U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2015-3006

February 2015

SAUDI

ARABIA

SYRIA

TURKEY

IRAQ IRAN

KUWAIT

PERSIAN

GULF

CASPIAN

 SEA

LAKE

URMIA

LAKE

RAZAZZA

LAKE 

THARTHAR

40° 45° 50°

35°

30°

0 150 300 MILES

0 150 300 KILOMETERS

Source: U.S. National Park Service TURKEY

IRAN

SYRIA

EGYPT

IRAQ

SAUDI

 ARABIA SUDAN YEMEN

OMAN

UAE

Area of

study

ARABIAN

SEA

EXPLANATION

International boundary

Sargelu Continuous Oil

 AU 2024 0261

System Series Formation

JURASSIC

UPPER

MIDDLE

LOWER

Chia Gara

Gotnia Barsarin

Sargelu

Adalyah Sehkanlyan

Sarki

Naokelekan

Alan

Najmah

Mus

Butmah

Assessment of Unconventional Oil and Gas Resources in the 

Jurassic Sargelu Formation of Iraq, 2014

Using a geology-based assessment methodology, 

the U.S. Geological Survey estimated means of 1.6 

billion barrels of unconventional oil and 0.96 trillion 

cubic feet of associated gas in the Jurassic Sargelu 

Formation of Iraq.

Introduction

The U.S. Geological Survey (USGS) quantitatively 

assessed the potential for unconventional (continuous) oil 

and gas resources within the Jurassic Sargelu Formation 

of Iraq (Sargelu Continuous Oil Assessment Unit; fig. 1). 

Organic-rich shales of the Jurassic Sargelu Formation are 

one of the main petroleum source rocks for conventional 

fields in the Arabian Peninsula (fig. 2) (Bordenave and 

Hegre, 2010). The Sargelu Formation consists of marine 

shales, with as much as 10 weight percent sulfur-rich 

Type IIS organic matter, deposited in a relatively deep, 

anoxic Jurassic depocenter. The potential for volumes of 

oil retained in the Sargelu Formation source-reservoir rock 

system following oil migration, cracking, or degradation 

is the focus of this assessment. Conventional oil and gas 

resources of Iraq were assessed by the USGS in 2012 (U.S. 

Geological Survey World Energy Assessment Team, 2012).

The USGS assessment methodology consists of a 

well-performance approach (Charpentier and Cook, 2011) 

that recognizes the geologic variability within assessed 

reservoirs. For non-U.S. assessments, the USGS assesses 

shale-gas or shale-oil reservoirs that (1) contain greater 

than 2 weight percent total organic carbon (TOC), (2) 

are within the proper thermal maturity window for oil 

or gas generation, (3) have greater than 15-m thickness 

Figure 1. Map showing location of the Jurassic 

Sargelu Formation Assessment Unit (AU) 

assessed in this study.

Figure 2. Jurassic stratigraphy within the study area of Iraq.

National and Global Petroleum Resource Assessment Project
ISSN 2327-6916 (print)

ISSN 2327-6932 (online)

http://dx.doi.org/10.3133/fs20153006

of organic-rich shale, and (4) contain Type I or II organic 

matter. These specific USGS criteria when applied to any 

given shale-oil or shale-gas reservoir might significantly 

reduce the potential resource assessment area compared to 

maps made with greater than 1 weight percent TOC.

Geologic Model for Assessment

The geologic model for the assessment is summarized 

as follows. Volumes of oil and gas generated from organicrich, deep-basin anoxic facies of the Jurassic Sargelu 

Formation in the Late Cretaceous remained in the source 

rock, migrated long distances, or re-migrated during the late 

Tertiary Zagros tectonic event, which is based on known 

Sargelu-sourced oil occurring in some Zagros-age structures. 

About 30 percent of the assessment unit (AU) area at the 

mode is estimated to contain adequate source facies that 

were relatively unaffected by structural movements and 

might have retained oil in the source-reservoir system. 

Upper Jurassic Gotnia Formation evaporites that overlie the 

Sargelu Formation likely provide a partial seal to the sourcereservoir system. Shales of the Naokelekan Formation 

were not included in this assessment, as the Naokelekan 

potentially is a separate source-reservoir rock system.

The input data for the Jurassic Sargelu Continuous Oil 

AU are shown in table 1.

Resource Summary

The USGS quantitatively assessed unconventional oil and 

gas resources in the Jurassic Sargelu Formation of Iraq (table 2). 

For unconventional oil resources, the mean total is 1,606 million 

barrels of oil (MMBO), with a range from 327 to 3,730 MMBO; for 

associated gas the mean total is 963 billion cubic feet (BCFG), with 

a range from 183 to 2,315 BCFG; and a mean total of 29 million 

barrels of natural gas liquids (MMBNGL), with a range from 5 

to 72 MMBNGL. These resource estimates are for undiscovered, 

technically recoverable volumes of oil and gas and do not reflect 

volumes of economically recoverable resources.

References Cited

Bordenave, M.L., and Hegre, J.A., 2010, Current distribution of oil and 

gas fields in the Zagros fold belt of Iran and contiguous offshore as 

the result of the petroleum systems, in Leturmy, P., and Robin, C., 

eds., Tectonic and stratigraphic evolution of Zagros and Makran 

during the Mesozoic–Cenozoic: Geological Society, London, 

Special Publications, v. 330, p. 291–353.

Charpentier, R.R., and Cook, T.A., 2011, USGS methodology for 

assessing continuous petroleum resources: U.S. Geological Survey 

Open-File Report 2011–1167, 75 p.

U.S. Geological Survey World Energy Assessment Team, 2012, 

Assessment of undiscovered conventional oil and gas resources of 

the Arabian Peninsula and Zagros fold belt, 2012: U.S. Geological 

Survey Fact Sheet 2012–3115, 4 p.

Publishing support provided by the U.S. Geological Survey

Denver and Tacoma Publishing Service Centers

Sargelu Assessment Team:

Christopher J. Schenk, Janet K. Pitman, Ronald R. Charpentier, 

Timothy R. Klett, Stephanie B. Gaswirth, Michael E. Brownfield, 

Heidi M. Leathers, Tracey J. Mercier, and Marilyn E. Tennyson.

For Further Information:

Assessment results are available at the USGS Energy 

Resources Program Web site, http://energy.usgs.gov/oilgas/.

Total Petroleum System 

(TPS) 

and Assessment Units (AUs)

AU 

Probability

Accumulation 

Type

Total Undiscovered Resources

Oil (MMBO) Gas (BCFG) NGL (MMBNGL)

F95 F50 F5 Mean F95 F50 F5 Mean F95 F50 F5 Mean

Sargelu TPS

Sargelu Continuous Oil 

 AU (20240261) 1.0 Oil 327 1,357 3,730 1,606 183 793 2,315 963 5 23 72 29

Total unconventional

 resources 327 1,357 3,730 1,606 183 793 2,315 963 5 23 72 29

Table 2. Jurassic Sargelu Formation assessment results.

[MMBO, million barrels of oil; BCFG, billion cubic feet of gas; MMBNGL, million barrels of natural gas liquids; TPS, total petroleum system; AU, assessment unit. 

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]

Assessment input data

Sargelu Continuous Oil AU

Minimum Mode Maximum Calculated mean

Potential production area of AU (acres) 13,037 3,911,100 13,037,000 5,653,712

Average drainage area of wells (acres) 80 160 220 153

Success ratios (%) 10 50 90 50

Average EUR (MMBO, oil; BCFG, gas) 0.04 0.08 0.2 0.086

Table 1. Key assessment input data for the unconventional assessment 

unit in the Jurassic Sargelu Formation, Iraq.

[EUR (estimated ultimate recovery per well), well drainage area, and success ratios 

are from U.S. shale-gas and shale-oil analogs. MMBO, million barrels of oil; BCFG, 

billion cubic feet of gas; AU, assessment unit; %, percent. The average EUR input is 

the minimum, median, maximum, and calculated mean]