far that true ocean crust may have formed. Several kilometres of sand and mud then filled the basins through the Eocene.

As the Neotethys began to close in the Eocene, the deep water went stagnant and organic-rich mud collected on the floor. It happened again from the Oligocene into the Miocene. Those two layers are the main source rocks today. Repeated falls in sea level since the Late Cretaceous carried sand past the shelves into deltas, slope channels and deep-sea fans — the main reservoirs in this assessment. When the ocean finally closed in the Neogene, many structures along the margins and ridges were squeezed and inverted. Out in the basins most were not, and they now form traps.

The USGS notes that this history is widely accepted but still loosely dated: when the ocean crust formed, how far it extends and when the main stretching happened all need more work.

Where the oil and gas would come from

Both have made thermogenic gas — gas cooked from buried organic matter by heat. Several offshore Romanian fields also hold biogenic gas, made by microbes, possibly from much younger Pleistocene and Holocene sediment. Jurassic, Cretaceous and Eocene marls and Miocene diatomite may be further sources, but there are not yet enough geochemical data to judge them.

Five assessment units

All five sit inside one Mesozoic–Cenozoic Composite Total Petroleum System.

Gas includes gas dissolved in oil fields as well as gas fields. Most seals are mudstones within the same formations. Some of the sandstones may be too poorly sorted or cemented to hold a field.

The western basin is expected to hold nearly half the oil and well over half the gas. The inputs behind these numbers — how many fields each unit might contain and how big — were set at up to 80 oil and 280 gas fields in the western basin, against 6 and 24 in the small Karkinit Trough.

Sources

Written from Assessment of Undiscovered Conventional Oil and Gas Resources of the Black Sea Area, 2023, USGS Fact Sheet 2024–3018, by C.J. Schenk and the USGS Black Sea Area Assessment Team, a work of the United States government in the public domain. The fact sheet draws on Simmons, Tari and Okay (eds.), Petroleum Geology of the Black Sea (Geological Society of London Special Publication 464, 2018), Nikishin and others (2015, 2017), Sosson and others (2016), Hippolyte and others (2018), Robinson and others (1996) and Yegorova and others (2013). Full input data are in Schenk (2024); more results are on the USGS Energy Resources Program site.