Petroleum Geology and Total Petroleum Systems of the Widyan Basin and Interior Platform of Saudi Arabia and Iraq

By James E. Fox and Thomas S. Ahlbrandt

U.S. Geological Survey Bulletin 2202–E

U.S. Department of the Interior U.S. Geological Survey

U.S. Department of the Interior Gale A. Norton, Secretary

U.S. Geological Survey Charles G. Groat, Director

VVersionersion 1.0, 1.0, 2001 2002

This publication is only available online at: http://geology.cr.usgs.gov/pub/bulletins/b2202-e

Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply endorsement by the U.S. Government

Manuscript approved for publication July 24, 2002 Published in the Central Region, Denver, Colorado Graphics by Margarita Zyrianova Photocomposition by Norma J. Maes Edited by L.M. Carter

Contents

Foreword...... 1 Abstract ...... 1 Geology of the Widyan Basin–Interior Platform Province...... 2 Structural Setting ...... 2 Petroleum Occurrence ...... 2 Paleozoic Qusaiba/Akkas/Abba/Mudawwara Total Petroleum System ...... 4 Stratigraphy ...... 6 Source Rocks ...... 7 Stratigraphy ...... 7 Petroleum Geochemistry...... 11 Maturation ...... 11 Migration ...... 11 Reservoir Rocks ...... 11 Unayzah Formation...... 11 Pre-Unayzah Formations ...... 12 Hydrocarbon Traps...... 13 Structural Traps ...... 13 Stratigraphic Traps...... 14 Seals and Overburden Rocks ...... 15 Timing and Critical Events...... 15 Jurassic Gotnia/Barsarin/Sargelu/Najmah Total Petroleum System...... 16 Source Rocks ...... 16 Stratigraphy...... 16 Petroleum Geochemistry...... 17 Oil to Source Rock Correlation...... 20 Maturation ...... 20 Migration...... 21 Reservoir Rocks ...... 21 Hanifa Formation, Tuwaiq Mountain Formation, and Arab Formation ...... 21 Sargelu Formation, Naokelekan Formation, and Najmah Formation...... 22 Hydrocarbon Traps...... 23 Platform Horst/Graben-Related Oil Assessment Unit (20230201)...... 23 Basinal Oil and Gas Assessment Unit (20230202)...... 24 Seals and Overburden Rocks ...... 24 Timing and Critical Events...... 24 Petroleum Assessment...... 24 Summary...... 25 References Cited ...... 25

Figures

1–4. Maps showing: 1. Structural provinces of Arabian Peninsula...... 3 2. Widyan Basin–Interior Platform Province; Paleozoic Qusaiba/Akkas/Abba/Mudawwara Total Petroleum System, and Horst/Graben-Related Oil and Gas Assessment Unit ...... 4 3. Major tectonic regions and petroleum fields of Iraq...... 6 4. Major petroleum fields in western Arabian Gulf region ...... 7 5. Paleozoic stratigraphic column of Jordan, Southwestern Desert of Iraq, and northern Saudi Arabia...... 9

III

6. Subsurface composite reference section for the Qalibah Formation in central Saudi Arabia...... 10 7. Map showing time at which significant volumes of oil were generated from Lower Silurian strata ...... 12 8. Map showing present-day levels of thermal maturity of Lower Silurian source rock in Arabian Peninsula ...... 13 9. Diagrammatic burial history model of Akkas-1 well of western Iraq ...... 14 10. Diagrammatic model for migration of hydrocarbons generated from basal Qusaiba “hot shale” in central Saudi Arabia ...... 15 11. Events chart showing events critical to petroleum accumulation in the Paleozoic Qusaiba/Akkas/Abba/Mudawwara Total Petroleum System ...... 16 12. Map showing Widyan Basin–Interior Platform Province, Jurassic Gotnia/ Barsarin/Sargelu/Najmah Total Petroleum System, Platform Horst/Graben- Related Oil Assessment Unit, and Basinal Oil and Gas Assessment Unit ...... 17 13. Map showing Gotnia, Arabian, and Southern Arabian Gulf Basins in which Jurassic hydrocarbon source rocks accumulated ...... 19 14. Correlation chart of Jurassic strata of western and southern Arabian Gulf region 20 15. Graph showing burial history of the Hanifa and Sargelu Formations ...... 21 16. Map showing distribution of the Jurassic Tithonian evaporite seal lithofacies in Arabian Gulf region ...... 22 17. Events chart for that part of the Jurassic Gotnia/Barsarin/Sargelu/Najmah Total Petroleum System that lies in Arabian Basin...... 23

Tables

1. Widyan Basin–Interior Platform, Province 2023, Assessment Results Summary— Allocated Resources ...... 5 2. Paleozoic Qusaiba/Akkas/Abba/Mudawwara, Total Petroleum System 202301— Assessment Results Summary...... 8 3. Jurassic Gotnia/Barsarin/Sargelu/Najmah, Total Petroleum System 202302— Assessment Results Summary...... 18

IV

Petroleum Geology and Total Petroleum Systems of the Widyan Basin and Interior Platform of Saudi Arabia and Iraq

By James E. Fox1 and Thomas S. Ahlbrandt

Foreword (3) generation, migration, and accumulation of hydrocarbons; and (4) preservation of hydrocarbons. This report was prepared as part of the World Energy A numeric code identifies each region, province, total Project of the U.S. Geological Survey. For this project, the world petroleum system, and assessment unit. These codes are uniform was divided into eight regions and 937 geologic provinces, throughout the World Energy Project and identify the same type which were then ranked according to the discovered oil and gas of entity in any of the publications. The code is as follows: volumes within each (U.S. Geological Survey World Energy Example Assessment Team, 2000). Of these, 76 “priority” provinces Region, single digit 2 (exclusive of the U.S. and chosen for their high ranking) and 26 Province, three digits to the right of region code 2023 “boutique” provinces (exclusive of the U.S. and chosen for their Total petroleum system, two digits to the right of anticipated petroleum richness or special regional economic province code 202301 importance) were selected for appraisal of oil and gas resources. Assessment unit, two digits to the right of The petroleum geology of these priority and boutique provinces petroleum system code 2023 01 is described in this series of reports. The codes for the regions and provinces are listed in U.S. The purpose of the World Energy Project is to assess the Geological Survey World Energy Assessment Team (2000), and quantities of oil, gas, and natural gas liquids that have the poten- in Klett and others (1997). tial to be added to reserves within the next 30 years. These vol- Oil and gas reserves quoted in this report are derived from umes either reside in undiscovered fields whose sizes exceed the Petroconsultants’ Petroleum Exploration and Production data- stated minimum-field-size cutoff value for the assessment unit base (Petroconsultants, 1996) and other area reports from Petro- (variable, but must be at least 1 million barrels of oil equivalent) consultants, Inc., unless otherwise noted. or occur as reserve growth of fields already discovered. Figure(s) in this report that show boundaries of the total The total petroleum system constitutes the basic geologic petroleum system(s), assessment units, and pods of active source unit of the oil and gas assessment, and is defined as including all rocks were compiled using geographic information system (GIS) genetically related petroleum that occurs in shows and accumula- software. Political boundaries and cartographic representations tions (discovered and undiscovered) that (1) has been generated were taken, with permission, from Environmental Systems by a pod or by closely related pods of mature source rock, and Research Institute’s ArcWorld 1:3 million digital coverage (2) exists within a limited mappable geologic space, along with (1992), have no political significance, and are displayed for gen- the other essential mappable geologic elements (reservoir, seal, eral reference only. Oil and gas field centerpoints, shown in and overburden rocks) that control the fundamental processes of these figures, are reproduced, with permission, from Petrocon- generation, expulsion, migration, entrapment, and preservation sultants, 1996. of petroleum. The minimum petroleum system is that part of a total petroleum system encompassing discovered shows and accumulations along with the geologic space in which the vari- Abstract ous essential elements have been proven by these discoveries. An assessment unit is a mappable part of a total petroleum Two total petroleum systems are associated with the system in which discovered and undiscovered fields constitute a Widyan Basin–Interior Platform Province in northern Saudi Ara- single, relatively homogeneous population such that the chosen bia and western Iraq. In the Paleozoic Qusaiba/Akkas/Abba/ methodology of resource assessment based on estimation of the Mudawwara Total Petroleum System, which consists of one number and sizes of undiscovered fields is applicable. A total assessment unit—the Horst/Graben-Related Oil and Gas Assess- petroleum system may equate to a single assessment unit, or it ment Unit—high-gravity, low-sulfur crude oil, as well as natural may be subdivided into two or more assessment units if each unit gas, occurs in horst/graben-related traps that formed prior to, is sufficiently homogeneous in terms of geology, exploration during, and after Hercynian deformation (Carboniferous). The considerations, and risk to assess individually. source of oil and gas is from organic-rich marine shale at the A graphical depiction of the elements of a total petroleum base of the Silurian sedimentary sequence (Qusaiba, Akkas, system is provided in the form of an events chart that shows the Mudawwara, and Abba Formations) that was deposited under times of (1) deposition of essential rock units; (2) trap formation; dysoxic to anoxic conditions in an intra-shelf basin located north ______of the Central Arabian Arch. Onset of oil generation in Iraq 1South Dakota School of Mines and Technology, Rapid City, S.Dak. began about 250 million years ago (Ma) and in eastern Saudi

1 Arabia about 160 Ma, reaching peak generation, expulsion, terranes define the northern and eastern margins of the plate and migration, and entrapment during the Jurassic Period. In Saudi form the Taurus thrust suture zone of southeastern Turkey and Arabia, petroleum migrated into fluvial and eolian quartzose the Zagros thrust suture zone that trends from Iraq to Oman. sandstones of the Carboniferous-Early Permian Unayzah Forma- The southern and western margins are rift basins of the Mediter- tion that overlies the Hercynian unconformity, filling in rifts and ranean Sea, Red Sea, and Gulf of Aden. Structural provinces of half-grabens to thicknesses ranging to more than 400 meters. the Arabian Peninsula include the Arabian Shield, surrounded by Combined stratigraphic-structural traps exist where the Unayzah the Arabian shelf and Arabian platform. Various basins and Formation is the reservoir, as is the case in central Saudi Arabia. uplifts of these provinces are shown in figure 1. Oil and gas are sealed in those reservoirs by overlying tight car- The focus of this report is on the Widyan Basin-Interior bonate-evaporite strata, and by subunconformity pinchouts of Platform Province (2023) and its two associated total petroleum Pre-Unayzah clastic reservoir units against impermeable facies. systems—the Paleozoic Qusaiba/Akkas/Abba/Mudawwara Total In Iraq, reservoirs are sandstones of the Ordovician Upper Kha- Petroleum System (202301) and the Jurassic Gotnia/Barsarin/ bour and Silurian Akkas Formations. Over most of the South- Sargelu/Najmah Total Petroleum System (202302). The prov- western Desert of Iraq, Lower Silurian shale is a seal for ince is situated on the Arabian Peninsula (fig. 2), adjacent to, and hydrocarbons in the underlying Ordovician Khabour Formation. east and north of, the Interior Homocline–Central Arch Province The Jurassic Gotnia/Barsarin/Sargelu/Najmah Total Petro- (2020). Undiscovered resources allocated to this province are leum System has two assessment units: the Platform Horst/Gra- shown in table 1. ben-Related Oil Assessment Unit and the Basinal Oil and Gas The Widyan Basin is separated from the Tabuk Basin on the Assessment Unit. All reservoirs are in the Upper Jurassic Naj- west by the low-lying Arabian Shield area and the Ha’il-Rutbah mah Limestone and Gotnia Formation in Iraq, and the correla- Arch (fig. 1). Paleozoic rocks ranging in age from Cambrian tive Arab Formation in Saudi Arabia, occurring as lenses of through Late Permian crop out and subcrop in an offlap manner marine bar or shelf-margin calcarenites, calcarenitic limestone, away from the Arabian Shield. and dolomite. These strata grade eastward into organic-rich To the north, the stable shelf zone of Iraq is a continuation source rocks that were deposited under anoxic and dysoxic con- of the Widyan Basin. It includes a large area referred to as the ditions in three restricted intra-shelf basins—from north to Western and Southwestern Deserts (fig. 3) (Agrawi, 1998). This south, the Gotnia, Arabian, and Southern Arabian Gulf Basins. zone occupies more than one-third of the area of Iraq, including Maturation of the Upper Jurassic source-rock formations the Khieisia and Rutbah-Ga’ara Uplifts and the Anah Graben. (Sargelu and Naokelekan Formations in Iraq) began around 90 Target reservoirs in the stable shelf zone are primarily of Paleo- Ma; peak generation took place from 85 to 13 Ma. With time, zoic age. Eastward, the structural style changes into the Meso- the oil migrated updip and was trapped in calcarenite lenses. potamian Basin, the folded, and the thrust zones. These Later, oil remigrated and was trapped in anticlines that began to structural zones are not within the area defined as the Widyan form in Early Cretaceous time. Younger Jurassic shale and anhy- Basin–Interior Platform Province. The surface of the Mesopota- drite seal rocks are distributed throughout the total petroleum mian Basin zone is flat and covered by Quaternary fluvial-plain system. deposits of the Tigris and Euphrates Rivers, marsh and lacustrine The Widyan Basin–Interior Platform Province (2023) ranks sediments of southern Mesopotamia, and eolian sediments. It 17th in the world, exclusive of the United States, with 62.5 bil- constitutes the richest petroleum basin in Iraq, with more than lion barrels of oil equivalent of total petroleum endowment two-thirds of the country’s total hydrocarbon reserves, mostly (cumulative production plus remaining petroleum plus estimated contained in Cretaceous reservoirs. mean undiscovered volumes). Mean estimates of undiscovered The folded zone (fig. 3), formed during the Late Cretaceous petroleum for the province, which includes both Paleozoic and and Tertiary, occupies the northern and northeastern mountain- Jurassic petroleum systems as well as portions of three addi- ous areas of Iraq. It is contiguous with the Alpine fold belts of tional total petroleum systems from adjacent provinces, are the Taurus and Zagros Ranges of Turkey and Iran, respectively. 21.22 billion barrels of oil, 94.75 trillion cubic feet of gas (15.8 The folded zone has been subdivided into the high-folded and billion barrels of oil equivalent), and 6.85 billion barrels of natu- low-folded (foothill) zones (Buday, 1980; Al-Sakini, 1992; ral gas liquids. The Paleozoic total petroleum system is domi- Agrawi, 1998). The low-folded subzone has thick sedimentary nantly gas prone, whereas the volumetrically larger Jurassic total cover; its long anticlinal folds are arranged in narrow belts sepa- petroleum system is oil prone—resulting in the characterization rated by broad flat synclines. Outcropping rocks are younger of the province as an oil province. The discovery maturity for than Neogene age. In contrast, the high-folded subzone is char- the province is a relatively low 31 percent, meaning that much of acterized by extensive exposures of sedimentary rocks as old as the province petroleum potential lies in the future. Jurassic, particularly in some high-amplitude folds and over- turned structures. The thrust zone is a restricted narrow strip along the northern and northeastern borders of Iraq. Geology of the Widyan Basin–Interior Platform Province (2023) Petroleum Occurrence Structural Setting Two large hydrocarbon-producing areas occur in Saudi The stable platform interior of the Arabian plate is sur- Arabia, divided by the Central Arabian Arch. Nearly all of the rounded by tectonically active margins (fig. 1). Compressional discovered oil reserves of Saudi Arabia and adjacent countries

2 Total Petroleum Systems, Widyan Basin and Interior Platform, Saudi Arabia and Iraq 35°E 40°E 45°E50°E 55°E 60°E EXPLANATION

S THRUST SUT ....lll RU UR CASPIAN TAU E Z Flexures ON ARD E SEA Thrust fault–Sawteeth M IN HIGH on upthrown block 35° EUPHRATES ANA Fault GRABENS KHLEVUSSIA

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l I A R . A Z ARCH N . R l O U N . N Country boundary C l T l . l . E H B l . l . B l A . . l A l . GULF . H l . S l . KUWAIT . I l TABUK N N . ARABIAN 0 200 KILOMETERS BASIN E B FARS A OMAN LINE R G TH

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E OMAN A FAUL C U S INDEX MAP B R IR T A A MOUNTAINSOM I C T W R A E A H AN N H K E N TR Q A E SYSTEM A M L D L I - AR IAN A AB H R S A K E RCH D HIGH H L N I A E E M H N C L I D R SAUDISAUDI S A 20° A S R A I F ARABIAARABIA L B A Q S KH U E L H B A N A RU IN A S M BA

N O SUDANSUDAN BASIN JID BIA OMANOMAN RA AR

WA A T CH OU R AM I HR F AD T H SADAH GRABEN 15° MARIB-JAWF YEMENYEMEN SUB-BASIN LA UKAL M H ARC

ADEN GULF OF

Figure 1. Structural provinces of Arabian Peninsula (modified from Al-Laboun, 1986). Coordinates are approximate.

are to the north of the arch, and on the Interior Platform (figs. 1, The three principal trends of petroleum accumulation just 2). A minor amount is present at the lower edge of the Interior discussed may represent concentrations of calcarenite lenses Homocline bordering the Interior Platform. The structural fea- that were deposited under bar or shelf margin conditions to form tures that control the accumulation are a series of north-south- reservoirs, while contemporaneous muddy lime deposition took trending anticlines. place to the east in the deeper part of the basin to form source Within Saudi Arabia, more than 90 percent of the total rocks. With increased burial and compaction, oil was expelled petroleum reserves lies in structures of the Ghawar field area from the muddy lime facies and trapped in the calcarenite (fig. 4). A second trend of accumulation is about 100 km (kilo- lenses. Later, the oil was concentrated in the anticlines by struc- meters) to the west and parallel to Ghawar field. The southern tural growth beginning probably in Early Cretaceous time and part of this trend is within Province 2023 and includes Khurais extending into the Tertiary. Structural growth was probably field and several smaller fields, namely Mazalij, Farhah, Qirdi, completed by Miocene time. and Abu Jifan. As discussed by Ayres and others (1982), the large reserves About 90 km to the northwest is a third north-south trend of oil that accumulated on the Arabian shelf in Jurassic reser- of small oil fields on short north-south aligned anticlines (fig. 4). voirs can be attributed to several factors: (1) widespread devel- Between Jaham field on the south to Rimthan field on the north, opment of thermally mature organic-rich facies underlying or a distance of about 300 km, are the Faridah, Al Haba, Wari’ah, juxtaposed with a carbonate-evaporite shelf containing laterally Suban, and Dibdibah fields. extensive reservoir units with high porosities and permeabilities; Geology of the Widyan Basin–Interior Platform Province (2023) 3 40°E 45°E 50°E

Mosul

2089 2074 2030 35°

2029 Baghdad Iran 2027 2028

Iraq 2024

Al Basra

30° 2023 2025 Al Kuwayt 2026 Kuwait Arabian Gulf

Saudi

Arabia 2020 Interior Homocline- Red Sea Central Arch 25° Province

Ar Riyad

EXPLANATION Total petroleum system boundary 0 100 200 KILOMETERS Assessment unit boundary Widyan Basin-Interior Platform Province boundary Other province boundary and number Country boundary Figure 2. Widyan Basin–Interior Platform Province (2023) (outlined in bold red); Paleozoic Qusaiba/Akkas/Abba/Mudawwara Total Petroleum System (202301) (outlined in dark purple); Horst/Graben-Related Oil and Gas Assessment Unit (20230101) (light- er purple).

(2) a cycle of deposition that concluded with a major regression migration was extremely efficient, and traps with access to rich in which the intra-shelf basins and adjacent shelves were cov- source rocks were filled because they drained large source areas. ered by continuous evaporite seals; and (3) tectonic activity that created large structural traps, but was not strong enough to dis- Paleozoic Qusaiba/Akkas/Abba/Mudawwara rupt oil migration paths or the evaporite caps. Total Petroleum System Murris (1980) attributed the richness of this habitat to the extraordinary horizontal extent of the basins, structural traps, The Paleozoic Qusaiba/Akkas/Abba/Mudawwara Total and lithologic units. As a result, horizontal hydrocarbon Petroleum System (202301) of the Widyan Basin–Interior

4 Total Petroleum Systems, Widyan Basin and Interior Platform, Saudi Arabia and Iraq field size assessed (MMBO or BCFG). Prob., probability (including both ed estimates. For gas fields, all liquids are included under the NGL (natural liquids) les are additive under the assumption of perfect positive correlation. Shading indicates Gas (BCFG) NGL (MMBNGL) Undiscovered Resources 00000000 27,032 73,698 129,997 75,600 2,079 5,672 10,281 5,868 27,032 73,698 129,997 75,600 2,079 5,672 10,281 5,868 Mean F95 F50 F5 Mean F95 F50 F5 Mean F5 Oil (MMBO) F50 000000000000 000000000000 7,984 20,708 36,026 21,215 6,710 18,243 34,658 19,146 315 906 1,901 980 7,984 20,708 36,026 21,215 6,710 18,243 34,658 19,146 315 906 1,901 980 F95 1.00 7,984 20,7080.00 36,026 21,215 33,742 91,941 164,655 94,746 2,394 6,577 12,183 6,849 1.00 0.00 1.00 1.00 7,984 20,708 36,026 21,215 33,742 91,941 164,655 94,746 2,394 6,577 12,183 6,849 (0-1) Total: Assessed onshore portions of Widyan Basin-Interior Platform Province Total: Assessed offshore portions of Widyan Basin-Interior Platform Province Grand Total: Assessed portions of Widyan Basin-Interior Platform Province Widyan Basin–Interior Platform, Province 2023, Assessment Results Summary—Allocated Resources. Total Total Total Oil Fields Oil Fields Oil Fields Type Code Gas Fields Gas Fields Gas Fields and Field MFS Prob. 2023 2023 2023 Table 1. [MMBO, million barrels of oil. BCFG, billion cubic feet gas. MMBNGL, natural gas liquids. MFS, minimum geologic and accessibility probabilities) of at least one field equal to or greater than the MFS. Results shown are fully risk category. F95 represents a 95 percent chance of at least the amount tabulated. Other fractiles are defined similarly. Fracti not applicable]

Paleozoic Qusaiba/Akkas/Abba/Mudawwara Total Petroleum System 5 40°E 42°E 44°E 46°E 48°E

TURKEY 0 100 KILOMETERS HIGH-FOLDED Sufaiyah ZONE Ain Zala Jabat Kand-1 THRUST ZONE Butmah Mosul Alan Afar Afshan-1 IRAN 36° SYRIA Jawan Najmah Kirkuk Qaiyarah Bai Hasan Khiesia-1 KHIESIA UPLIFT Khabbaz Jambur Hamrin Saddam LOW-FOLDED Pulkhana (FOOTHILL) ANAH Tikrit ZONE Akkas-1 GRABEN 34° Naft Khaneh IRAQ Balad

WESTERN DESERT Baghdad East Baghdad KH 5/1 RUTBAH-GA'ARA Fallujah Ti gri UPLIFT s R iv er ABU JIR

JORDAN FAULTS Abu Gharah KH 5/6 West Kifil-1 Jabal Fauqi 32° Bazurgan Ekhaider MESOPOTAMIAN BASIN ZONE EXPLANATION E uph West Qurnah rates High folded zone Rive Rumaila Majnoon Samawa r North Northwest low-folded zone Nahr (Triassic, Jurassic, and Cretaceous) Diwan-1 Rumaila-172 Umr Basrah Rest of the low-folded zone Rafawi (mainly Tertiary) SOUTH Zubair Mesopotamian basin zone EUPHRATES Luhais FAULTS 30° (mainly Cretaceous) Stable shelf zone Rumaila (Paleozoic) South Oil field SOUTHWESTERN DESERT Well SAUDI ARABIA Abu Khema City KUWAIT Country boundary

Figure 3. Major tectonic regions and petroleum fields of Iraq. Widyan Basin includes Western and Southwestern Deserts of Iraq (shown in yellow), the only region of Iraq included in this report (modified from Agrawi, 1998).

Platform Province (2023) has one assessment unit, identified as the Khuff Formation then blanketed the region, reflecting tec- the Horst/Graben-Related Oil and Gas Assessment Unit tonic stability. During the Mesozoic and early Cenozoic, the (20230101) (fig. 2), and assessed undiscovered resources are region remained tectonically stable. shown in table 2. The sedimentary sequence thickens from west to east across the Arabian Plate, ranging from the erosional edge along the Precambrian outcrop to more than 15,000 m Stratigraphy (meters) in central Iran. Strata also thicken to the north to more than 7,000 m. Cambrian to Devonian nonmarine clastic sedimentary rocks Buday (1980) subdivided the Paleozoic sedimentary col- crop out along the edge of the Arabian Shield and were depos- umn of Iraq into three major sedimentary cycles (fig. 5): Cam- ited prior to the breakup of the eastern platform into a series of bro-Ordovician to Silurian, Devonian to Lower Carboniferous, north-trending horsts and grabens during the Hercynian Orogeny and Upper Carboniferous to Upper Permian. Of particular (Al-Laboun, 1986). Carboniferous and Lower Permian clastics interest are hydrocarbon source rocks of the Silurian Mudaw- of the Unayzah Formation then covered the eroded and truncated wara-Akkas-Qalibah Formations, clastic reservoir rocks of the edges of older Paleozoic strata (fig. 5). Permian carbonates of Ordovician Risha Formation in Iraq, clastic reservoir rocks of

6 Total Petroleum Systems, Widyan Basin and Interior Platform, Saudi Arabia and Iraq 50°E55°E

IRAQ Pazanan Gachasaran Luhais Rumaila Rag-E- Safid Shour Abdali EXPLANATION 30° Raudhatain Hendijan Hakimeh Ash-Shaham Bibi Bahrgansar Binak Jurassic oil field Mutriba Bahrah Sabiriyah Nowruz Jurassic gas field KUWAIT Medina Abouzar Dorood (Darius) Non-Jurassic field Rugei Ahmadi (Ardeshir) Dorra Soroush Minagish (Cyrus) Hout Lulu Esfandiar Kuh-E-Kaki W Rimthan Fawaris Marjan Dibdibah Maharah IRAN North Pars Harqus Suban Manifa Habari Sharar Karan Wari'ah Watban Juraybi'at Jana Jurayd Al Haba ah Berri Faridah Dhib Abu Sa'fah Jaladi Samin Qatif Al-Rayyan Dammam Abqaiq BAHRAIN Awali Arabian Jaham

Ghawar Dukhan Gulf Khurais QATAR 25° Abu Jifan Qirdi Harmaliyah Farhah

Mazalij

Dilam Sahba Raghib Abu Shidad Lughfah Shiblah Abu Rakis UNITED ARAB Jawb Hilwah Mulayh EMIRATES Abu Markhah Khuzama OMAN Burmah Nuayyim Hawtah Hazmiyah Nisalahf SAUDI ARABIA Umm Al Jurf Layla Usaylah

0 200 KILOMETERS

Figure 4. Major petroleum fields in western Arabian Gulf region (modified from Al-Husseini, 1997). the Carboniferous Unayzah-Ga’ara Formations of Saudi Arabia, sea-level rise that resulted from melting of the Gondwana polar and widespread seals in carbonates and evaporites of the Per- ice cap (Al-Husseini, 1991). The basal Qusaiba Member in cen- mian Khuff and Chia Zairi Formations. tral and south Arabia accumulated in a trough that subsided from Late Ordovician through Early Devonian time. Abundant grapt- olites in the Qusaiba Member indicate an Early to Middle Lland- Source Rocks overian age for the lower Qusaiba (fig. 6; Mahmoud and others, 1992). The general paleogeographic pattern is that of a major Stratigraphy rapid transgression during the early Llandoverian followed by a regression (Al-Husseini, 1991), as reflected in a depositional Lower Silurian clastic sediments of the Qalibah Formation sequence consisting of, in ascending order, organic-rich shale, were deposited on a broad continental shelf of Gondwana during sandy and shaly micaceous gray siltstone, and gray to dark-gray

Paleozoic Qusaiba/Akkas/Abba/Mudawwara Total Petroleum System 7 field size assessed (MMBO or BCFG). Prob., probability (including both ed estimates. For gas fields, all liquids are included under the NGL (natural liquids) les are additive under the assumption of perfect positive correlation. Gas (BCFG) NGL (MMBNGL) Assessment Results Summary. Undiscovered Resources 202301— 19,888 58,628 106,688 60,562 1,566 4,656 8,690 4,843 Mean F95 F50 F5 Mean F95 F50 F5 Mean F5 Oil (MMBO) F50 754 2,438 5,141 2,632 1,102 3,615 7,943 3,948 61 211 502 237 754 2,438 5,141 2,632 1,102 3,615 7,943 3,948 61 211 502 237 F95 1.00 1.00 7541.00 2,4381.00 5,141 754 2,632 2,438 20,989 5,141 62,243 114,631 2,632 64,510 20,989 62,243 1,627 114,631 4,867 64,510 9,191 1,627 5,080 4,867 9,191 5,080 (0-1) 5 30 19,888 58,628 106,688 60,562 1,566 4,656 8,690 4,843 Total: Paleozoic Qusaiba/Akkas/Abba/Mudawwara Total Petroleum System Total Petroleum Total: Paleozoic Qusaiba/Akkas/Abba/Mudawwara Paleozoic Qusaiba/Akkas/Abba/Mudawwara, Total Petroleum System

Total Total

Oil Fields Oil Fields Type Code Gas Fields Gas Fields and Field MFS Prob. 20230101 Horst/Graben-Related Oil and Gas Assessment Unit 202301 [MMBO, million barrels of oil. BCFG, billion cubic feet gas. MMBNGL, natural gas liquids. MFS, minimum geologic and accessibility probabilities) of at least one field equal to or greater than the MFS. Results shown are fully risk category. F95 represents a 95 percent chance of at least the amount tabulated. Other fractiles are defined similarly. Fracti Shading indicates not applicable] Table 2.

8 Total Petroleum Systems, Widyan Basin and Interior Platform, Saudi Arabia and Iraq SAUDI PERIOD JORDAN IRAQ WSWARABIA

Chia Zairi Khuff

Unayzah Ga'ara and pre-Unayzah

"Hercynian" erosion

Carboniferous Permian Harur Berwath Ora Kaista Pirispiki Jubah

EXPLANATION Jawf Carbonate Devonian Shale Tawil Sandstone

Kisha Suffi Silurian hiatus

Silurian Mudawwara Akkas Qalibah

Sarah-Zarqa Risha ? ? ? Khabour Dubaydib Qasim Hiswab ? Ordovician Umm Sahm Saq Disi

Burj Ram Group

Cambrian Salib Precambrian Saramuj

Figure 5. Paleozoic stratigraphic column of Jordan, Southwestern Desert of Iraq, and northern Saudi Arabia (modified from Agrawi, 1998). Correlations queried where uncertain. and black fissile micaceous shale with black carbonaceous Cole, Halpern, and Aoudeh (1994) have recognized three organic matter (fig. 6). The overlying Sharawa Member of the distinct gamma-ray response zones (cool, warm, and hot) on Qalibah Formation is siltstone and sandstone that is generally well logs through the Qusaiba Member of the Qalibah Forma- coarser than the underlying Qusaiba Member of the Qalibah tion. Cool zones with low gamma-ray response are not consid- Formation. ered to be a source rock and were deposited under oxic

Paleozoic Qusaiba/Akkas/Abba/Mudawwara Total Petroleum System 9 ROCK UNIT GAMMA DENSITY RAY NEUTRON STAGE 100 m GRAMS/CM3 WELL 2.0 2.5 3.0 0 VERTICAL SCALE SERIES API UNITS LITHOLOGY NEUPOR. % MEMBER

FORMATION 200 100 04515-15 SYSTEM Lat 23 UDAYNAN WELL - 1 GED ° PRIDOLI 28' N., long 48 UPPER SILURIAN LOWER DEVONIAN SUB-TAWIL DISCONFORMITY ° 30' E. STRATIGRAPHIC WELL - 39 Lat 23 ° 14' N., long 47 QUSAIBA SHARAWA QALIBAH FORMATION (955 m)QALIBAH FORMATION FM. TAWIL LOWER SILURIAN - LLANDOVERIAN ° HOT 07' E. EXPLANATION SHALE (70 m) SUB-QALIBAH DISCONFORMITY Sandstone ? SARAH FM. Shale QASIM FM. Siltstone UPPER ORDOVICIAN

Figure 6. Subsurface composite reference section for the Qalibah Formation in central Saudi Arabia, showing lithology, biostratigraphy, and gamma ray-density-neutron well-log patterns (modified from Mahmoud and others, 1992). Note “hot shale” unit of basal Qusaiba Member. NEUPOR. %, neutron log porosity in percent.

conditions. Moderate gamma-ray response zones (warm zones) south of Ar Riyad and thinner and less widespread in the north- contain moderate to excellent source-rock quality of dysoxic ori- ern Tabuk Basin (Jones and Stump, 1999). The Qusaiba Mem- gin. “Hot shales,” of dysoxic to mainly anoxic origin, are excel- ber in northern Saudi Arabia is as much as 1,200 m thick in the lent source rocks for the Paleozoic Qusaiba/Akkas/Abba/ Widyan Basin (Agrawi, 1998). The “hot shale” is at least 60 m Mudawwara Total Petroleum System. thick in the southern basin, south of Ar Riyad. In the Tabuk Qusaiba Member source rocks occur primarily in two sub- Basin the maximum thickness is about 30 m. The maximum basins, being thickest and most widespread in the southern basin thickness of age-equivalent strata in western Iraq (Akkas

10 Total Petroleum Systems, Widyan Basin and Interior Platform, Saudi Arabia and Iraq Formation) is about 2,000 m. In the Akkas-1 and Khiesia-1 wells Migration (fig. 3), the Silurian “hot shale” has an average thickness of about 65 m. As discussed by McGillivray and Al-Husseini (1992), the Qusaiba Shale subcrops beneath the Unayzah reservoir in cen- tral Saudi Arabia (fig. 10). The overlying Unayzah is an effec- tive reservoir and provides a potential migration route for Petroleum Geochemistry transport of hydrocarbons from the structurally deeper mature The “hot shale” source rock in the Tabuk Basin area, sepa- source-rock areas. Permeability enhancement is evident locally rately assessed as the Jafr-Tabuk Basin Province (2026) (fig. 2), in sandstones below the pre-Unayzah unconformity and updip has total organic carbon content (TOC) that exceeds 3 percent from the Qusaiba subcrop. Possible, but much less likely, is and has mixed oil/gas-prone to oil-prone potential (Alsharhan that subcropping pre-Qusaiba reservoirs could be charged from and Nairn, 1997). Microscopic study of “hot shale” source rocks the overlying Unayzah. In both Iraq and Saudi Arabia the by Monnier and others (1990, written commun.) identified pri- direction of migration is toward the west (Cole, Halpern, and marily oil prone amorphous Type-II organic material. A marine Aoudeh, 1994). shale origin is demonstrated by high rearranged to regular ster- In Iraq, along the eastern flank of the Akkas field, Al- ane ratios, tricyclic terpane distributions typical of marine Habba and Abdullah (1989) concluded that hydrocarbons shales, and relatively high diahopane to hopane ratios. Silurian- migrated downward from the Lower Silurian source into the sourced oils from Saudi Arabia are isotopically light. Ordovician upper Khabour fractured sandstone reservoir. At Akkas-1 and Khiesia-1 wells the Silurian “hot shale” has Paleozoic reservoirs of trapped oil that followed similar migra- TOC values as much as 16.6 percent, and a hydrocarbon yield of tion pathways are expected in Saudi Arabia, and the Southwest- 49 kg (kilograms) per ton of rock, one of the highest yields in the ern Desert of Iraq. world (Al-Gailani, 1996). Petrographic analysis of the kerogen extracted from these shales reveals a marine origin, rich in organic components such as algae, graptolites, and chitins. Reservoir Rocks Geochemical analysis of kerogens from the “hot shale” of the Western Desert revealed that the saturated and aromatic Unayzah Formation hydrocarbons exceed 96 percent of their components, which characterizes them as having low molecular weights (C3-C20) In north-central Saudi Arabia, the primary Paleozoic reser- and limited amounts of asphaltic materials (about 3.89 percent). voirs are the correlative Unayzah and Ga’ara Formations of They are almost free of H S. Similar characteristics are reported 2 Early Permian–Carboniferous age (fig. 5), which are bounded by for both the extracts of the Silurian “hot shales” and the hydro- two regional unconformities—the underlying pre-Unayzah carbons being produced from the Paleozoic reservoirs of Central (Hercynian) and the overlying pre-Khuff unconformity. The Saudi Arabia and Jordan. Unayzah Formation progressively overlaps, from east to west approaching the Arabian Shield, older inclined strata below the pre-Unayzah (Hercynian) unconformity, and fills in rifts and Maturation half-grabens. Thickness of the Unayzah Formation is variable, ranging Silurian “hot shale” source rocks began generating hydro- from less than 2 m on the west end of the Central Arabian Arch carbons possibly during the late Paleozoic in the region of to more than 400 m in the Widyan Basin (Al-Laboun, 1987). northern Arabia into Iraq, and during the Triassic in the region The Unayzah is predominantly fluvial and alluvial fine- to of the present-day Arabian Gulf (fig. 7; Bishop, 1995). Peak coarse-grained quartz sandstone, flood-plain and distal fan silt- generation, migration, and entrapment occurred through the stone, and varicolored flood-plain, distal fan, and playa clay- Jurassic and ended in Early Cretaceous. As the rate of oil gen- stone that contains stringers of carbonate in the upper part. It eration declined, gas saturation continued to form. Folding that grades upward from alluvial fan conglomerate at the base into developed during the late Tertiary when Arabia and Eurasia numerous braided channel fills, and is capped by distal fan and collided caused gas to displace oil that was already trapped in playa facies. Eolian sandstone beds are also present. Reservoir reservoirs. Major gas yield and entrapment ended in the deep facies are predominantly sandstone beds that are fine to coarse basin of eastern Iraq and northeastern Saudi Arabia by early grained and poorly cemented to friable. Thus, primary porosity Neogene, but gas is still presently generated in updip regions and permeability are highly variable, and are reduced locally by (fig. 8). Silurian source rocks are presently immature at shal- kaolinite and illite. Polkowski (1997) noted the presence of low depths. authigenic clay minerals within the Unayzah reservoir rock that In the Western Desert area, Silurian strata at Akkas-1 and cause adverse effects during well drilling, completion, produc- Khiesia-1 wells are currently in the oil-generating zone. Agrawi tion, and (or) injection. Generally, porosity exceeds 20 percent (1998) (fig. 9) showed a burial history model of the Akkas-1 well and permeability may be several darcys. in which the Silurian “hot shale” source rock entered the oil zone During compaction and hydrocarbon generation, fluids rich in late Paleozoic and remains there today. in hydrocarbons and organic acids were expelled from shale

Paleozoic Qusaiba/Akkas/Abba/Mudawwara Total Petroleum System 11 34°E 44°E 54°E 64°E

TURKEY Caspian EXPLANATION Sea Possible Carboniferous fill Tau rus Possible Paleozoic yield

Z a g Triassic yield ed ro erod ? s ded 131 Line of equal time ? ro e (Ma) @ level of maturity 9 n n a ? i 36° ria r ? u lu l Boundary of Paleozoic i IRAQ Si th S ru st hydrocarbon system 66 IRAN

anean

0 SYRIA Thrust fault–Sawteeth on upthrown block

Sea Country boundary su

tu d r

Mediterr e

e 0 250 KILOMETERS d

o r e

n a ri S u ilu E il Mesozoic r ia S z n on Cenozoic e e r d o JORDAN e d d Flu e o e sh zon r S d ed e i 66 lu r ia 0 n

n

a

i A

r

u l r i a

S b 28 e i ° r a Paleozoic o n

eroded d

e G d u lf Gulf of

S Oman

i

l u

SAUDI r i

210 a

ARABIA n

131 e

r o d e Arabian d EGYPT Red Sea Shield Cretaceous

Precambrian 0 210 66 131

66 Jurassic 0 Cenozoic

Fa cie s lim al 20 it of nd se ° Silurian as a source a Precambrian OMAN

Arabian YEMEN SUDAN Sea

Figure 7. Time at which significant volumes of oil were generated from Lower Silurian strata (modified from Bishop, 1995). Note that this was possibly Carboniferous and late Paleozoic in northern Arabian Peninsula, and Triassic to the south in Arabian Gulf area. Queried boundaries indicate uncertain extent. Coordinates are approximate. units of the Qusaiba Formation. These fluids migrated into the transgressive Khuff and Unayzah Formations. Such rocks Unayzah reservoir rocks and enhanced reservoir quality by are mainly clastics, and their reservoir quality may have been leaching pore throats, corroding framework grains, and remov- enhanced by weathering during the development of the Her- ing carbonate cement (Aktas and Cocker, 1995). cynian unconformity. The thickness of pre-Unayzah clastic rocks is as much as 1,000 m in the Widyan Basin (Al-Laboun, Pre-Unayzah Formations 1986). The main Paleozoic reservoir rocks in the Western Desert Reservoir continuity may exist in dipping strata such as of Iraq and East Jordan (Risha field) are sandstones of the the Qusaiba and underlying reservoirs such as the Sarah, Ordovician Upper Khabour and Silurian Akkas Formations (fig. Qasim, and Saq Formations (fig. 5). These units are truncated 5). In the Akkas field, the Akkas-1 well contains high gravity oil by the pre-Unayzah unconformity and are in contact with the (42° API) from the Silurian Akkas sandstone, and sweet gas overlapping Unayzah or basal Khuff sandstone. Few data are from the upper sandstones of the underlying Ordovician Kha- available on pre-Unayzah Paleozoic potential reservoirs below bour Formation (Agrawi, 1998). Thirty to forty percent of all

12 Total Petroleum Systems, Widyan Basin and Interior Platform, Saudi Arabia and Iraq 34°E 44°E 54°E 64°E

TURKEY Caspian EXPLANATION Sea Immature Tau rus Mature for oil

Z Gas generation zone a g ed ro Exhausted erod s ded ro e 10.5 Line of equal level of 13 n n a i a r maturity 36° ri u u l l i IRAQ Si th S ru Boundary of Paleozoic st IRAN hydrocarbon system anean Thrust fault–Sawteeth on Sea upthrown block su Country boundary

tu

Mediterr d re

e

d 0 250 KILOMETERS o 10.5 r e

10.5 13 13 n a ri u ISRAEL il S 10.5 8 zo ne d JORDAN e d Flu e o S sh on r i ed z e lu r ia n

n A

a i

r r

8 u l a i b

e S i

r a

28 o ° Paleozolic n

d eroded

e G

d ulf 13 Gulf of

S Oman

i l

SAUDI u

r i

ARABIA a n

e 13 r o d e d 10.5 EGYPT Arabian Shield Precambrian 8 13

10.5 Fa Red Sea cie s li l 20 mit o d sea ° f Siluria rce an Precambrian n as a sou OMAN

Arabian YEMEN SUDAN Sea

Figure 8. Present-day levels of thermal maturity of Lower Silurian source rock in Arabian Peninsula (modified from Bishop, 1995). Note areas that are currently mature for oil and gas generation in northern Saudi Arabia and western Iraq. Level of maturity (LOM) values shown. Coordinates are approximate. known reservoirs in Iraq occur within 600 m of the surface, and Precambrian basement fault blocks (fig. 10). Major structural most of the remaining are within 3,000 m of the surface. The growth occurred during the Late Devonian to Carboniferous recent Paleozoic discovery at Akkas was at a depth of about Hercynian Orogeny (Simms, 1995). Paleozoic traps are moder- 3,650 m (Agrawi, 1998). ate-relief, fault-generated structures with 30 to 100 m of clo- sure. Prominent high-angle reverse faulting in the older Paleozoic section does not extend above the Carboniferous-Per- mian Unayzah Formation, and gentle drape folds occur in the Hydrocarbon Traps overlying strata. Strata younger than early Eocene do not reflect deeper structures and there is little or no topographic expression Structural Traps of these long linear folds. Ghawar oil field is located on one of these structures, which has a total length of about 370 km (fig. 4). Structural features that control accumulation of petroleum Two other north-trending structural features are present about are north-trending block-faulted anticlines reactivated above 100 km and 200 km west of the Ghawar field. In addition to these

Paleozoic Qusaiba/Akkas/Abba/Mudawwara Total Petroleum System 13 Tri- Paleozoic assic Jurassic Cretaceous Tertiary Lower Miocene Oligocene Upper middle Eocene "hot shale" Silurian/Lower Devonian Lower Eocene/Paleocene Maastrichtian 1,000 Permian/Upper Carboniferous Ordovician Upper Devonian/ Lower Carboniferous

2,000 DEPTH, IN METERS 3,000

Basement

EXPLANATION 4,000 Oil zone Gas zone

450 400 350 300 250 200 150 100 50 TIME, IN MILLION YEARS Figure 9. Burial history model of Akkas-1 well of western Iraq (modified from Agrawi, 1998). Basal Silurian “hot shale” source rock is shown to have remained in the oil generation window since the late Paleozoic. Location of the Akkas-1 well shown in figure 3. major anticlines, many shorter ones are scattered over the Ara- targets involve Triassic and Paleozoic formations in prospects bian shelf, most showing generally north south orientation. with variable shape and size. Only 10 wells have been drilled in Within Iraq, 526 structures have been identified and classi- the stable shelf zone. Among the major field discoveries are Abu fied as potential hydrocarbon prospects with demonstrable clo- Khema, Diwan, Samawa, Ekhaidher, and Akkas. The major dis- sures (Al-Gailani, 1996). Most of these structures are elongated covery in Paleozoic strata at Akkas is on a structure that is more anticlines that range in length from 1 to 110 km. However, about than 27 km long and is the result of basement faulting. High one-third of the 526 structures are oval to circular in outline. gravity oil (42° API) is present in the Silurian Akkas sandstone Iraq has been divided into four tectonic settings with characteris- reservoirs, and sweet gas occurs in the upper sandstones of the tically different structures (fig. 3) (Agrawi, 1998). These include Ordovician Khabour Formation. thrust zone, folded zone, Mesopotamian Basin zone, and stable shelf zone (Western and Southwestern Desert). Stratigraphic Traps The area of Iraq included in this assessment, the stable shelf zone, has about 155 prospects that range in length from 1 km to Stratigraphic traps and combined stratigraphic-structural 46 km, with seven longer than 20 km. All of the exploration traps have also been identified in central Saudi Arabia; they

14 Total Petroleum Systems, Widyan Basin and Interior Platform, Saudi Arabia and Iraq SOUTHWEST NORTHEAST DILAM ABU JIFAN

1,000

Hercynian unconformity

UNAYZAH FM

2,000

ZARQA/SARAH FM KHUFF FM DEPTH, IN METERS 3,000 QASIM FM MIGRATION KHUFF BASEMENT

QALIBAH FM ?SAQ FM 4,000 QASIM

SAQ BASAL QUSAIBA HOT SHALE

5,000

Figure 10. Diagrammatic model for migration of hydrocarbons generated from basal Qusaiba “hot shale” in central Saudi Arabia (modified from McGillivray and Al-Husseini, 1992). Note that petroleum (red) migrates up- ward to the Hercynian unconformity (wavy line) and laterally into the Unayzah Formation, which acts as the con- duit for updip migration. This model may be applicable to a similar situation in the Widyan Basin to the north.

typically are updip pinchouts of Unayzah sandstone. The verti- Triassic strata. Also, local seals may consist of tight, nonfrac- cal closure mapped in many of these fields is insufficient to tured dense limestone within the Khuff and Chia Zairi Forma- account for the observed hydrocarbon column, indicating a tions. Local seals formed as a result of several cycles of shelf stratigraphic component. Evans and others (1997) studied the carbonate and evaporite deposition on the passive margin plat- recent discovery of Arabian super light oil (49° API) in the form in what is now the foreland portion of the Arabian/Ira- Usaylah-1 well at Usaylah field (fig. 4). Production is from an nian Basin. Also, Lower Silurian shale regional seals extend updip pinchout of an upper Unayzah eolian dune sandstone unit. from northern Saudi Arabia over most of the Southwestern A second well at Usaylah field was drilled in a wadi fill channel Desert of Iraq (Agrawi, 1998). of the equivalent-age dune sands at the Usaylah-1 well. Wells drilled into the finer grained Unayzah facies have poor produc- tion rates. Similar types of traps in the Unayzah are anticipated Timing and Critical Events to occur within the Widyan Basin–Interior Platform Province. The sequence of favorable events leading to the entrapment Seals and Overburden Rocks of petroleum include (1) truncation of Paleozoic sandstone res- ervoirs against faults, (2) drape of Unayzah clastics as reservoir The most effective cap is the regional salt and evaporite and carrier beds, overlain by Upper Permian Khuff or Triassic seal, either within the Permian Khuff reservoir of Saudi Arabia tight formations, and (3) maturation and migration of hydrocar- and the age-equivalent Chia Zairi reservoir of Iraq (fig. 5) or in bons in the Triassic and Jurassic. Lower Silurian source rocks

Paleozoic Qusaiba/Akkas/Abba/Mudawwara Total Petroleum System 15 TOTAL PALEOZOIC MESOZOIC CEN. Carboniferous Carboniferous Cretaceous Cretaceous Ordovician Cambrian Devonian Permian Jurassic T Silurian T T Lower Upper PETROLEUM Lower Upper Lower Upper ertiary ertiary riassic SYSTEM EVENTS

SOURCE ROCK

RESERVOIR ROCK

SEAL ROCK

TRAP FORMATION

GENERATION/ OIL DRY WET MIGRATION GAS GAS

Figure 11. Timing of events critical to petroleum accumulation in the Paleozoic Qusaiba/Akkas/Abba/Mudawwara To- tal Petroleum System of the Widyan Basin–Interior Platform Province (modified from Wender and others, 1998). Unayzah clastics were deposited on horst and graben features that formed during the Hercynian disturbance. Traps resulted where uplifted fault blocks formed anticlines into which petroleum migrated and was trapped by impermeable beds in the Khuff Formation or overlying Triassic strata. entered the oil window as early as the Triassic and continued Basins—formed on the passive margin platform of the generating oil until the Tertiary. These same units entered the Tethys sea after Permo-Triassic rifting. They were sepa- gas window during the later Jurassic and remain in the gas win- rated from each other and from the open Tethys sea to the dow at present (Wender and others, 1998) (fig. 11). east by shoals of calcarenite grainstones (Ayres and others, 1982). In the Gotnia Basin, the largest and deepest of the three, Jurassic Gotnia/Barsarin/Sargelu/Najmah Total source rocks are primarily bituminous limestone and shale of the Sargelu Formation (Middle Jurassic) (fig. 14). Similarly, Petroleum System source rocks in the Arabian and Southern Arabian Gulf Basins are primarily organic rich, thin-laminated peloidal carbonate of The Jurassic Gotnia/Barsarin/Sargelu/Najmah Total Petro- the Tuwaiq Mountain Formation (Upper Jurassic) and overlying leum System (202302) of the Widyan Basin–Interior Platform Hanifa (Diyab/Dukhan) Formation. The richest source rocks in Province (2023) has two assessment units. These are the Plat- all three basins occur primarily as condensed sections deposited form Horst/Graben-Related Oil Assessment Unit (20230201), in the early phase of transgressive cycles. Circulation was and Basinal Oil and Gas Assessment Unit (20230202) (fig. 12, restricted, allowing for preservation of large quantities of table 3). organic matter, mostly algal, under periodically anoxic bottom conditions in the basin centers. Source-rock facies of Callovian and Oxfordian ages in the Arabian Basin of eastern Saudi Ara- Source Rocks bia, with total organic carbon (TOC) > 1.0 wt. percent, may reach a thickness of more than 90 m. In contrast, higher energy shallow-water grainstone and dolomitic limestone (the reservoir Stratigraphy facies) accumulated at the basin margins. Evaporitic facies formed the seal for the hydrocarbons that accumulated in the Most of the petroleum is sourced from cyclically bed- high-energy shoal deposits, charged from the organic-rich shale ded Jurassic shales and carbonates (Callovian-Oxfordian- and mudstone in the basin center. In latest Jurassic (Tithonian Kimmeridgian ages) that accumulated in three restricted age) the seas eventually became shallower and evaporation rates intra-shelf basins (fig. 13). These basins—from north to increased, resulting in precipitation of evaporites that covered south, the Gotnia, Arabian, and Southern Arabian Gulf the organic-rich source and reservoir facies.

16 Total Petroleum Systems, Widyan Basin and Interior Platform, Saudi Arabia and Iraq 40°E 45°E50°E

Mosul

2089 2074 2030 35° Syria Iran

2029 Baghdad 2027 2028

Iraq 2024

Al Basra Jordan 30° 2023 2025 Al Kuwayt Saudi Arabia Kuwait Arabian Gulf

Red Sea 2020 Interior Homocline- Central Arch 25° Province Ar Riyad

EXPLANATION Total petroleum system boundary 0 100 200 KILOMETERS Platform Horst/Graben-Related Oil Assessment Unit Basinal Oil and Gas Assessment Unit Widyan Basin–Interior Platform Province boundary Other province boundary and number Country boundary

Figure 12. Widyan Basin–Interior Platform Province (2023) (outlined in bold red); Jurassic Gotnia/Barsarin/Sargelu/Najmah Total Petroleum System (202302) (outlined in green); Platform Horst/Graben-Related Oil Assessment Unit (20230201) (blue); Basinal Oil and Gas Assessment unit (20230202) (purple).

Petroleum Geochemistry Cole, Carrigan, and others (1994) have reported on Upper Jurassic source rocks in the Arabian and Southern Arabian Gulf In the Gotnia Basin of Iraq, Upper Jurassic source rocks of Basins, specifically at Khurais, Al Haba, Ghawar, and Abu the Sargelu, Naokelekan, and Gotnia Formations (Qara Chauq-1 Hadriya fields (fig. 4). Oil in these fields has API gravity of 25° well) and Naokelekan, Barsarin, and Chia Gara Formations to 35°. At Khurais field in eastern Saudi Arabia, the Tuwaiq (Kirkuk-109 well) have TOC values that range from about 2 to 5 Mountain Formation, the primary source rock, is 142 m thick. percent of Type-II kerogen (Sadooni, 1997). For the four oil fields just listed, source rocks of the Hanifa,

Jurassic Gotnia/Barsarin/Sargelu/Najmah Total Petroleum System 17 field size assessed (MMBO or BCFG). Prob., probability (including both geologic itive under the assumption of perfect positive correlation. Shading indicates not applicable] ates. For gas fields, all liquids are included under the NGL (natural liquids) category. Gas (BCFG) NGL (MMBNGL) Undiscovered Resources 4,602 15,688 30,254 16,364 189 667 1,425 719 202302—Assessment Results Summary. Mean F95 F50 F5 Mean F95 F50 F5 Mean F5 Oil (MMBO) F50 1,530 4,605 8,038 4,682 1,128 3,570 6,952 3,748 63 207 446 225 2,102 6,439 11,390 6,569 1,960 6,279 12,168 6,578 109 365 780 395 F95 1.00 1.00 1,5301.00 4,6051.00 8,038 5721.00 4,682 1,8341.00 2,320 3,352 2,102 8,154 1,886 6,439 17,003 11,390 4,242 8,741 13,813 6,569 112 25,419 6,561 14,201 21,967 403 42,422 187 917 22,942 629 444 299 1,288 1,032 670 2,205 1,114 (0-1) 5 3020 572 1,834 3,352 1,886 832 1,192 2,709 4,584 5,216 10,052 2,830 4,993 47 49 157 195 334 471 170 219 otal: Jurassic Gotnia/Barsarin/Sargelu/Najmah Total Petroleum System 120 3,410 11,105 20,202 11,371 141 472 953 500 Basinal Oil and Gas Assessment Unit Jurassic Gotnia/Barsarin/Sargelu/Najmah, Total Petroleum System Total Total Total Oil Fields Oil Fields Oil Fields Type Code Gas Fields Gas Fields Gas Fields and Field MFS Prob. 20230201 Platform Horst/Graben-Related Oil Assessment Unit 20230202 202302 T Table 3. [MMBO, million barrels of oil. BCFG, billion cubic feet gas. MMBNGL, natural gas liquids. MFS, minimum and accessibility probabilities) of at least one field equal to or greater than the MFS. Results shown are fully risked estim F95 represents a 95 percent chance of at least the amount tabulated. Other fractiles are defined similarly. Fractiles are add

18 Total Petroleum Systems, Widyan Basin and Interior Platform, Saudi Arabia and Iraq 45°E50°E 55°E 60°E

Caspian Sea EXPLANATION Euxinic intraplatform basin Mixed evaporitic/carbonate platform Shallow carbonate shelf 35° Deep carbonate shelf/basin margin Z a g Erosional limits r IRAQ o Country boundary s

Gotnia t h Basin r u s t s u IRAN t u r e 30° z o n e A rab ian G u lf Arabian Basin Southern Arabian BasinGulf 25° Gulf of Oman

SAUDI ARABIA

OMAN

20°

Red Sea

YEMEN 0 500 KILOMETERS

Figure 13. Gotnia, Arabian, and Southern Arabian Gulf Basins in which Jurassic hydrocarbon source rocks accumulated (modified from Alshar- han and Kendall, 1986). Sawteeth on upthrown block of Zagros thrust suture zone.

Tuwaiq Mountain, and Dhruma Formations (fig. 14) were com- composed primarily of amorphous Type-II kerogen (from blue- pared by Cole, Carrigan, and others (1994); they reported TOC green algae), which makes them capable of generating large averages as follows: Hanifa 2.60 percent, Tuwaiq Mountain amounts of petroleum, particularly oil (hydrogen indices exceed 3.52 percent, and Dhruma 1.80 percent. Source rocks are 600 mg (milligrams) of hydrocarbons per gram (g) of TOC).

Jurassic Gotnia/Barsarin/Sargelu/Najmah Total Petroleum System 19 EARLY AGE and SEQUENCE KUWAIT SAUDI ARABIA BAHRAIN CRETACEOUS BERRIASIAN SULAIY

HITH HITH 1st Anh - Salt TITHONIAN ARAB A 2nd Anh - Salt ARAB B

3rd Anh - Salt GOTNIA ARAB C LATE 4th Anh - Salt ARAB D

KIMMERIDGIAN JUBAILA HANIFA OXFORDIAN NAJMAH TUWAIQ MOUNTAIN NAOKELEKAN CALLOVIAN Upper Dhruma DHRUMA SARGELU SARGELU BATHONIAN Middle Dhruma DHRUMA Dhibi Lst DHRUMA J U R A S I C A J U R BAJOCIAN Dhruma Shale AALENIAN HIATUS A Upper Marrat B TOARCIAN MARRAT MARRAT C Lower Marrat

EARLY MIDDLE D-E EARLY JURASSIC HIATUS

LATE TRIASSIC MINJUR

Figure 14. Correlation of Jurassic strata of western and southern Arabian Gulf region (modified from Al-Husseini, 1997).

Pyrolysis yields average 22.2 mg hydrocarbons/g rock. Most of Maturation these source rocks are within the oil-generating window and thus have excellent petroleum generation potential. Maturation of the Upper Jurassic source-rock formations in Iraq (Sargelu and Naokelekan Formations) began around 90 Ma, Oil to Source Rock Correlation with peak generation from 85 to 13 Ma (fig. 15; Murris, 1981). Generally, trap growth in Iraq took place through most of the Based on selected chromatographic, carbon isotopic, and Mesozoic, although Late Cretaceous and Miocene to Recent are biomarker parameters studied by Cole, Carrigan, and others the principal periods of growth. Ayres and others (1982) have (1994), a clear correlation exists between Jurassic Hanifa and estimated the degree of maturation of the kerogen within the Tuwaiq Mountain source rocks and Arab and Hanifa Formation Gotnia and Arabian Basins from atomic hydrogen/carbon (H/C) oils. Arab and Hanifa oils are high-sulfur (1–4 percent), aro- ratios, and from calculated maturity based on Lopatin’s time- matic-intermediate oils that are consistent with a marine-carbon- temperature index (TTI) method; from these data, they inter- ate source. Comparison of gas chromatograms between Arab oil preted that maturation and migration of oils from Jurassic source and Hanifa source-rock extract from the same field show great rocks began in early Tertiary time. similarity. Alsharhan and Nairn (1997) also compared C 27–29 Because of a slower rate of deposition in the Arabian and regular steranes from Upper Jurassic oils with Hanifa and Southern Arabian Gulf Basins than to the northeast in Iraq, Tuwaiq Mountain source-rock extracts. Their similarity also petroleum generation and migration from the Hanifa Formation supports the interpretation that the Tuwaiq Mountain and Hanifa in the Arabian and Southern Arabian Gulf Basins were not wide- are the source of Jurassic oil. spread until about 55 Ma.

20 Total Petroleum Systems, Widyan Basin and Interior Platform, Saudi Arabia and Iraq Quaternary J U R A S S I C C R E T A C E O U S T E R T I A R Y LowerMiddle Upper Lower Upper Lower Upper 200 x 106 years 100 x 106 years 0 METERS

50o 1,000 60o Hanifa 70o Sargelu 80o VR/E 0.62 90o 2,000

100o

110o VR/E 0.81 ASSUMPTIONS: 3,000 Surface temperature = 20 o C Temperature gradient/100 m Jurassic Present 3.55 o C 3.4 o C

4,000

Figure 15. Burial history of the Hanifa and Sargelu Formations (modified from Murris, 1981). Mature zone is colored yellow. VR/E, calculated maturity in vitrinite reflectance equivalent units. Periods of uplift and erosion shown (dashed line); corrected burial history curve shown (wavy red line).

Migration the high-energy calcarenite-grainstone facies that accumulated around the Gotnia, Arabian and Southern Arabian Gulf Basins Petroleum generated from Jurassic source rocks in the Got- (Alsharhan and Kendall, 1986). nia Basin migrated updip to the west and south into basement- The Hanifa, Tuwaiq Mountain, and Arab Formations related, block-faulted structures that moved periodically since exhibit similar cyclic sedimentation—transgressive-regressive Precambrian. Ayres and others (1982) stated that the relatively depositional cycles that consist of a shallowing-upward marine large number of oil reservoirs implies extensive vertical migra- carbonate sequence overlain by anhydrite. Each carbonate cycle tion of oils from a more limited number of rich source rocks. concludes with an ooidal-pelletoidal grainstone. These grain- For example, evidence in the Ghawar field suggests that the Han- stones contain most of the oil in Jurassic reservoirs (Arab A, B, ifa reservoir is in hydraulic continuity with the Arab D reservoir, C, D; Lower Kimmeridigian to Tithonian) in Saudi Arabia. They probably maintained through vertical fractures. Presence of oil are cyclical calcarenites (grainstone, coarse lime sand to gravel, in the upper Arab reservoirs implies that, at least locally, the oolitic and peloidal), aphanitic limestone, anhydrite, and dolo- Arab anhydrite interbeds were absent or in communication mite. A typical depositional cycle began with regression of the through fractures or fault offsets with underlying carbonate sea that resulted in lateral shift of reservoir grainstone facies bas- rocks. The Hith Anhydrite appears to be an effective seal every- inward (Cole, Carrigan, and others, 1994). Further regression where except in the northeast offshore area. exposed the shelf surrounding the intra-shelf basins where water became euxinic and source rocks were deposited. As sea level rose during the next transgression, euxinic conditions ended and Reservoir Rocks reservoir grainstones (high energy) were deposited on intra-shelf basinal source rocks. This type of cyclic stratigraphy was ongo- Hanifa Formation, Tuwaiq Mountain Formation, and Arab Formation ing throughout the Middle to Late Jurassic. Most Jurassic reservoirs lie at depths ranging from 1,400 to Jurassic reservoirs are almost exclusively cyclic interbed- 3,750 m (Alsharhan and Nairn, 1997). Primary porosity in the ded calcarenite, calcarenitic limestone, dolomite, and organic- reservoir grainstones has been reduced by secondary recrystalli- rich carbonate source rocks (fig. 14). The best reservoirs are in zation and dolomitization. Porosity and permeability are variable

Jurassic Gotnia/Barsarin/Sargelu/Najmah Total Petroleum System 21 45°E50°E 55°E 60°E Caspian Sea EXPLANATION Evaporitic platform Z Evaporite/carbonate platform a g Evaporite basin r o Shallow carbonate shelf 35° s Continental slope t h r Deep open marine u IRAN s Thrust zone IRAQ t Thrust fault—Sawteeth on s upthrown block u t u Erosion limit r e Country boundary

30° z o n e

A ra bi an G ul f Gulf of 25° Oman

SAUDI ARABIA

OMAN 20 LT ° SA LI KHA MIT OF SEB

Red Sea

YEMEN 0 250 KILOMETERS ?

Figure 16. Distribution of the Jurassic Tithonian evaporite seal lithofacies (in shades of pink to purple) in Arabian Gulf region (modified from Murris, 1981). Queried boundaries shown. in the Arab Formation reservoirs, ranging from 1 to 30 percent Limestone is the primary reservoir; this unit consists of oolitic and from about 0.1 millidarcy (mD) to greater than 5,000 mD, limestone, dolomite, and anhydrite that were deposited in a shal- respectively. low marine and transitional marine setting of lagoons and shoals, similar to the Arabian and Southern Arabian Gulf Basins. Locally, oolitic and sandy, marly, argillaceous limestone reser- Sargelu Formation, Naokelekan Formation, and Najmah Formation voir facies of the Sargelu and Najmah Formations are laterally juxtaposed to argillaceous limestone marl and shale source-rock In the Gotnia Basin, source rocks are organic-rich beds facies of the Sargelu. As the seas regressed, the basin was cov- within the Sargelu, Naokelekan, and Najmah Formations (fig. ered with seal rocks of anhydrite intercalated with limestone and 14). However, where porous and permeable interbeds exist with dolomite. a local seal, they can also be a minor oil reservoir, such as at The overlying Gotnia Formation consists of evaporite, Hamrin and Alan fields (fig. 3). Generally, the Najmah anhydrite, oolitic limestone, and bituminous shale that were

22 Total Petroleum Systems, Widyan Basin and Interior Platform, Saudi Arabia and Iraq Total Petroleum Jurassic Cretaceous Paleogene Neogene System Events Source (Tuwaiq Mtn. and Hanifa) Reservoir (Arab carbonates)

Seal (Hith evaporites)

Overburden

Trap development

Generation and expulsion

Generation Expulsion Preservation

Figure 17. Events chart for that part of the Jurassic Gotnia/Barsarin/Sargelu/Najmah Total Petroleum System (202302) that lies in Arabian Basin (modified from Cole, Carrigan, and others, 1994).

deposited in an evaporite basin and lagoonal setting. The Gotnia principal periods of growth. Reservoir rocks and seals are simi- is locally a minor oil reservoir. The anhydrite is a cap rock. Oil lar for both assessment units. tested in Fallujah-1 well in Fallujah field comes from the inter- bedded subordinate limestone units. In the Gotnia Basin, the Gotnia Formation is nearly isochronous with the Arab Formation Platform Horst/Graben-Related Oil Assessment Unit (20230201) (fig. 14) but has thicker evaporite cycles (Yousif and Neuman, The main producing areas of Saudi Arabia are located on the 1997). northeastern part of the Central Arabian Arch. The basement of Arabia was broken into blocks during Precambrian time. These blocks were reactivated in several stages: (1) the Hercynian Orogeny, (2) following Permo-Triassic rifting, (3) during Late Hydrocarbon Traps Cretaceous plate collision and nappe abduction, and (4) during late Miocene to recent. The Jurassic Gotnia/Barsarin/Sargelu/Najmah Total Petro- In the Widyan Basin, traps are on fault-block anticlines, leum System (202302) has two assessment units, the Platform with elongated generally north south trends, that occur over reac- Horst/Graben-Related Oil Assessment Unit (20230201) and the tivated, deep-seated Precambrian/Hercynian faults with some Basinal Oil and Gas Assessment Unit (20230202). Their bound- “drape” compaction (Ayres and others, 1982; Christian, 1997). aries are shown in figure 12, and their assessed undiscovered vol- The anticlines have flat tops and long linear flanks with relatively umes are shown in table 3. Both assessment units have traps that steep dips in the upper beds (as much as 10°). Ghawar oil field, formed on reactivated basement fault blocks with a drape com- east of this assessment area, is on one such structure that has a ponent resulting in elongate and broad anticlinal structures. total length of about 370 km. The region to the north of the west- Additionally, the Basinal Oil and Gas Assessment Unit contains ern Widyan Basin and extending to the Tigris/Euphrates drainage oil and gas that accumulated in folds resulting from plate colli- area may have good source and reservoir facies but limited conti- sion and nappe abduction that began in the Late Cretaceous. nuity of evaporite seal facies. Numerous prospects and structures Generally, trap growth took place through most of the Mesozoic, have been identified in Iraq, and many of them have irregular although Late Cretaceous and Miocene to Recent are the forms. In addition to major anticlines, there are many shorter

Jurassic Gotnia/Barsarin/Sargelu/Najmah Total Petroleum System 23 anticlines scattered over the Arabian shelf, all showing generally Timing and Critical Events north south orientation. Critical events are summarized in figure 17. The northern Gotnia source basin was buried deeper during the Cretaceous Basinal Oil and Gas Assessment Unit (20230202) than the Arabian platform and therefore passed into the oil win- dow in Early Cretaceous, some 35 million years before the Han- Numerous prospects and structures have been identified in ifa of the Arabian Basin (fig. 15). By Tertiary time, the Hanifa Iraq. Oil and gas are trapped in elongate north-south-trending was in the oil window, and perhaps a second migratory phase or fault-block anticlines over reactivated, deep-seated faults in the the original migratory phase began. Mesopotamian Basin region. Also, there may have been early The Tuwaiq Mountain Formation source rock reached entrapment when Zagros folds were beginning to form during early-stage maturity for oil generation at about 75 Ma and Late Cretaceous and Tertiary, coincident with peak petroleum attained peak oil expulsion in the deeply buried areas by Late migration. Later (during the Miocene-Pliocene) oil generation Cretaceous and early Tertiary; hydrocarbons then began to accu- may have been renewed, associated with thick molasse deposi- mulate in broad, gentle structures. By 25 Ma, most of the hydro- tion to the east. Tectonic elements of the Mesopotamian Basin, carbons generated in the Tuwaiq Mountain and Hanifa Zagros Fold Belt, and Zagros Thrust Zones, along with the Formations north and south of the basin center had also begun to northwest-trending compressive folds accompanying the Zagros migrate into basin margin structures (Alsharhan and Nairn, collision zone, may contain significant volumes of petroleum 1997; Cole, Carrigan, and others, 1994). There are presently that were derived from Jurassic source rocks. large areas of active oil-generation. Hanifa source rocks were in the oil window and migration occurred by Tertiary time (fig. 15). Source rocks to the north in the Sargelu Formation of the Gotnia Basin passed into the oil Petroleum Assessment window during the Early Cretaceous. If oil migrated southward Within the boundary of the Widyan Basin–Interior Platform from this basin into the region of Province 2023, an additional Province (2023), portions of five total petroleum systems con- 35 million years of migration and accumulation may have tribute to the petroleum assessment of this province. Two of occurred, but this would have to predate the folding that took these total petroleum systems are dominant in the province and place during Campanian time in the early Zagros collisional are described in this report, that is, the Paleozoic Qusaiba/ zone. Previously trapped oil in the Gotnia Basin may have been Akkas/Abba/Mudawwara TPS and the Jurassic Gotnia/Barsarin/ remobilized and migrated into Zagros-related folds. Sareglu/Najmah TPS. Portions of three other petroleum systems are also found within the province. Portions of two petroleum systems allocated to the Greater Ghawar Uplift Province (2021) Seals and Overburden Rocks extend into Province 2023, including the Central Arabia Qusaiba Paleozoic TPS and the Arabian Sub-Basin Tuwaiq/Hanifa-Arab The regional seal of the Arabian shelf is the Upper Jurassic TPS. The Zagros-Mesopotamian Cretaceous-Tertiary TPS Tithonian Hith Anhydrite, a massive 90- to 120-m-thick anhy- assigned to the Zagros Fold Belt Province (2030) also extends drite that formed when the south margin of the Tethys sea broke into Province 2023. Allocations were made to Province 2023 up (fig. 16). Where the anhydrite pinches out in the southern from these three TPS whose areal extent crosses into Province Arabian Gulf region, upward and lateral leakage of hydrocar- 2023 (table 1). bons took place from below into younger reservoirs. Upward Previous assessments of Saudi Arabia by Masters and oth- seepage through the salt caps is facilitated only where perme- ers (1994) did not include estimates for the Paleozoic Total ability is enhanced, resulting from fracturing, faulting, or facies Petroleum System; thus direct comparisons are not made with changes. The Upper Jurassic Hanifa and Jubaila Formations that assessment. The significant discoveries of Paleozoic have interbedded shale and carbonate reservoirs, with the shale hydrocarbons, particularly in Iraq at the Akkas field, signifi- beds providing seals to trap hydrocarbons in the carbonate cantly alter the petroleum potential for this province. Petro- reservoirs. In central Saudi Arabia, local seals are tight anhydrite leum-migration modeling studies were undertaken to understand and dolomite in the Arab Formation. The thin layers of anhy- the hydrocarbon potential for both Paleozoic and Jurassic Total drite that separate the four units of the Arab Formation were Petroleum Systems in the province. These modeling studies deposited in restricted intra-shelf depressions on a silled shelf in (Steinshour and others, 2001) coupled with proprietary prospect an arid, low-latitude location. information were critical to clarification of the undiscovered To the north in Iraq, the Najmah and Gotnia-Barsarin are potential of these systems. less cyclic. There, the Gotnia Formation is about 200 m of anhy- Known petroleum volumes of 18.7 BBOE (17.4 BBO, 7.4 drite with subordinate shale and limestone, and forms a tight seal TCF gas or 1.23 BBOE, and <0.1 BBNGL) give the province a for local oil and gas accumulations in the underlying Najmah ranking of 23rd in the world, exclusive of the United States Limestone. (Klett and others, 1997). The recent U.S. Geological Survey

24 Total Petroleum Systems, Widyan Basin and Interior Platform, Saudi Arabia and Iraq assessment estimated the total petroleum endowment (cumula- half-grabens. In Iraq, reservoirs are in sandstones of the Ordov- tive production plus remaining reserves plus estimated mean ician Upper Khabour and Silurian Akkas Formations. undiscovered volumes) for this province at 62.54 BBOE (38.7 Structural features that control accumulation of petroleum BBO, 102.1 TCF gas or 17.1 BBOE, and 6.84 BBNGL; U.S. in Saudi Arabia are moderate-relief, fault-generated structures Geological Survey World Energy Assessment Team, 2000). This with 30–100 m of closure in north-trending block-faulted anti- petroleum volume gives the province a ranking of 17th in the clines (horsts and grabens) that were reactivated over Precam- world exclusive of the U.S. It is classified as an oil province brian basement blocks during the Late Devonian to (Ahlbrandt and others, 2000). Carboniferous Hercynian Orogeny. Hydrocarbons migrated ver- The estimated mean undiscovered volumes of the Paleozoic tically along reactivated basement-rooted faults or associated Qusaiba/Akkas/Abba/Mudawwara TPS (202301) are 2.63 BBO, fracture zones along the flanks of these structures. Combined 64.51 TCF gas or 10.75 BBOE, and 5.08 BBNGL. Recent data stratigraphic-structural traps may also exist where the Carbonif- from Paleozoic reservoirs of this TPS, particularly in Jordan and erous–Lower Permian Unayzah Formation is the reservoir, as is Iraq, indicate relatively high thermal maturity, which is also the case in central Saudi Arabia. reflected in dominance of gas and natural gas liquids within the Petroleum is sealed in the Unayzah reservoirs by overly- system (table 2). ing tight carbonate-evaporites within the Upper Permian Khuff The mean estimates of undiscovered volumes of the Juras- Formation or in overlying Triassic strata, and by sub-unconfor- sic Gotnia/Barsarin/Sargelu/ Najmah TPS (202302) are 6.60 mity pinchouts of pre-Unayzah clastic reservoir units against BBO, 22.94 TCF gas or 3.82 BBOE, and 1.11 BBNGL (table 3). impermeable facies. Lower Silurian shale extends over most of This system is dominantly oil, reflecting the lower thermal matu- the Southwestern Desert of Iraq and acts as a seal for hydrocar- rity of the Jurassic source rocks of this TPS when compared to bons in the underlying Ordovician Khabour Formation. Silurian source rocks of the Paleozoic system. Mean undiscov- The Jurassic Gotnia/Barsarin/Sargelu/Najmah Total Petro- ered oil estimates for the province, which include both Paleozoic leum System in the Widyan Basin–Interior Platform Province and Jurassic petroleum systems as well as portions of the other has two assessment units: (1) Platform Horst/Graben-Related three TPS allocated to the Widyan Basin–Interior Platform Prov- Oil, and (2) Basinal Oil and Gas. Reservoirs are developed in the ince (2023) (U.S. Geological Survey World Energy Assessment Upper Jurassic Najmah Limestone and Gotnia Formation (in Team, 2000), are 21.22 BBO, 94.75 TCF gas (15.8 BBOE) and Iraq), and equivalent Arab Formation (in Saudi Arabia) as lenses 6.85 BBNGL (table 1). of marine bar or shelf margin calcarenites, calcarenitic lime- The province discovery maturity of 31 percent demon- stone, and dolomite. These grade eastward into organic-rich strates that much of the province’s petroleum potential for muddy limestone hydrocarbon source rocks that were deposited both oil and gas lies in the future (Ahlbrandt and others, under anoxic and dysoxic conditions in three restricted intra- 2000). shelf basins—from north to south, the Gotnia, Arabian, and Southern Arabian Gulf Basins. Maturation of the Upper Jurassic source-rock formations (in Iraq named Sargelu and Naokelekan) Summary began around 90 Ma, with peak generation from 85 to 13 Ma. Oil was first trapped in calcarenite lenses, then was concentrated The Paleozoic Qusaiba/Akkas/Abba/Mudawwara Total in anticlines by structural growth that may have begun in Early Petroleum System has one assessment unit in the Widyan Basin– Cretaceous time and completed by Miocene time. Younger Interior Platform Province, identified as the Horst/Graben- Jurassic shale and anhydrite seal rocks are distributed through- Related Oil and Gas Assessment Unit. High-gravity crude oil out the total petroleum system. and natural gas occur in horst/graben-related traps that formed Major factors contributing to the large reserves of oil in prior to, during, and after the Hercynian Orogeny (Carbonifer- Jurassic reservoirs on the Arabian shelf are (1) a carbonate- ous). Petroleum is sourced from organic-rich marine “hot shale” evaporite shelf containing extensive high porosity and perme- at the base of Silurian strata (Qusaiba, Akkas, Mudawwara, and ability reservoirs adjacent to widespread and thermally mature Abba Formations) that were deposited under dysoxic to anoxic organic-rich source rocks; (2) cyclic sedimentation in which conditions in an intra-shelf basin located north of the Central most cycles are capped with widespread evaporite seals Arabian Arch. Onset of oil generation in Iraq began about 250 deposited during regressions; and (3) growth of large struc- Ma and in eastern Saudi Arabia about 160 Ma, reaching a peak tural traps coincident with long phases of oil generation and with accompanying migration and entrapment through the Juras- migration. sic. By the early Tertiary maturation had largely ceased. The Zagros collision of the Arabian and Eurasian plates in the Miocene resulted in reactivation of maturation and generation, References Cited and increased trap capacity. In Saudi Arabia, fluvial and eolian quartzose sandstones of Agrawi, A.A.M., 1998, Paleozoic stratigraphy and petroleum systems of the Carboniferous–Lower Permian Unayzah Formation overlie the Western and Southwestern Deserts of Iraq: GeoArabia, v. 3, no. the Hercynian unconformity and were deposited in rifts and 2, p. 229–247.

Summary 25 Ahlbrandt, T.S., Charpentier, R.R., Klett, T.R., Schmoker, J.W., and Environmental Systems Research Institute Inc., 1992, ArcWorld 1:3M dig- Schenk, C.J., 2000, Analysis of assessment results, Chapter AR in ital database: Environmental Systems Research Institute, Inc. 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26 Total Petroleum Systems, Widyan Basin and Interior Platform, Saudi Arabia and Iraq