GEOLOGICAL SURVEY CIRCULAR 881

Assessment of Undiscovered 

Conventionally Recoverable 

Petroleum Resources of the 

Arabian-Iranian Basin
Assessment of Undiscovered 

Conventionally Recoverable 

Petroleum Resources of the 

Arabian-Iranian Basin

By Charles D. Masters, H. Douglas Klemme, and 

Anny B. Coury

GEOLOGICAL SURVEY CIRCULAR881

A resource assessment and a brief 

description of the petroleum geology, 

including play distribution, that accounts 

for the largest accumulation of 

petroleum in the world.

1982
United States Department of the Interior

JAMES G. WATT, Secretary

Geological Survey

Dallas L. Peck, Director

Library of Congress catalog-card No. 82-600659

Free on application to Distribution Branch, Text Products Section, 

U. S. Geological Survey, 604 South Pickett Street, Alexandria, MA 22304
ASSESSMENT OF ENERGY RESOURCES

The World Energy Resources Program of the U.S. Geological Survey (USGS) 

intends to develop reliable and credible estimates of undiscovered petroleum 

resources throughout the world. Initial program efforts have focused on the major producing areas of the world in order to gain a broad geological understanding of the characteristics of petroleum occurrence for purposes of resource assessment as well as for analysis of production potential. Investigations of production 

potential are carried out in cooperation with other U.S. Government agencies; 

specifically, the studies of the main free world exporting nations, of which this 

study is a part, are carried out in cooperation with the Foreign Energy Supply 

Assessment Program of the Department of Energy.

The program seeks to investigate resource potential at the basin level, primarily through analogy with other petroleum regions and does not necessarily require, 

therefore, current exploration information that is commonly held proprietary. In 

conducting the investigations, we intend to build a support base of publicly available 

data and geologic syntheses against which to measure the progress of exploration and thereby validate the assessment. Most of these investigations will lead 

directly to quantitative resource assessments, which, like exploration, to be effective, must be ongoing processes taking advantage of changing ideas and data 

availability-the results produced are progress reports reflecting on a state of 

knowledge at a point in time. Because the program is coordinated with the USGS 

domestic assessment program and because both use similar techniques for assessment, the user can be assured of a thread of consistency permitting comparisons 

between the various petroleum basins of the world, including the United States, 

that have been assessed in the overall USGS program.

In addition to resource estimates, the program provides a regional base of 

understanding for in-country exploration analysis and for analysis of media reports 

regarding the exploratory success or failure of ventures in studied areas.

Other USGS publications relating to the assessment of undiscovered conventionally recoverable petroleum resources include the following:

Open-File Report 81-0986-Assessment of conventionally recoverable petroleum resources of Persian

Gulf basin and Zagros fold belt (Arabian-Iranian basin)

Open-File Report 81-1027-Assessment of conventionally recoverable petroleum resources, VolgaUrals basin, U.S.S.R.

Open-File Report 81-1142 - Assessment of conventionally recoverable petroleum resources of Indonesia 

Open-File Report 81-1143-Assessment of conventionally recoverable petroleum resources of northeast Mexico.

Open-File Report 81-1144-Assessment of conventionally recoverable petroleum resources of southeastern Mexico, northern Guatemala, and Belize

Open-File Report 81-1145-Assessment of conventionally recoverable petroleum resources of Trinidad 

Open-File Report 81-1146-Assessment of conventionally recoverable petroleum resources of Venezuela 

Open-File Report 81-1147-Assessment of conventionally recoverable petroleum resources of the West

Siberian basin and Kara Sea basin, U.S.S.R.

These reports are available from Open File Services Section, Branch of Distribution, USGS, Box 25425, Federal Center, Denver, CO 80225.
ACKNOWLEDGMENT

The resource assessment for this report was prepared in collaboration with the 

Resource Appraisal Group of the Branch of Oil and Gas Resources.
CONTENTS

Page

Abstract ______________________________________________ 1

Introduction ___________________________________________ 1

Regional geology _________________________________________ 2

Petroleum geology ________________________________________ 2

Resource assessment _______________________________________ 6

Selected references _______________________________________ 12

ILLUSTRATIONS

FIGURE 1. Location map for the Arabian-Iranian basin ___________________ 3 

2. Diagram of generalized stratigraphy of Arabian-Iranian basin showing position 

of five major plays and their associated reservoir rock, cap rock, and source 

rock _______________________________________ 4 

3-16. Graphs showing probability distribution of resource assessment of the ArabianIranian basin __________________________________ 9

3. Recoverable oil _____________________________ 9

4. Recoverable total gas _________________________ 9

5. Saudi Arabia recoverable oil ______________________ 9

6. Saudi Arabia recoverable total gas ___________________ 9

7. Iran recoverable oil ___________________________ 10

8. Iran recoverable total gas _______________________ 10

9. Iraq recoverable oil ___________________________ 10

10. Iraq recoverable total gas _______________________ 10

11. United Arab Emirates recoverable oil _________________ 11

12. United Arab Emirates recoverable total gas _____________ 11

13. Kuwait recoverable oil _________________________ 11

14. Kuwait recoverable total gas _____________________ 11

15. Oman recoverable oil __________________________ 12

16. Oman recoverable total gas _______________________ 12

TABLES

Page

TABLE 1. Assessment of undiscovered conventionally recoverable petroleum resources of

the Arabian-Iranian basin ____________________________ 7

2. Supplementary and comparative data supporting this resource assessment of the

Arabian-Iranian basin ______________________________ 7

3. Mean estimated undiscovered conventionally recoverable resources by country 

considering only that portion of a country included in the Arabian-Iranian 

basin _________________________________________ 8
Assessment of Undiscovered Conventionally

Recoverable Petroleum Resources of the

Arabian-Iranian Basin

By Charles D. Masters, 1 H. Douglas Klemme,2 and Anny B. Coury3

ABSTRACT

The estimates of undiscovered conventionally recoverable 

petroleum resources in the Arabian-Iranian basin at probability 

levels of 95 percent, 5 percent, and statistical mean are for oil 

(in billions of barrels): 72, 337, and 174; and for gas (in trillions 

of cubic feet): 299, 1792, and 849.

The occurrence of petroleum can be accounted for in five 

definitive geological settings or plays. The assessment of undiscovered resource potential assumes that the new discoveries 

will expand the occurrence of petroleum in these basic plays; no 

additional plays with significant petroleum potential were 

recognized. The five plays listed by geologic age are: (I) Upper 

Cretaceous and Tertiary, (II) Lower and Middle Cretaceous 

sandstone, (III) Lower and Middle Cretaceous limestone, (IV) 

Jurassic, and (V) Permian. The Permian play, located in the 

south-central Arabian Gulf region and extending northeastsouthwest from southern Iran to the Ar Rub' al Khali in Saudi 

Arabia, accounts for over four-fifths of the mean estimate of undiscovered gas. The remainder of the gas is divided about equally among the other four plays. The Jurassic play, located on the 

south side of the Arabian Gulf, accounts for slightly less than 

one-third of the estimated undiscovered oil, which is split equally between Saudi Arabia and Iraq. The Lower and Middle 

Cretaceous limestone play is located in the southern Gulf region 

and accounts for about one-fifth of the undiscovered oil, most of 

which is located in Saudi Arabia and the remainder in the 

United Arab Emirates. The Lower and Middle Cretaceous sandstone play is centralized in Kuwait at the head of the Arabian 

Gulf with significant potential extending to the northwest in 

Iraq; the play accounts for about one-third of the undiscovered 

oil, the great majority of which is estimated to be in Iraq with 

the remainder divided between Saudi Arabia and Kuwait. The 

upper Cretaceous-Tertiary play is located in the Zagros fold belt 

of Iran and Iraq and accounts for about one-fifth of the undiscovered oil.

Estimation of the resources in this vast area is hindered by 

lack of data from certain broad regions, abandoned wells, and 

several significant dry holes, but the regional geology is well 

enough reported that geologic projections can be made and inferences from selected wells can be drawn to permit estimation 

of the resource potential.

'U.S. Geological Survey, Reston, Va.

2Weeks Petroleum Corporation, Westport, Conn.

3U.S. Geological Survey, Denver, Colo.

INTRODUCTION

The investigation of the petroleum resource 

potential of the Arabian-Iranian basin was performed under contract to Weeks Petroleum Corporation (Contract No. 14-08-001-17919) by Dr. 

H. Douglas Klemme. Sources of data include 

Petroconsultants Ltd. and published literature. 

The resource assessment was conducted by the 

Resource Appraisal Group (RAG) of the USGS, 

Branch of Oil and Gas Resources, following the 

standard procedures developed since 1974 for 

domestic petroleum resource analysis. The technique, briefly, requires study of a given area, paying 

particular attention to the geologic factors controlling the occurrence, quality, and quantity of the 

petroleum resource. Standardization of critical 

elements of the investigations is achieved by the 

preparation of data forms for each basin, which 

call for specific volumetric, areal, and rock-quality 

measurements, as well as the determination of 

basin analogs for comparison purposes. In addition, finding-rate histories and projections are constructed, when possible. From these data and 

analyses, various analytical techniques are used to 

calculate a set of resource numbers.

The assessment process itself is subjective; the 

results of the geological investigation and of the 

resource calculations are presented to a team of 

USGS assessment specialists, who make their personal estimates conditional upon recoverable 

resources being present. Initial assessments are 

made for each of the assessed provinces as follows:

(a) A low-resource estimate corresponding to 

a 95 percent probability of more than 

that amount; this estimate is the 95th 

fractile (F95).

(b) A high-resource estimate corresponding to 

a 5 percent probability of more than
that amount; this estimate is the 5th 

fractile (F5).

(c) A modal (most likely) estimate of the 

quantity of resource associated with 

the greatest likelihood of occurrence.

The individual estimates are then posted and 

averaged, and the results debated from the 

perspective of the personal experiences of the individual assessors; a second and third iteration of 

the procedure may follow depending on consensus. 

If no commercial oil has been heretofore discovered in the basin, then a marginal probability is 

subjectively assessed reflecting the probability 

that any commercial oil will ever be discovered.

The results of the final estimates are averaged, 

and those numbers are computer processed using 

probabilistic methodology (Crovelli, 1981) to show 

graphically the resource values associated with a 

full range of probabilities and to determine the 

95th fractile, the 5th fractile, and the mean, as well 

as other statistical parameters.

REGIONAL GEOLOGY

The Arabian-Iranian basin lies on the northeast 

flank of the Arabian craton (fig. 1), which, along 

with a proximal ocean margin of the Tethys seaway, provided the physical framework that controlled the sedimentary processes during 

Paleozoic, Mesozoic, and most of Cenozoic time. 

The tectonic behavior of the area during most of 

this time was as a modified passive margin. The 

continental mass, of which the Arabian Shield and 

associated sedimentary basins were a part, moved 

from the general South Pole region to the Equator 

(Scotese and others, 1979). Clearly, in the resultant closing of the Tethys, subduction of the oceanic 

crust must have taken place north of the advancing 

Arabian craton, but that process does not appear 

to have affected the Arabian-Iranian basin 

sedimentation (except perhaps in whatever role it 

may have played in the crustal movements that 

controlled sedimentation) until the Late 

Cretaceous, at which time ophiolites were obducted in the area of Oman (Glennie and others, 

1974, p. 393), and clastic sediment derived from 

the north and east was deposited as wedges (Murris, 1980, p. 598). In latest Tertiary, coincident 

with the final closing of this portion of the Tethys, 

the Zagros Mountains were uplifted and thrust to 

the south, shedding sediments into a Neogene foreland basin over the earlier accumulated sedimentary rocks on the passive margin of the ArabianIranian basin. In the process, the foreland belt of 

sediments was tightly folded into a series of long 

linear northwest-trending parallel folds (Murris, 

1980).

The location of the Arabian craton near the 

Equator during the Phanerozoic provided optimal 

environmental conditions for carbonate sedimentation on platform areas and for deposition of 

organic shales in marginal continental slope and 

deep water marine environments. Periodically, the 

dominant carbonate/shale depositional environment received an influx of siliceous clastic 

sediments from the shield or experienced such 

restrictions of water movement that evaporite 

deposition ensued. The combination of favorable 

depositional conditions for reservoir rock, seals, 

and source rock, in a tectonic environment of large 

anticlinal structures caused initially by gentle basement crustal movements and later by continental 

plate collision, produced the Arabian-Iranian basin 

petroleum province.

PETROLEUM GEOLOGY

The Arabian-Iranian basin is the richest 

petroleum province in the world. Other basins, 

such as the Los Angeles basin, are richer per unit 

volume, but their volumes are relatively small. The 

individual factors that have contributed to this 

great accumulation are well known, but just how 

they all fit together to result in such extraordinary 

quantities of trapped, recoverable petroleum can 

only be hypothesized.

Essential factors in petroleum occurrence and 

entrapment are reservoir rock, source rock, trap, 

seal, and the timing and geometry of their interrelationship. Major oil and gas occurrence in the 

Arabian-Iranian basin is associated with five time 

periods during which the relationship between 

essential geologic factors was satisfactory for 

significant oil generation and entrapment; it is 

possible to define five discrete plays that account 

for most of the petroleum occurrence: (I) Upper 

Cretaceous and Tertiary, (II) Lower and Middle 

Cretaceous sandstone, (III) Lower and Middle 

Cretaceous limestone, (IV) Jurassic, and (V) Permian (fig. 2). Each of the plays is geographically 

restricted to areas where all of the essential factors were present, and for the most part the boundaries of these areas are predictable.

The Upper Cretaceous and Tertiary play (I) is 

concentrated in the Zagros fold belt of Iran and
40

35'

TURKEY

EXPLANATION

~ SYRIA

Assessment region boundary 

Non-prospective area within

assessment region 

Oil field 

Gas field

-\j Play boundaries (see figure 2)

I Cretaceous and Tertiary

II Cretaceous sandstone

III Cretaceous limestone

IV Jurassic

V Permian

I I 

a/eozoic and Mesozoic 0 50100 200 3DO KILOMETERS

outcrop

Neutra 

Zone I

NITE

ARAB 

EMIRATES/

SAUDI 

ARABIA

Mesozoic 

outcrop

PEOPLES' DEMOCRATIC 

YEMEN ARAB REPUBLIC OF YEMEN

REPUBLIC

The boundary lines on this chart are for purposes of illustration 

only, and do not necessarily reflect the position or views of the 

United States with respect to the boundaries involved.

Modified from Petroconsultants

FIGURE 1. - The Arabian-Iranian basin assessment region including the locations of most oil and gas fields and petroleum play limits.
w/sw E/NE

EXPLANATION

j Conglomerate

:'] Sandstone

j Shale

l Bituminous shale

l Marl

] Limestone

Dolomite

Evaporite deposit

Volcanic rock

FIGURE 2.-Generalized stratigraphy of Arabian-Iranian basin showing position of five major plays (Roman numerals I-V) and their associated reservoir rock (R), cap rock (C), 

and source rock (S). Note source rock for play V is Cretaceous and Jurassic. (Modified from Murris, 1980, p. 598.)
Iraq. Structures that derive from the collision tectonics associated with the closing of the Tethys 

trend are arrayed northwest-southeast in the area 

northeast of the Arabian Gulf and the Euphrates 

River valley. More than three-fourths of the 

petroleum potential lies in Iran. The reservoir rock 

distribution is controlled by fractures associated 

with the Zagros folding. Regional evaporites effectively seal the traps. The major reservoir rock unit 

is the Asmari limestone of Oligocene to early 

Miocene age. The Asmari is a neritic limestone, 

with shelly intercalations, that is locally altered to 

dolomite and anhydrite. The degree of permeability of the reservoir rocks is due entirely to fracturing; porosity ranges from 9 to 14 percent. The 

Asmari is 100 to 1,500 ft thick, and its productive 

section commonly comprises 25 to 75 percent of 

the overall thickness. The Asmari is absent in the 

southern fold belt owing to facies change or nondeposition, but, there, other fractured Tertiary 

limestones provide substitute reservoirs. The 

source rocks are the same Cretaceous and possibly 

Jurassic shales that sourced the Arabian platform 

facies. Here, the oil has migrated vertically 

through fractures to a zone of principal concentration in the Asmari. To the southeast, the source 

rocks are absent because of facies change or 

nondeposition, just as is the Asmari limestone. 

Throughout the fold-belt region, large structures 

are pervasive; hence a principal limitation on oil occurrence appears to be source rock distribution. In 

the southern Zagros, however, where Jurassic and 

Cretaceous source rock are absent, the lower 

Paleozoic source rocks have generated large quantities of natural gas, but the gas fields are yet to be 

developed.

The Lower and Middle Cretaceous sandstone play (II) is located centrally in the ArabianIranian basin; Kuwait and the giant field Burgan 

are in the middle of the play area. The boundaries 

of the controlling reservoir sandstone facies extend through the Neutral Zone into Saudi Arabia, 

northwest into Iraq and northeast into the fold belt 

of Iran. The reservoir rock consists of pure quartz 

sandstone, clean, well sorted, medium- to coarsegrained, commonly glauconitic, with minor secondary cementation and shale interbeds. 

Porosities range from 10 to 30 percent, and 

permeabilities are commonly in excess of one darcy. Thickness of the delta sandstone wedge varies 

from 200 to 1,200 ft. Source rocks are most likely 

facies equivalent, prodelta shales deposited

seaward (east) of the delta. Traps are found in upward continuations of the same structural lineaments responsible for oil accumulation in the 

Jurassic play in Saudi Arabia. They are basement 

controlled, generally north-south oriented, and 

commonly enhanced by salt flowage. The limiting 

factor in Cretaceous oil occurrence appears to be 

the distribution of reservoir facies and structural 

lineaments. Though the present producing delta 

facies appears to have been delineated, it is possible that the poorly explored regions to the northwest in Iraq could contain one or more deltas; it is 

not known from the literature on Iraq available to 

us whether or not anticlinal traps exist in this 

region of Iraq.

The Lower and Middle Cretaceous limestone 

play (III) is concentrated in southeastern Saudi 

Arabia, the United Arab Emirates, and in northern 

Oman. The reservoir rock consists of mixed carbonate facies, including packstone, wackestone, 

lime mudstone, and bioclastic fore-reef debris 

deposited under general cyclic conditions between 

slope and carbonate platform edge localities. The 

reservoir units, up to 3,000 ft thick, generally have 

been recrystallized and possess 10 to 20 percent 

porosity and 0 to 500 md permeability. The age of 

the source rocks is uncertain, but the petroleum 

probably was generated in shale facies that are 

equivalent in age to the reservoir rock. Traps are 

associated with salt diapirs. Cap rocks consist of 

shale and evaporite that are common throughout 

the area. Petroleum occurrence is strongly facies 

and structure controlled. Field sizes are modest by 

Middle East standards and are limited generally by 

trap size.

The Jurassic play (IV) occupies a large area in 

northeast Saudi Arabia, extending northeast to approximately the Iranian border, southeast to the 

United Arab Emirates, and north to Kuwait (fig. 

1). The reservoir rocks, principally in the Upper 

Jurassic Arab zone (fig. 2), are commonly poorly 

cemented bioclastic, oolitic, limey sandstones with 

high porosity (20-30 percent) and permeability 

(several hundred millidarcies). Off structure, these 

same units are strongly altered by recrystallization 

to dolomite and anhydrite. Pay zones range from 

30 to 200 ft, averaging 90 ft. The Arab D zone contains the principal reserves of the Jurassic section. 

Production per well averages 12,000 b/d. Source 

rocks include the Hanifa and Sargelu formations of 

Middle and Late Jurassic age. The source rocks interfinger with the reservoir rocks at the southern
end of their distribution, but for the most part they 

were deposited to the northeast beyond the edge of 

the platform carbonate bank; the oil migrated updip to the south and west into the early formed 

structures that controlled reservoir deposition. 

The Qatar-south fars arch was probably a limiting 

factor in migration to the south. The dominant 

traps for Upper Jurassic oil are north-trending, 

broad anticlinal structures that are controlled by 

gentle basement block movements, some of which 

are enhanced by salt movement. The size of the 

traps and the fact that their early growth contributed to favorable reservoir rock sedimentation 

have resulted in a close grouping of the largest oil 

fields in the world. The largest of these fields is 

Ghawar with original reserves in excess of 80 

billion barrels of oil (BBO). Another significant factor in Jurassic oil occurrence is an overlying 

regional evaporite seal of Late Jurassic age, the 

Hith formation (fig. 2). Limiting factors in Jurassic 

oil occurrence are reservoir facies distribution, 

structure size, and migration paths from principal 

source rock areas.

The Permian play (V) is located in the southcentral Arabian Gulf region and extends in a 

southwest-northeast direction from the Arabian 

platform to the Zagros fold belt (fig. 1). Reservoir 

rocks are in the Khuff formation, a platform carbonate with interbedded evaporite deposits and a 

basal conglomerate (fig. 2). Thicknesses of the 

Khuff range up to 2,000 ft. Reservoir rocks in the 

Zagros fold belt are fractured. The source rocks 

are unconformably underlying widespread Ordovician and Silurian graptolitic shales, up to 3,000 ft. 

thick, which have been deeply buried and thermally 

matured to the gas generation stage. The distribution of the Hercynian unconformity that permits 

juxtaposition of lower Paleozoic source rock with 

upper Paleozoic reservoir rock controls the play 

boundaries. In general, the critical uncomformity 

distribution follows the Qatar-south fars arch (fig. 

1). The traps on the shield are basement-controlled 

anticlines trending north, and some are enhanced 

by the movement of Cambrian salt. The traps in 

the Zagros fold belt are long, linear anticlines. 

Seals are provided by the interbedded Permian 

evaporite deposits as well as by regional Triassic 

evaporite deposits (fig. 2). Limiting factors in the 

play appear to be (1) the distribution of the critical 

unconformity that brings source and reservoir in 

juxtaposition, (2) structure size, and (3) drilling 

depth.

RESOURCE ASSESSMENT

Introduction.-The location of the ArabianIranian basin assessment region is shown in figure 

1. U.S. Geological Survey (USGS) estimates of oil 

and gas resources in this region are given in table 1 

and figures 3 through 16. Supplementary data of 

interest in analyzing these estimates are supplied 

in table 2.

At the time of the assessment, the country-bycountry distribution of resources was estimated on 

the basis of play distribution by country and on the 

proportion of total estimated resources assigned to 

each play (table 3). This estimate was considered to 

be an allocation of the mean quantity of resources, 

and a curve was fitted using the mean estimate to 

determine the resource values associated with a 

full range of probabilities for each country from 

which we selected, for reporting, a 95 to 5 percent 

probability range and a statistical mean, standard 

deviation (S.D.), median, and mode (see figs. 3-16).

Commodities assessed.-The assessment of 

undiscovered conventionally recoverable 

petroleum resources includes those resources that 

can be extracted using conventional methods 

assuming a continuation of present economic and 

technologic conditions (Dolton and others, 1981). 

The assessment does not include inferred resources, which may yet be found in new pay zones 

or extensions of existing fields. Also excluded from 

the assessment, even if present, are unconventional resources such as heavy oil deposits, tar 

deposits, and oil shales, as well as gas in low 

permeability (tight) reservoirs, gas occluded in 

coal, gas in geopressured reservoirs and brines, 

and natural gas hydrates.

Factors in the assessment.-Assessing 

petroleum resources in the Middle East is limited 

by the lack of analog basins that can be used for 

comparison. Considering the quantity of petroleum, there is no comparable area on the face of 

the Earth. Given the absence of analogs, one must 

rely on judgments as to maturity of exploration, 

but this method is limited by the paucity of published data on dry holes and the uncertainty of 

reserve data. Nonetheless, much pertinent data 

have been published, and broad levels of field sizes 

can be determined. Furthermore, because large 

fields are so dominant, some judgments as to exploration maturity for giant and supergiant fields 

can be considered. Clearly, there is another lower 

level of field sizes not included in assessment
TABLE 1.- Assessment of undiscovered conventionally recoverable petroleum resources of the Arabian-Iranian basin

[Resource assessment by USGS as of 4/1/81; see also figures 3 through 16.]

Crude oil Natural gas in trillions of cubic feet 

in billions of barrels (Tcf) and billions of barrels of oil 

(BB) equivalent (BBOE) @ 6,000 cuft/bbl.

Estimate

Low High Mean Low

Foe1 Fe1 Fas1

72 337 174 Tcf 299

BBOE 50

High

1,792 

299

Mean

849 

142

Distribution by country of undiscovered conventionally recoverable petroleum resources in Arabian-Iranian basin2

Crude oil (BB) Natural gas (Tcf)

Saudi Arabia

Iran

Iraq

UAE

Kuwait

Oman

Qatar

Bahrein

Low

24

11

32

2

2

1

High

Fa1 

111

50

150

13

9

4

Mean

57

26

78

7

4

2

0

Low

F-,6 1

71

197

11

13

4

3

High

Fs1

423

1183

66

80

23

17

Mean

201

560

31

38

11

8

0

72 337 174 299 1792 849

1 F95 denotes the 95th fractile; the probability of more than the amount F95 is 95 percent. F5 is defined similarly.

2 Neutral Zone potential of 2.2 BB of oil and 17 Tcf of gas is equally divided between Kuwait and Saudi Arabia.

3 Not included in totals.

TABLE 2. -Supplementary and comparative data supporting this resource assessment of the Arabian-Iranian basin1

Crude oil Natural gas 

in billions of barrels (BB) in trillions of cubic feet (Tcf)

Cumulative production to 1/1/79

102-108 +

Identified Reserves to 1/1/793

Demonstrated ____________ 356 517 

Inferred________________ 59 563

415 1,080

BBOE 180

Original recoverable resources (ultimate) of the basin

Oil Gas (BBOE)

Cumulative production ________ 108 + 2

Identified reserves __________ 415 1,080

Undiscovered resources _______ 174 849

697 1,929 +

BBOE 322 +

Total oil and gas 1,019 + BBOE

Basin richness

Original resources 1,019 + BBOE = 5600 000 BOE/miS 

Basin volume 1,700,000 mi3

1 Cumulative production and reserves are composited estimates from various sources.

2 Quantity positive but data unavailable.

3 Follows terminology outlined in USGS Circular 831. Demonstrated is equivalent to API Proved plus Indicated additional. Inferred represents anticipated reserves growth in existing fields.
TABLE 3. -Mean estimated undiscovered conventionally recoverable resources by country considering only that portion of a country 

included in the Persian Gulf basin and Zagrosfold belt (Arabian-Iranian basin) 1

OIL in billions of barrels (BB) 

Mean estimate from this study: 174 BB

GAS in trillions of cubic feet (Tcf) 

Mean estimate from this study: 849 Tcf

I.

II.

III.

IV.

V.

Percentage 

Plays distribution 

by play

Upper Cretaceous and 20 

Tertiary.

Lower and Middle 32 

Cretaceous (sandstone).

Lower and middle 19

Cretaceous (limestone).

Jurassic 28

Permian 1

Distribution Distribution 

of mean Percentage of mean Percentage 

estimation distribution estimation distribution 

by play by country2 by country by play

35 In 

Iq

55 Iq 

On

SA

K

(NZ

33 SA

UAE

49 Iq 

SA

2 SA

On

75 

25

80 

3 

9

8 

4)

80

20

50 

50

80

20

26.3 4

8.7

44.0 3 

1.6 

5.0

4.4 

(2.2)

26.4 4

6.6

24.5 5 

24.5

1.6 84

.4

Distribution 

of mean Percentage 

estimation distribution 

by play by country

34 In 

Iq

25 Iq 

On

SA

75 

25

90 

3

7

Distribution 

of mean 

estimation 

by country

25 

9

22 

1 

2

Q<13 <13

34 SA

UAE

43 SA 

UAE

713 In

SA

UAE

K

80

20

60 

40

75

20.5

2

1.5

Q<13 

On 1.0

(NZ 2.0)

27

7

26

17

535

146

14

11

7

(17)

100 174 174 100 849 849

1 Neutral Zone (NZ) potential is equally divided between Kuwait and Saudi Arabia and is included in those totals.

2 SA=Saudi Arabia, On=Oman, Iq=Iraq, UAE= United Arab Emirates, In=Iran, Q=Qatar, NZ=Neutral Zone, K=Kuwait.

3 Not included in totals.

thinking today that will likely sustain Middle East 

energy needs for decades to come; the geology 

relating to possible smaller field sizes is insufficiently understood for those resources to be 

knowingly included in this assessment. It does not 

follow that the assessed quantities of oil and gas 

necessarily will change significantly with new data 

on smaller fields, because the quantities of oil and 

gas contained in the large fields still will dominate 

the estimates.

Though most areas in the Arabian-Iranian basin 

region have been significantly explored, vast areas 

still remain almost undrilled, especially to the 

south in the Ar Rub' al Khali and to the north in 

southern Iraq. In addition, deep target objectives 

have not been adequately tested in several areas. 

We believe, however, that enough is known of 

regional geology to make reasoned projections. To 

the south, in the Ar Rub' al Khali, we assume the 

potential for some extension of the Jurassic limestone play and for significant discovery of Permian

gas. We are concerned, however, and have factored into the assessment probability considerations relative to the changing reservoir facies, the 

lack of known structures, and the increasing depth 

of the play. In Iraq, we are concerned about the 

absence of known reservoir facies and structure required for a continuation of the Lower and Middle 

Cretaceous sandstone play but assume that the 

probabilities are good for play extension. Because 

of the limited data available in Iraq, we possibly 

have underestimated the deep-gas possibilities, but 

it is not likely that gas fields, if present, will be 

developed any time soon. A final problem area in 

Middle East assessment is inferred gas reserves. 

We have excluded them from the assessment of undiscovered resources but consider that the potential for undeveloped Permian gas on Ghawar and 

other major fields in appropriate areas is of 

supergiant proportions and must be clearly considered in any analysis of future gas supply in the 

area.

8
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MODE 

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MflTES 

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 113.52 

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 213.75 

337.00 

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0.0 65.0 130.0 195.0 260.0 325.0 390.0 455.0

BILLION BflRRELS RECOVERflBLE OIL

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BILLION BflRRELS RECO

FIGURE 3.-Arabian-Iranian basin recoverable oil as assessed April 1,1981. FIGURE 5.-Arabian-Iranian basin-Saudi A

April 13, 19

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5TIMRTES 

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TRILLION CUBIC FEET RECOVERRBLE GflS

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TRILLION CUBIC FEET 

FIGURE 4.-Arabian-Iranian basin recoverable total gas as assessed April 1,1981. FIGURE 6.-Arabian-Iranian basin-Saudi A

April 13
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FIGURE 7.-Arabian-Iranian basin-Iran recoverable oil as assessed April 13, 1981. FIGURE 9.-Arabian-Iranian basin-Iraq recov

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TRILLION CUBIC FEET RECOVERRBLE 6RS

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TRILLION CUBIC FEET 

FIGURE 8.-Arabian-Iranian basin-Iran recoverable total gas assessed 

April 13,1981.

FIGURE 10.-Arabian-Iranian basin-Iraq reco

April 13,1981.
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BILLION BflRRELS RECOVERflBLE OIL

20.0 22.5

FIGURE 11.-Arabian-Iranian basin-United Arab Emirates recoverable oil as 

assessed April 13, 1981.

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FIGURE 13.-Arabian-Iranian basin-K

April 13, 

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32.80 

13.40 

22.72 

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47.34 

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22.33

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FIGURE 12.-Arabian-Iranian basin-United Arab Emirates recoverable total gas as 

assessed April 13,1981.

FIGURE 14.-Arabian-Iranian basin-Kuw

April 13, 
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BILLION BflRRELS RECOVERflBLE OIL

MflTES 

2.02 

1.81 

0.84 

1.32 

1.81 

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1.46 

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7.2 8.

FIGURE 15.-Arabian-Iranian basin-Oman recoverable oil as assessed 

April 13, 1981.

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6.81 

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TRILLION CUBIC FEET RECOVERflBLE

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GflS

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FIGURE 16.-Arabian-Iranian basin-Oman recoverable total gas as assessed 

April 13, 1981.

SELECTED REFERENCES

Crovelli, R. A., 1981, Probabilistic methodology for oil and gas

resource appraisal: U.S. Geological Survey Open-File

Report 81-1151, 77 p. 

Dolton, G. L., and others, 1981, Estimates of undiscovered

recoverable conventional resources of oil and gas: U.S.

Geological Survey Circular 860, 87 p.

Glennie, K. W., and others, 1974, Geology of the Oman mountains: Ned. Geol. Mijnbouwkd, Genoot., Verh., v. 31, 423 p.

Murris, R. J., 1980, Middle East and stratigraphic evolution and 

oil habitat: American Association Petroleum Geologists 

Bulletin, vol. 64, no. 5, p. 597-618.

Scotese, C. R., Bambach, R. K., Barton, C., Van Der Voo, R., 

and Ziegler, A. M., 1979, Paleozoic base maps: The Journal 

of Geology, v. 87, no. 3, p. 217-277.

12