U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2016–3001

January 2016 Printed on recycled paper

National and Global Petroleum Assessment

Assessment of Undiscovered Gas Resources of the Thrace Basin, 

Turkey, 2015

Using a geology-based assessment methodology, the U.S. Geological Survey assessed undiscovered, technically recoverable mean 

resources of 787 billion cubic feet of conventional gas and 1,630 billion cubic feet of unconventional gas in the Thrace Basin, Turkey.

The geologic model for the 

Paleogene Tight Gas AU has gas generated 

from shales of the Hamitabat Formation 

in the overpressured zone of the central 

part of the Thrace Basin migrating locally 

into tight (low-permeability) sandstones 

and siltstones of the Hamitabat Formation 

and into possibly tight sandstones of 

the Ceylan and Mezardere Formations. 

The geologic model for the Oligocene 

Biogenic Gas AU is for biogenic gas to be 

retained in the coals of the Danişmen and 

possibly the Osmancik Formation in the 

central, undeformed part of the basin.

Key assessment input data for three 

AUs are summarized in table 1. Data for 

estimated ultimate recovery (EUR), well 

drainage area, and success ratio were 

based on U.S. analogs for tight-gas and 

biogenic-gas reservoirs (U.S. Geological 

Survey U.S. Continuous Resources 

Assessment Team, 2015).

Undiscovered Resource Summary

The USGS assessed potential undiscovered, technically recoverable conventional resources and unconventional 

(continuous) gas resources in the Thrace 

Introduction

The U.S. Geological Survey (USGS) 

assessed the potential for undiscovered, 

technically recoverable conventional 

and unconventional (continuous) gas 

resources within the Thrace Basin, Turkey 

(fig. 1). The Thrace Basin, with as much 

as 9,000 meters of Eocene to Holocene 

sedimentary rock in the basin center, is 

interpreted as a supradetachment basin 

formed on an exhumed and extended 

Rhodope metamorphic complex that was 

subsequently modified by Neogene strikeslip deformation (Kilias and others, 2013). 

Shales of the Eocene Hamitabat Formation 

are the principal petroleum source rocks in 

the Thrace Basin (Hoşgörmez and Yalҫin, 

2005; Gürgey, 2009); other potential source 

rocks include shales of the Eocene Ceylan 

Formation, Oligocene Mezardere Formation, 

and Oligocene coals (for biogenic gas) of 

the Danişmen and Osmancik formations. 

The USGS defined a Paleogene Composite 

Total Petroleum System (TPS) that includes 

the (1) Paleogene Conventional Reservoirs 

Assessment Unit (AU); (2) Paleogene 

Tight Gas AU; and (3) Mezardere Shale Oil 

AU. The Oligocene Biogenic Gas AU was 

defined within the Biogenic Gas TPS.

Geologic Models for Assessment

The geologic model for the assessment 

of the Paleogene Conventional Reservoirs 

AU has gas generated within organicbearing Paleogene shales migrating into 

sandstone reservoirs of the Hamitabat, 

Osmancik, and Danişmen Formations, and 

into carbonate reservoirs of the Soğucak 

Formation (fig. 2). Reservoirs are within 

complex traps that originated as Eocene 

extensional structures modified by Neogene 

deformation. This assessment model 

assumes that structures and reservoirs in the 

deeper part of the basin are underexplored 

when compared to shallower structures.

Figure 1. Location of the Thrace Basin in western Turkey and boundaries of the three 

assessment units defined in this study. AU, assessment unit.

26° 27° 28° 29° 30°

42°

41°

40°

TURKEY

TURKEY

TURKEY

GREECE

BULGARIA

SEA OF MARMARA

AEGEAN SEA

BLACK SEA

Istanbul

Source: National Park Service 0 25 50 MILES

0 25 50 KILOMETERS

International boundary

Paleogene Tight Gas AU

EXPLANATION

Paleogene Conventional Reservoirs AU

Oligocene Biogenic Gas AU GREECE

BLACK SEA

TURKEY

BULGARIA

ROMANIA

Area 

of study GREECE
Basin, Turkey. For conventional resources, 

the USGS assessed a mean of 787 billion 

cubic feet of gas (BCFG) and 3 million barrels of liquids (MMBNGL) (table 2). The 

ranges of resource estimates, F95–F5 (317 to 

1,471 BCFG; 1 to 6 MMBNGL) reflect geologic uncertainty. For unconventional tight-gas 

resources, the mean is 1,261 BCFG with an 

F95–F5 range from 378 to 2,303 BCFG. For 

biogenic gas resources, the mean is 369 BCFG, 

with an F95–F5 range from 0 to 724 BCFG 

(table 2).

Other unconventional oil and gas accumulations may be present in the Thrace Basin, 

including shale oil within source rocks of the 

Mezardere Formation (Gürgey, 2015). However, 

available data for total organic carbon content 

indicate that most of the Mezardere Formation 

does not pass the threshold for the USGS definition of a potential shale-oil reservoir (greater 

than 2 weight percent total organic carbon), 

and therefore the Mezardere Formation was not 

quantitatively assessed in this study.

References Cited

Gürgey, Kadir, 2009, Geochemical overview and undiscovered gas resources generated from Hamitabat petroleum system in the Thrace Basin, Turkey: Marine and Petroleum Geology, v. 26, no. 7, 

p. 1240–1254.

Gürgey, Kadir, 2015, Estimation of oil in-place resources in the lower Oligocene Mezardere Shale, 

Thrace Basin, Turkey: Journal of Petroleum Science and Engineering, v. 133, p. 543–565.

Hoşgörmez, Hakan, and Yalҫin, M.N., 2005, Gas–source rock correlation in Thrace Basin, Turkey: 

Marine and Petroleum Geology, v. 22, no. 8, p. 901–916.

Kilias, Adamantios, Falalakis, George, Sfeikos, Aristides, Papadimitriou, Eleftheria, Vamvaka, Agni, 

and Gkarlaouni, Chara, 2013, The Thrace Basin in the Rhodope province of NE Greece—A tertiary 

supradetachment basin and its geodynamic implications: Tectonophysics, v. 595–596, p. 90–105.

U.S. Geological Survey U.S. Continuous Resources Assessment Team, 2015, U.S. Geological Survey 

assessments of continuous (unconventional) oil and gas resources, 2000 to 2011: U.S. Geological 

Survey Digital Data Series DDS–69–MM, 46 p.

Table 1. Key assessment input data for two continuous assessment units and one 

conventional assessment unit in the Thrace Basin, Turkey.

[AU, assessment unit; %, percent; EUR, estimated ultimate recovery per well; MMBO, million barrels of oil; 

BCFG, billion cubic feet of gas. The average EUR input is the minimum, median, maximum, and calculated 

mean. Shading indicates not applicable]

Assessment input data—Continuous AUs

Paleogene Tight Gas AU Minimum Mode Maximum Calculated mean

Potential production area of AU (acres) 1,000 226,000 452,000 226,333

Average drainage area of wells (acres) 40 80 120 80

Success ratio (%) 60 70 80 70

Average EUR (MMBO) 0.4 0.6 1.0 0.619

AU probability 1.0

Oligocene Biogenic Gas AU Minimum Mode Maximum Calculated mean

Potential production area of AU (acres) 1,000 406,000 609,000 338,667

Average drainage area of wells (acres) 40 60 80 60

Success ratio (%) 50 60 70 60

Average EUR (MMBO) 0.09 0.15 0.3 0.157

AU probability 0.9

Assessment input data—Conventional AU

Paleogene Conventional Reservoirs AU Minimum Mode Maximum Calculated mean

Number of undiscovered fields 1 30 90 31

Sizes of undiscovered fields (BCFG) 3 12 600 23

AU probability 1.0

Table 2. Assessment results for undiscovered continuous and conventional resources 

of the Thrace Basin, Turkey.

[AU, assessment unit; BCFG, billion cubic feet of gas; MMBNGL, million barrels of natural gas liquids. Results 

shown are fully risked estimates. For gas accumulations, all liquids are included under the NGL (natural gas 

liquids) category. F95 represents a 95 percent chance of at least the amount tabulated. Other fractiles are defined 

similarly. Fractiles are additive under assumption of perfect positive correlation. Shading indicates not applicable]

Total petroleum system 

and AU

AU

probability

Accumulation 

type

Total undiscovered resources

Gas (BCFG) NGL (MMBNGL)

F95 F50 F5 Mean F95 F50 F5 Mean

Paleogene Composite Total Petroleum System

Paleogene Tight Gas AU 1.0 Gas 378 1,211 2,303 1,261 1 5 10 5

Mezardere Shale Oil AU 0.1 Oil Not quantitatively assessed

Biogenic Gas Total Petroleum System

Oligocene Biogenic Gas AU 0.9 Gas 0 372 724 369 0 0 0 0

Total continuous resources 378 1,583 3,027 1,630 1 5 10 5

Paleogene Composite Total Petroleum System

Paleogene Conventional Reservoirs AU 1.0 Gas 317 722 1,471 787 1 3 6 3

Total conventional resources 317 722 1,471 787 1 3 6 3

Total undiscovered resources 695 2,305 4,498 2,417 2 8 16 8

Figure 2. Generalized stratigraphic column for the 

Thrace Basin, Turkey. Modified from Gürgey (2015).

Chronostratigraphy Unit Lithology Thickness (meters) Age (m.y.)

Quaternary Miocene Oligocene

Yeni Muhacir

Upper Lower Lower Middle Upper

Eocene

Upper

Lower

Upper

Middle

Lower

Cretaceous

Paleozoic

Metamorphics and Melange

5

10

15

20

25

30

35

40

45

50

55

Unnamed

Kirazli

Ergene

Group

Osmancik

Ceylan

Sogucak

Hamitabat

Mezardere

100500 0–600 300–500 500–1,200 0–1,000 0–300 500–4,000 600

?

Danismen

Pliocene

EXPLANATION

Conglomerate

Sandstone and shale

Sandstone

Coal

Shale

Limestone

Metamorphics

Melange

Thrace Basin Assessment Team

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.

For Further Information

Assessment results also are available at the USGS Energy Resources Program 

Web site at http://energy.usgs.gov.

ISSN 2327-6916 (print) ISSN 2327-6932 (online) http://dx.doi.org/10.3133/fs20163001