Undiscovered Gas in the Thrace Basin, Turkey: A USGS Assessment, 2015

The Thrace Basin in western Turkey and the three assessment units (AU) defined in the study. USGS; base map, National Park Service.

Using a geology-based method, the U.S. Geological Survey assessed mean undiscovered, technically recoverable resources of 787 billion cubic feet of conventional gas and 1,630 billion cubic feet of unconventional gas in the Thrace Basin of Turkey.

The basin

The Thrace Basin holds as much as 9,000 metres of Eocene to Holocene sedimentary rock at its centre. It is read as a supradetachment basin, formed on the uplifted and stretched Rhodope metamorphic complex and later reshaped by Neogene strike-slip faulting. Its main petroleum source rocks are shales of the Eocene Hamitabat Formation; others may include shales of the Eocene Ceylan and Oligocene Mezardere Formations, and Oligocene coals of the Danişmen and Osmancik Formations, which could yield biogenic gas.

Four assessment units

The USGS defined a Paleogene Composite Total Petroleum System containing three assessment units, and a Biogenic Gas Total Petroleum System containing a fourth:

• Paleogene Conventional Reservoirs AU. Gas from Paleogene shales migrated into sandstones of the Hamitabat, Osmancik and Danişmen Formations and carbonates of the Soğucak Formation, in complex traps that began as Eocene extensional structures and were altered by Neogene deformation. The model assumes the deeper structures are less explored than the shallow ones.
• Paleogene Tight Gas AU. Gas from Hamitabat shales in the overpressured centre of the basin moved locally into low-permeability sandstones and siltstones of the Hamitabat, and possibly into tight sandstones of the Ceylan and Mezardere Formations.
• Mezardere Shale Oil AU. Not assessed quantitatively: total organic carbon data show that most of the Mezardere Formation falls below the USGS threshold for a shale-oil reservoir, more than 2 percent by weight.
• Oligocene Biogenic Gas AU. Biogenic gas held in coals of the Danişmen, and possibly the Osmancik, Formation in the undeformed centre of the basin.

Inputs

Estimated ultimate recovery, well drainage area and success ratio came from U.S. analogs for tight-gas and biogenic-gas reservoirs. Values are minimum / mode / maximum (calculated mean).

The table gives the recovery for these two gas units in million barrels of oil (MMBO).

For the Paleogene Conventional Reservoirs AU: 1 / 30 / 90 undiscovered fields (mean 31), of 3 / 12 / 600 billion cubic feet each (mean 23); probability 1.0.

Results

Fully risked; F95 is the amount with a 95 percent chance of being at least that large, and F50 and F5 likewise. All liquids in gas accumulations count as natural gas liquids (NGL). Fractiles add up only on the assumption of perfect positive correlation.

The conventional NGL range was 1 to 6 million barrels; the tight-gas NGL range 1 to 10.

Sources

• Christopher J. Schenk, Timothy R. Klett, Marilyn E. Tennyson, Janet K. Pitman, Stephanie B. Gaswirth, Phuong A. Le, Heidi M. Leathers-Miller, Tracey J. Mercier, Kristen R. Marra, Sarah J. Hawkins and Michael E. Brownfield, Assessment of Undiscovered Gas Resources of the Thrace Basin, Turkey, 2015, USGS Fact Sheet 2016–3001, January 2016. https://doi.org/10.3133/fs20163001
• The map and tables are taken from the fact sheet's PDF; its stratigraphic column, modified from a journal article, is not reproduced here.