Deepwater shallow depth of burial gas, once considered a drilling hazard, has become an exploration focus for several companies around the world, including CNOOC in the Qiongdongnan Basin, Reliance Industries in the Krishna Godavari Basin and TPAO in the Western Black Sea. Companies exploring, appraising and developing the play have encountered a unique combination of technical challenges and commercial opportunities. This article summarises the exploration history of the play in the Black Sea and some key insights from recent well results.
Deepwater Exploration and Development in the Black Sea (1999-2026)
Figure 1 illustrates the distribution of oil and gas discoveries and fields in the Black Sea along with their discovery date. Prior to 1999, Black Sea oil and gas exploration was constrained to the shallow water (<100m) shelf areas by a handful of Soviet era jack-ups.
Taking a drillship or semi-submersible rig through the Bosphorus Strait was technically challenging and commercially prohibitive. Deepwater exploration began in 1999 when TPAO drilled the Limanköy-1 and 2 wells. So far, 36 deepwater exploration wells have resulted in 5, possibly six, commercial discoveries. Only 4 exploration wells have been drilled in the deepwater Eastern Black Sea. By comparison, 3800 exploration wells have been drilled in the deepwater Gulf of Mexico which is three times the size of the Black Sea.
Seismic data (Figure 2) demonstrate a passive post-rift fill that can be clearly imaged all the way to the deepest parts of the Black Sea Basin. The major extensional rift event that simultaneously creates the accommodation of up to 14 km of sediment (Figure 3) and 2 km of water present day is early Tertiary with most of the fill being Oligocene to Pleistocene in age. The basin is subdivided into the West and East by the Mid Black Sea High (Figure 3). Most of the significant structures (Domino, Pelican South and Sakarya) have been formed because of later reactivation of the extensional rift faults (Figure 2).
In terms of depth of burial there are two main plays in the Black Sea. A deep play comprising Oligocene and older structural traps with thermogenic gas and oil derived from Oligocene to Eocene Maykop marine shales.
The shallow play comprises Plio-Miocene structural traps with either biogenic gas derived from the surrounding shales (which also act as the top seal) or Maykop derived thermogenic gas leaking from underlying structures. Several of the larger oil and gas discoveries in shallow waters have been brought into production. Ana and Doina are examples of small, shallow water, gas discoveries within the Plio-Miocene play that were once considered uneconomic. Today they are part of the Midia Gas Development operated by Black Sea Oil and Gas, that is producing about 3.1 Mm³/day of gas, equivalent to 12% of Romanian demand.
The early (1999-2012) focus of deepwater Black Sea exploration was for oil in deeper structures. Shallow depth of burial gas was avoided or ignored. The first significant milestone for the deepwater gas play was in 2012, when Exxon and partners made the 2 Tcf Domino discovery. The integration of the Domino well results with 3D seismic data helped identify several multi-Tcf prospects in structural-stratigraphic traps. Seismic amplitudes (potentially indicative of gas-bearing sands) extended beyond structural closures.
Early optimism was short-lived, only one additional commercial discovery (Pelican South in 2015) was established after the drilling of 9 deepwater exploration and appraisal wells. Appraisal drilling at Domino resulted in a smaller resource size than the pre-drill estimate. Operator, Exxon, sold their interests in Romania (including the Neptun deep development project) in 2022 to Romgaz for over $1 billion. OMV Petrom took over operatorship and today the Neptun deep project comprises Domino and Pelican South, combined resources are 100 Bcm (3.5 Tcf). The discoveries will be tied back to an unmanned platform in shallow water, a 160 km 30” pipeline will take the gas to the onshore terminal at Tuzla. First gas is expected in 2027 with a plateau production of 21 Mm³/day or 140,000 boepd.
The next and possibly most significant discovery came in 2020 when TPAO discovered Sakarya. Since then, TPAO have expanded their drilling fleet to six vessels. Their deepwater Western Black Sea exploration campaign has resulted in the Amasra discovery in 2021, Çaycuma in 2022 and Göktepe in 2024. Sakarya Phase 1 has been producing since 2023. Phases 1, 2 and 3 comprise 49 producers and the development of Amasra and Göktepe with a combined peak capacity of around 40 Mm³/day in 2034. TPAO have drilled a number of deepwater exploration wells in the play without any official comment, these are interpreted to be dry holes or commercial failures.
The recent Krum and Vinekh dry holes, drilled in the OMV operated Han Asparuh offshore Bulgaria, have highlighted the challenges and risks associated with stratigraphic traps in the play.
Key Technical Insights
Some key observations from the shallow depth of burial gas Plio-Miocene Gas Play are outlined below:
1. Seismic imaging challenges – the overburden in the Western Black Sea is very complex and heterogeneous. The stratigraphy comprises the Danube levee complex, canyons, shallow gas channels, and mass transport complexes. As a result, overburden velocities are very heterogeneous resulting in amplitude dimming, wipe-outs and structural distortions in the target reservoir sections.
2. DHIs – no conformance of amplitudes to structure. Residual gas is often present throughout the shallow depth of burial section. The seismic response of full saturation and residual saturation gas sands is difficult to distinguish. There is often no conformance of seismic amplitudes to structure due to the presence of residual gas below the free water level.
3. Trap, Seal and Charge Model – sealing lithologies are silty shales that leak over time. Simultaneously with trap leakage biogenic gas is being generated from surrounding shales and thermogenic gas is leaking from deeper traps. Traps can be underfilled due to charge versus leakage. Higher capillary pressures and smaller pore throats of deepwater shallow depth of burial mudstones can result in a higher sealing capacity compared to shallow water shallow depth of burial systems.
4. Reservoir sands can be hard to identify. Data acquisition in the top-hole section of older deepwater exploration wells is often limited and involves no mud returns and only LWD gamma ray and resistivity. Hot sands have abnormally high gamma ray response due to the presence of radioactive minerals with limited difference to the surrounding shales and silts. Sands can be missed or interpreted as being shale prone.
5. Gas pay can be hard to identify. Sands are thinly bedded and highly laminated. Alternating millimetre- to centimetre-thick layers of resistive gas-bearing sands and conductive mudstone/silts with capillary-bound water result in a suppressed resistivity response. Residual gas saturations are present outside the gas column due to present day biogenic gas generation and/or ongoing thermogenic migration from depth. The average resistivity of gas pay above can be as low as 1–10 Ωm. Such low resistivity gas sands can be missed or ignored on wireline log data. They cannot be resolved by electromagnetic surveys. Establishing reliable water and gas gradients from pressure data can be challenging due to the presence of capillary bound water in the gas leg and residual gas in the water leg.
6. Unconsolidated sands. Shallow depth of burial sands are unconsolidated and susceptible to invasion and formation damage. Cased hole gravel packed completions are required; drawdown limits should be established to avoid sand production. Clays can be smectite rich and swelling - oil-based muds are used to prevent swelling.
7. Flow assurance – biogenic gas is generally very dry and can be easier to process. This can allow for a simplified subsea development tie-back directly to shore e.g. Sakarya Phase 1. Potential hydrate formation in the flowlines requires injection of chemical inhibitors like methanol or monoethylene glycol (MEG).
Summary
Exxon, OMV Petrom and TPAO have been instrumental in exploring and developing the shallow depth of burial Plio-Miocene gas play in the Western Black Sea. TPAO has signed MoUs with Exxon, Chevron, BP and Total. Information sharing between countries and companies is improving but is limited, which is hindering the understanding and development of the play.
Some of the technical challenges associated with play have been outlined in this paper. TPAO’s aggressive exploration campaign is helping Türkiye to lower their emissions, reduce gas imports and secure energy independence. Large parts of the basin however remain underexplored or unexplored including offshore Eastern Türkiye, Ukraine, Russia and Georgia.
To replicate the success of Türkiye and TPAO, other countries need to create an environment that allows for the aggressive exploration and development of the play.
Outlook
Although exploration of the Black Sea remains at an early stage compared with mature deepwater provinces, recent discoveries demonstrate the significant potential of the Plio-Miocene gas play. Continued advances in seismic imaging, drilling technology and geological understanding are likely to play a key role in unlocking remaining resources across the basin.
