paleozoic (pre-khuff) hydrocarbon geology of the ghawar

273

Paleozoic Hydrocarbon Geology of Ghawar, Saudi Arabia

Paleozoic (Pre-Khuff) Hydrocarbon Geology of the Ghawar Area,Eastern Saudi Arabia

Lawrence E. Wender, Jeffrey W. Bryant, Martin F. Dickens,Allen S. Neville and Abdulrahman M. Al-Moqbel

Saudi Aramco

ABSTRACT

Saudi Aramco is conducting an exploration program to discover additional non-associated gas reserves in the Ghawar Area. The program has successfully discoveredsignificant sweet gas and condensate reserves in the pre-Khuff siliciclastics and hasfurther increased our understanding of the Paleozoic petroleum system.

The Lower Permian Unayzah Formation is the principal pre-Khuff hydrocarbonreservoir in the Southern Ghawar Area, where it contains both oil and gas. TheUnayzah consists of fluvial to marginal marine sandstones. The Devonian JaufFormation is the principal pre-Khuff reservoir in the Northern Ghawar Area, whereit hosts the recently discovered giant Hawiyah gas-condensate field. The Jauf consistsof shallow marine sandstones that exhibit unusually high porosities considering theburial depths. The primary source rock for pre-Khuff hydrocarbons is the basal “hotshale” of the Lower Silurian Qalibah Formation. Maturation modeling of these shalesindicates hydrocarbon generation began in the Middle Triassic (oil) and continues tothe present (dry gas).

Pre-Khuff hydrocarbon traps are found in simple four-way closures as well as morecomplex structural-stratigraphic traps on the flanks of Hercynian structures. Trapformation and modification occurred in four main phases: (1) Carboniferous(Hercynian Orogeny); (2) Early Triassic (Zagros Rifting); (3) Late Cretaceous (First orEarly Alpine Orogeny); and (4) Tertiary (Second or Late Alpine Orogeny). Structuresin the Ghawar Area show differences in growth histories, which have impacted theamount and type of hydrocarbons contained.

INTRODUCTION

As a result of the increasing demand for natural gas in the Kingdom of Saudi Arabia, Saudi Aramcohas carried out an aggressive exploration program to discover additional non-associated gas reservesnear existing facilities. The first phase of this program began in early 1994 and focused on the pre-Khuff of the Ghawar Area (Figures 1 and 2), where non-associated sweet gas accumulations wereknown to exist.

The initial exploration well of this program, located on the eastern flank of Central Ghawar (HawiyahWell-200), was completed in August of 1994 as a deeper pool gas-condensate discovery. Since theHawiyah discovery, several pre-Khuff penetrations have been drilled, resulting in significant additionsto Saudi Aramco’s non-associated gas and condensate reserves base.

In addition to the new wells, several thousand line kilometers of 240-fold split spread 2-D seismic datausing longer offsets of 5 to 6 kilometers (km) have been acquired since the program was initiated. Thenew information gained from the drilling, testing, and seismic operations has substantially increasedour knowledge of the pre-Khuff geology of the Ghawar Area.

The purpose of this paper is to synthesize this data to provide an insight into the Paleozoic hydrocarbongeology of the Ghawar Area. Furthermore, this paper addresses the growth history of Ghawar andsurrounding structures to determine the relationship of structural growth to the hydrocarbonoccurrences in the Ghawar Area. The generalized geologic map of the southern part of the ArabianPeninsula with the Ghawar Study Area is shown in Figure 1.

GeoArabia,Vol. 3, No. 2, 1998Gulf PetroLink, Bahrain

Downloaded from https://pubs.geoscienceworld.org/geoarabia/article-pdf/3/2/273/4552715/wender.pdfby gueston 02 September 2019

274

Wender et al.

STRATIGRAPHIC FRAMEWORK AND BASIN EVOLUTION

The pre-Khuff sediments in the Ghawar Area are composed of a massive sequence of siliciclasticsediments with a thickness range from approximately 8,000 feet (ft) in the basinal areas to 2,000 ft onthe crest of Ghawar, where significant erosion has occurred. This erosion is the result of uplift associatedwith the Hercynian Orogeny, which we consider to be of Carboniferous age in the Ghawar Area.These clastics were deposited in terrestrial to shallow marine environments on the stable passive marginof Gondwana (Beydoun, 1991).

With the exception of the Hercynian Unconformity, which is overlain by the Unayzah or Khuffformations, the stratigraphic succession of the pre-Khuff in the Ghawar Area is generally conformable.Pre-Hercynian thickness variations observed are thought to be due to broad epierogenic uplifts of theArabian Plate, creating intracratonic sag basins with intervening gentle arches. Figure 3 shows thegeneralized Paleozoic stratigraphic column of the Ghawar Area. A composite log showing the wirelinelog characteristics and major reservoirs of the Paleozoic is shown in Figure 4. Figure 5 shows thesubcrop of the pre-Unayzah and pre-Khuff (where the Unayzah Formation is absent) unconformities.

Precambrian-Early Cambrian

The Precambrian to Lower Cambrian “basement rocks” of the Ghawar Area are composed of steeplydipping, fractured metasediments. The basement complex of the Arabian Shield was subjected tocompressional movements associated with the Idsas Orogeny (McGillivray and Husseini, 1992). Thisevent created the dominant present-day north-south structural grain in the Ghawar Area. Subsequentrejuvenation of the north-south structural elements has controlled the structural development of thesedimentary cover rock and has had a major influence on the hydrocarbon distribution of the area.

Figure 1: Generalized geologic map of the Arabian Peninsula showing the Ghawar study area.

ARABIAN GULF

RE

D S

EA

RUB 'AL K

HALI

NAFUD

Jeddah

Riyadh

GHAWARAREA

Km

5000

PALEOZOIC

TERTIARYMESOZOIC

QUATERNARY-TERTIARYVOLCANICS

QUATERNARY

PRECAMBRIANBASEMENT

35° 40° 45° 50° 55° 60°

15°

20°

25°

30°

65°

GULF OF OMAN

ARABIAN SEA

GULF OF ADEN


275


Figure 2:Location map ofthe Ghawarstudy areashowing pre-Khuffpenetrations (inblack).

The north-south orientation of major basement-involved structures such as Ghawar can readily beseen on gravity maps, where structural highs are generally associated with positive gravity anomalies.Northwest-southeast structural elements which run parallel to the Najd-fault system (Stern, 1985;Husseini and Husseini, 1990) are also present in the Ghawar Area, but are of secondary importance.

The basement complex has been penetrated by two wells in the Ghawar Area. In both wells, thebasement is composed of highly-fractured metasediments, which have a distinctive high resistivity,high velocity (high sonic travel times), and moderate gamma-ray wireline log character. Thesemetasediments have been radiometrically dated at 671 and 604 million years (Ma).

Cambrian-Ordovician

Following peneplanation of eastern Arabia and the formation of the pre-Saq Unconformity (PSU),terrestrial to marginal marine sediments of the Saq Formation were deposited. The Saq in the GhawarArea consists of arkosic sandstones and red micaceous siltstones in the lower part with a generallycleaner, skolithus-bearing sandstone unit at the top. This upper, shallow-marine sandstone is welldeveloped in the Haradh Area (Figure 2), and is informally referred to as the “Saq-A sandstone”. Theupper Saq at Ghawar has sporadic hydrocarbon shows, but is a poor reservoir in the Ghawar Areadue to silica cementation.

Abqaiq

Shedgum

'Uthmaniyah

Haradh

Sahba

Waqr Tinat

'Ain Dar

Ghawar

Udaynan

Farzan

Hawiyah

0 50

Km

50°

26°

24°

25°

49°

D

D'

Figure 19

C

C'

Figure 17

A

A'

Z

Z'

Y

X

X'

Y'

Harmaliyah

Figure 9

Figure 10

Figure 11Figure 13

NibanProspect


276

Wender et al.

Non

-Mar

ine

M

arin

e

GH

AW

AR

CR

ES

T

PK

U /

PU

U

PE

RM

IAN

DE

VO

NIA

NS

ILU

RIA

NO

RD

OV

ICIA

NC

AM

BR

IAN

LAT

EE

AR

LYLA

TE

MID

DLE

EA

RLY

LAT

EE

AR

LYLA

TE

MID

DLE

EA

RLY

LATE

MIDDLE

EARLY

KHUFF

UNAYZAH

JUBAH

JAUF

TAWIL

QALIBAH

SARAH

QASIM

SAQ

PRE-SAQ

UNDIV.

BASEMENT

KAZANIAN

TATARIAN

KUNGURIAN

SAKMARIAN

ARTINSKIAN

ASSELIAN

FAMENNIAN

FRASNIAN

GIVETIAN

EIFELIAN

SIEGENIAN

EMSIAN

GEDINNIAN

PRIDOLI

LUDLOW

WENLOCK

LLANDOVERY

ASHGILL

CARADOC

LLANDEILO

LLANVIRN

ARENIG

TREMADOC

"D-3B Zone"

Sharawra

Qusaiba

"Hot Shale"

Quwarah

Ra'an

Kahfah

Hanadir

A

B

C

D

SILURIAN HIATUS

SYSTEM

SERI

ES LITHOLOGY TECTONO-STRATIGRAPHIC

EVENTFORMATION MEMBER /

UNIT S N

"D" Anhydrite

STAGE

A

B

Neo-TethysOpeningS

helf

Car

bona

tes

Marine Transgression

Marine Influenced

Gondwana

Rapid Marine Transgression

Glaciation

Meg

a C

oars

enin

g U

pwar

d C

ycle

Passive Margin

Gondwana

Peneplanation

Najd Rifting

Idsas Orogeny/Island Arc Accretion

Passive Margin

Arid/Semi-AridLowstand Deposition

Infilling ofRelict Topography

Hercynian Orogeny

P C

PUU

PZSU

PSU

Source

PTU

Show

Reservoir

Figure 3: Generalized Paleozoic stratigraphic column of the Ghawar Area.


277


SUDAIR

JUBAH

A

A

B

B

C

D A

NH

YD

RIT

E

PUU

DE

VO

NIA

NP

ER

MIA

NT

RIA

SS

IC

KH

UF

FJA

UF

UN

AY

ZA

HTA

WIL

JAU

F

TAW

ILDE

VO

NIA

N

SARAH

QA

SIM

Qusa

iba M

em

ber

Shara

wra

Mem

ber

SIL

UR

IAN

QA

LIB

AH

OR

DO

VIC

IAN

GAMMA RAY(API)

PE

RIO

D

FORMATION/RESERVOIR

0.0 200.0

LITHOLOGY(interpretedwhole core)

0.20

DEN./NEU PORRESERVOIR

2,000.0

RESOLUTION(ohm-meters)

DENSITY (grams/cm)2.0 3.0

45.0 -15.0

NEWPORC (%)

GAMMA RAY(API)

PE

RIO

D

FORMATION/RESERVOIR

0.0 200.0 0.20

DEN./NEU PORRESERVOIR

2,000.0

RESOLUTION(ohm-meters)

DENSITY (grams/cm)2.0 3.0

45.0 -15.0

NEWPORC (%)

LITHOLOGY(interpretedwhole core)

500

Fee

t

Figure 4: Composite log of the Paleozoicof the Ghawar Area showing majorreservoirs (in red).


278

Wender et al.

ABQAIQ

SHEDGUM

HARADH

TINAT

WAQR

GHAWAR

HARMALIYAHY'

Y

X

X'

A

A

BC D

BC

D

Z

Z'

A BCD

E

F

0

Km

20

'AIN DAR

HAWIYAH

'UTHMANIYAH

LEGEND

Field Outline

Pre-Khuff WellUsed in Section

X

B

StructuralCross-section

X'

Dev

onia

nJubah Fm.

Jauf Fm.Tawil Fm.

Silurian Qalibah Fm.

Cambro-Ordovician,Undifferentiated

The Qasim embraces two distinct shale units in Northern Ghawar, the Ra’an and Hanadir, whichgrade into siltstone and sandstones to the south. This southward increase in sand content, which isobserved in several pre-Khuff units, is thought to be due to epierogenic uplift (ancestral Central ArabianArch?) in the southern Ghawar Area.

Following deposition of the Qasim Formation in the Late Ordovician, glaciation affected most of present-day western Arabia. Glacial and periglacial sediments have been recognized in the outcrop belt ofSaudi Arabia, and consist of tillites, sandstones, and siltstone units of variable thickness (McClure,1978; Vaslet, 1987, 1989). Recent field work in the Qasim Area has demonstrated the presence ofstriated glacial pavements, faceted and striated basement boulders, and tillites of Late Ordovician age(Senalp and Laboun, 1996). These deposits have been collectively referred to as the “Sarah-ZarqaFormation”, though recently it has been argued whether this terminology should be maintained (Senalpand Laboun, 1996).

These glaciogenic sediments do not appear to extend to the Ghawar Area, where this interval isrepresented by a relatively clean, fine- to medium-grained sandstone referred to as the “SarahFormation”. The Sarah Formation occurs immediately beneath the Qusaiba Member and represents a

Following Saq deposition, a sequence of micaceous sandstones with subordinate shales was deposited,which is assigned to the Qasim Formation (Williams et al., 1986). The Qasim was deposited in ashallow marine environment, and is made up of stacked sequences reflecting eustatic sea level changes.

Figure 5: Pre-UnayzahUnconformity (PUU) subcropmap with locations ofstructural cross sections. SeeFigure 9 (XX’), Figure 10 (YY’)and Figure 11 (ZZ’) forcross-sections.

LEGEND

Field Outline

Pre-Khuff WellUsed in Section

X

B

StructuralCross-section

X'D

evon

ianJubah Fm.

Jauf Fm.

Tawil Fm.

Silurian Qalibah Fm.

Cambro-Ordovician,Undifferentiated


279


lowstand deposit associated with Gondwana glaciation. The base of the formation represents anerosional unconformity (Type 1 sequence boundary), and is known as the pre-Zarqa/SarahUnconformity (PZSU).

Numerous shows have been reported from the Qasim and Sarah formations in the Ghawar Area, but,as yet, no measurable hydrocarbon flow rates have been established. The best shows have beenobserved in the Sarah, which flowed gas to surface. However, no sustainable flow rates have beenobtained, which is due to low permeability.

Silurian

In the Early Silurian (Llandoverian), sea level rose due to deglaciation and resulted in the widespreaddeposition of the Qalibah Formation. The upward-coarsening, progradational Qalibah Formationconsists of a lower Qusaiba Member and an upper Sharawra Member (Mahmoud et al., 1992).

At the base of the Qusaiba Member, an organic-rich shale persists and is regionally correlatable. Thisshale, with its distinctive high gamma-ray signature on logs, is referred to as the “hot shale”, and isconsidered the principal source rock for Paleozoic hydrocarbons in Saudi Arabia (McGillivray andHusseini, 1992; Mahmoud et al., 1992; Cole et al., 1994; Halpern and Carrigan, 1995 unpublished report).The “hot shale” was deposited during the rapid Early Silurian transgression over Gondwana followingglaciation, and is thought to represent a condensed section with the top of the “hot shale” correspondingto the maximum flooding surface.

The thickness of the “hot shale” in the Ghawar Area varies considerably, and ranges from 229 ft inSouthern Ghawar to 9 ft in Northern Ghawar. A significant acoustical impedance contrast occurs atthe base of the Qusaiba, resulting in a persistent seismic reflection. This reflector is the most reliableseismic event in the pre-Khuff section.

A sandstone/siltstone unit, informally referred to as the “Mid-Qusaiba Sand”, is present as a keystratigraphic marker within the Qusaiba throughout most of the Ghawar Area. The unit is composedof a crudely-thickening and coarsening-upward sequence and most probably represents aprogradational basin floor fan system. These sandstones have had significant gas shows in the GhawarArea, and have flowed gas at commercial rates in one well in Southern Ghawar (Haradh).

The overlying Sharawra Member consists of micaceous sandstones, siltstones, and shales representinga continuation of the progradational, coarsening-upward sequence. Sporadic hydrocarbon showshave been encountered in the Sharawra, but formation analyses indicate poor reservoir development.

A Late Silurian hiatus is observed in the Ghawar Area, as part of the uppermost Silurian (Wenlockian-Ludlovian) appears to be missing. This missing section is thought to reflect non-deposition ratherthan tectonic uplift and erosion (Mahmoud et al., 1992).

Devonian

The Tawil Formation was deposited after the “Silurian Hiatus”, and consists of coarser clastics. TheTawil is primarily composed of fine- to medium-grained gray to reddish sandstones with subordinatemicaceous siltstones and shales. These thin, discontinuous micaceous siltstones and shales yield aseries of sharp “gamma spikes” that give the Tawil a characteristic log signature. The Tawil wasdeposited in a fluvial to marginal marine environment.

The age of the Tawil is generally considered to be Early Devonian, though recent biostratigraphicanalyses indicate the Tawil to include the latest Silurian (Pridolian). The Tawil varies considerably inthickness, ranging from 1,010 ft to 225 ft in the Ghawar Area. This thickness variation is thought to bedue to depositional control, though an erosional unconformity at the base of the Jauf Formation cannotbe ruled out. The Tawil is commonly tightly cemented with silica and kaolinite, and is considered apoor reservoir.


280

Wender et al.

After deposition of the Tawil, the shallow marine sands of the Jauf Formation were deposited over abroad shelf. The Jauf sandstones have the most favorable pre-Khuff reservoir development in theGhawar Area, and are the host sands for the recently discovered giant gas-condensate field at Hawiyah.The Jauf consists of fine- to medium-grained sandstones that lack the silica cement that is so prevalentin other pre-Khuff sandstones of the Ghawar Area. Instead, the reservoir sandstones are weakly-cemented with authigenic illite clay. The Jauf Reservoir is further discussed in the “Reservoir Rock”section in this paper.

The reservoir sandstones of the Jauf are capped by a very distinctive shaley/silty unit informallycalled the “D-3B” stratigraphic marker. On wireline logs, it is characterized by a shaley/silty zonewith relatively high resistivities and velocities which rests directly on porous sands of the Jauf Reservoir(Figure 8). This zone is progressively better developed to the northeast, where it forms the seal for theJauf gas-condensate accumulation at the offshore Abu Safah field, which is located approximately 70km north of the island of Bahrain. The unit is characterized by an abundance of marine sphaeromorphacritarchs of Saudi Aramco’s “D-3B” palynozone.

The D-3B marker unit is considered to have been deposited during a sea level rise and may representa condensed section with a maximum flooding surface. Regional log and biostratigraphic correlationsindicate this unit to be correlative to the Hammamiyat Limestone Member of Northwest Arabia (Al-Hajri et al., 1996). The age of the Jauf Formation is Early to Middle Devonian (Emsian-Early Givetian).

Overlying the Jauf Formation is the Middle to Upper Devonian Jubah Formation. The Jubah ischaracterized by a rather monotonous sequence of very fine- to medium-grained sandstones withsubordinate siltstones and shales. The thickest occurrence of the Jubah is at Abqaiq, where 1,451 ftwere penetrated. Sporadic shows in the Jubah have been observed, but log and test analyses indicatethe sands to be of poor reservoir quality.

Carboniferous-Early Permian

As a result of the collision of Gondwana and Laurentia in Early Carboniferous(?) time, the northeastmargin of Gondwana was transformed from a passive to an active margin (Beydoun, 1991). Thistectonic event is termed the Hercynian Orogeny, and has had a dramatic effect on the geology ofArabia. Major north-south trending, basement-involved horst blocks formed in Central and EasternSaudi Arabia during this event, and are the sites of significant amounts of erosion. On the crestalportion of Ghawar, for example, approximately 3,500 ft of section was removed. The fault-boundedHercynian structures, such as Khurais (located 100 km west of Ghawar) and Ghawar, contain significantoil and gas reserves, and are generally well-expressed on gravity maps as positive anomalies.

Sediments of Early Carboniferous age are unknown in the Ghawar Area. During this time an extensiveerosional surface developed, which is referred to as the “pre-Unayzah Unconformity” (PUU) orHercynian Unconformity. The generalized pre-Permian geology of the area is shown on the PUUSubcrop Map (Figure 5). This map illustrates the significant amount of uplift and erosion at Ghawarduring the Hercynian Orogeny.

Deposition of the Middle Carboniferous(?) to Lower Permian Unayzah Formation followed theHercynian Orogeny. The basal Unayzah Formation consists of fine- to coarse-grained fluvial/alluvialsands that filled the relict topography due to differential erosion of the Hercynian structures (Al-Laboun, 1987). In Southern Saudi Arabia and Oman, glacial sediments of the Lower Unayzah(?), andthe Omani equivalent Al-Khlata Formation, were deposited (McClure, 1980; McClure et al., 1988;Hughes-Clarke, 1988; Levell et al., 1988). These glaciogenic sediments may be represented in theGhawar Area by a southeasterly-thickening wedge of undated, uncorrelatable siliciclastics in theHaradh-Tinat Area.

In the Southern Ghawar Area, the Unayzah forms the principal pre-Khuff hydrocarbon reservoir, andis divided into a lower Unayzah-B Reservoir, a middle Siltstone Member, and an upper Unayzah-AReservoir. The Unayzah is missing in Northern Ghawar, where the base of the Khuff Formation restsunconformably on pre-Hercynian sediments.


281


extensional phase resulted in the reactivation of earlier Hercynian faults and therefore formed animportant growth event for pre-Khuff hydrocarbon-bearing structures in the Ghawar Area (seediscussion on “Trap Formation”).

Jurassic-Early Cretaceous

Following the extensional structural adjustments in the Triassic, the Arabian Plate returned to a majorperiod of tectonic stability. A thick sedimentary sequence dominated by shallow water carbonatesaccumulated on a broad, stable platform (Alsharhan and Kendall, 1986). These rocks contain remarkablyuniform and aerially extensive source, seal, and reservoir units that host the world’s largest oil reserves(Murris, 1980).

In the Ghawar Area, the platform was west-dipping into an intrashelf basin located to the west ofGhawar. This westerly dip was reversed in Late Cretaceous time, when the Arabian Plate started totilt eastward due to differential loading of the plate margin.

Late Cretaceous-Tertiary

In Late Cretaceous time, tectonic activity again interrupted passive margin depositional conditions onthe Arabian Plate (Grabowski and Norton, 1995). At this time, the subduction complex of the Neo-Tethys collided with Oman, resulting in uplift and emplacement of the Semail Ophiolite Complex inOman (Glennie, 1995). This tectonic episode has been termed the First Alpine Event (Loosveld et al.,1996). In Eastern Saudi Arabia, this tectonic event led to significant structural growth and is representedstratigraphically by the pre-Aruma Unconformity (PAU). Most structures in the Ghawar Area showgrowth during this period. Additionally, in Late Cretaceous time, the dominant westward tilt of theGhawar Area was reversed, with the regional dip changing to the east.

Late Alpine tectonic movements affected Arabia from the Middle to Late Tertiary. At this time, Arabiaseparated from Africa along the Red Sea Rift and collided with Eurasia, resulting in compression ofthe Arabian Plate (Beydoun, 1991). In Oman, this collision resulted in the main uplift of the OmanMountains and has been termed the Second Alpine Event (Loosveld et al., 1996). In Eastern SaudiArabia, this event led to major reactivation of pre-existing structures and is represented stratigraphicallyby the pre-Neogene Unconformity (PNU). Additionally, the regional eastward tilting of the ArabianPlate, initiated in the Late Cretaceous, continued.

The significance of these tectonic events, and the effects on the growth histories of structures in theGhawar Area, is further discussed in the section entitled “Trap Formation” later in this paper.

Late Permian-Triassic

Following deposition of the Unayzah Formation, a major transgression occurred throughout Arabia,during which the predominantly carbonate Khuff Formation was deposited. This transgressioncoincided with Late Permian rifting along the Zagros suture and the opening of the Neo-Tethys ocean(Sengör et al., 1988; Beydoun, 1991).

The Khuff consists predominantly of cyclic shallow water carbonates and evaporites that generallythicken to the east. In the Ghawar Area, the Khuff ranges in thickness from approximately 1,400 ft atHaradh to 1,750 ft at Abqaiq. The Khuff carbonates contain major non-associated gas reserves withvarying amounts of hydrogen sulfide. The hydrogen sulfide content, in general, increases to the northin the Ghawar Area, concurrent with increasing reservoir temperatures.

At the base of the Khuff Formation, transgressive sands and shales occur which are commonly referredto as the “Basal Khuff Clastics”. The regionally extensive shales, along with associated tight carbonates,form the major regional seal for pre-Khuff hydrocarbon accumulations in Central and Eastern SaudiArabia.

At the end of Khuff deposition in the Early Triassic, shales and subordinate carbonates of the SudairFormation were deposited. This was followed by deposition of a mixed assemblage of shales,carbonates, and sands of the Jilh Formation during continued opening of the Neo-Tethys. This


282

Wender et al.

HYDROCARBON GEOLOGY

Exploration History

Saudi Aramco has drilled approximately 45 pre-Khuff wells in the Ghawar Area as of mid-1997. Thesepre-Khuff penetrations are shown in Figure 2. Depths to the base of the Khuff range from approximately12,000 to 15,000 ft.

Pre-Khuff hydrocarbons in the Ghawar Area were first discovered at Shedgum in 1980. The Shedgumdiscovery well flowed sweet gas and condensate from the Devonian Jauf Reservoir. The next majorpre-Khuff gas discovery was at Haradh in 1982. The discovery well flowed significant quantities ofsweet gas from both the Permian Unayzah-A and Unayzah-B Reservoirs. Also in 1982, light, sweet oilwas discovered at Tinat in the Unayzah-A Reservoir. In 1984, non-associated sweet gas was discoveredat Sahba in the Unayzah-A Reservoir (Figure 2).

The next major discovery in the pre-Khuff in the Ghawar Area did not occur until 1994, when theHawiyah-200 well discovered gas-condensate in the Jauf Reservoir on the east flank of Ghawar.Hawiyah-200 flowed 20.2 million cubic feet gas per day (mmcfgpd) of sweet gas with 3,286 barrels perday (bpd) of condensate. Hawiyah-200 was the first test of the recently defined Jauf flank play atGhawar, and has provided a major impetus to explore for similar structural/stratigraphic traps alongthe flanks of Ghawar, as well as other deeply-eroded Hercynian structures.

In early 1997 another major gas-condensate discovery was made at the Waqr structure, locatedsouthwest of Ghawar. The discovery well flowed in excess of 40 mmcfgpd from the Unayzah Formation.The Waqr structure is located along a separate north-south structural trend which, like Ghawar,originated in the Hercynian (Figure 2).

The pre-Khuff petroleum system is graphically depicted in Figure 6, and shows the essential elementsof a prospective hydrocarbon system: source, reservoir, seal, and timing of trap formation andhydrocarbon generation.

Source Rock

Based on carbon isotope and chromatographic fingerprinting techniques, the primary source rock forPaleozoic hydrocarbons in the Ghawar Area is the basal “hot shale” of the Lower Silurian QusaibaMember of the Qalibah Formation (Mahmoud et al., 1992; Cole et al., 1994; Halpern and Carrigan,1995, unpublished report). These basal Qusaiba shales have an average total organic carbon (TOC)

Figure 6: Pre-Khuff petroleum system of the Ghawar area.

366795140204245283312360410440500Ma

PALEOZOIC MESOZOIC CEN.

∈ O S D CL CU P TR J KL KU TL TU

GENERATION/MIGRATION

TRAP FORMATION

SEAL ROCK

RESERVOIR ROCK

SOURCE ROCK

PETROLEUM SYSTEMEVENTS

?

OIL WETGAS

DRYGAS


283


Figure 7: Unayzah Formation type log showing major subdivisions.

Pre-KhuffUnconformity

Pre-UnayzahUnconformity

100

Fee

tGAMMA RAY

(API)

0.0

2.0 3.0NEUPORC (%)

0.20 2,000.0 45.0 -15.0200.0

LITHOLOGYINTERPRETED

RESISTIVITYDENSITY-POROSITY

RESDILD

DENSITY (grams/cm)

(OHM-METER)

QusaibaMember

KhuffFormation

Unayzah-AReservoir

UnayzahSiltstoneMember

Unayzah-BReservoir


284

Wender et al.

richness of 3%, with a maximum observed TOC of 6.15% (Monnier et al., 1990, unpublished report;Cole and Colling, 1994, unpublished report). The organic matter in the Qusaiba has been identified aspredominantly amorphous Type II kerogen.

Other potential Paleozoic source rocks have been evaluated, but were determined to be poor sourcerock candidates due to low organic richness, insufficient thickness, and/or poor lateral continuity.These analyses include shales from the basal Khuff, Unayzah, Jauf, and Qasim formations.

Reservoir Rock

The major pre-Khuff hydrocarbon reserves discovered to date in the Ghawar Area are from the LowerPermian Unayzah Formation and the Lower to Middle Devonian Jauf Formation. The UnayzahFormation is the principal pre-Khuff hydrocarbon reservoir in the Southern Ghawar Area (Haradh,Sahba, Waqr and Tinat), while the Jauf Formation is the primary reservoir to the north (Hawiyah,‘Uthmaniyah and Shedgum).

The Unayzah Reservoir is generally divided into two units: the upper Unayzah-A Reservoir and thelower Unayzah-B Reservoir (Figure 7). These reservoir units are separated by a siltstone member,which is reddish to gray in color in the Southern Ghawar Area. This siltstone separator is well-developedin Southern Haradh, Sahba, and Tinat, but is poorly-developed to the north. The Unayzah is missingin Central and Northern Ghawar and thickens to the south. Thickness ranges from 0 ft in SouthernHawiyah to nearly 1,600 ft at Tinat, where a lower Unayzah unit (or its equivalent) is present beneaththe Unayzah-B Reservoir.

The Unayzah-A Reservoir has tested gas at Haradh, Sahba, and Harmaliyah. Premium crude oiloccurs in the Unayzah-A at Tinat, and is the only oil occurrence thus far found in the pre-Khuff of theGhawar Area. Porosities range from 5-25%, and average around 12% in the pay zones. Gas flow ratesfrom the Unayzah-A Reservoir are highly variable, ranging from less than 5 mmcfgd to more than 40mmcfgd.

The Unayzah-B Reservoir has only flowed measurable gas in southern Haradh. Porosities in this well,and surrounding Unayzah-B penetrations, are generally low and average approximately 6%. Higherflow rates in the Unayzah-B are attributable to fracture-enhanced permeability.

The Jauf Reservoir in the Ghawar Area is very well developed and displays unusually high porositiesconsidering the depth of burial. The Jauf Reservoir is the productive pre-Khuff unit at Shedgum,Hawiyah, ‘Uthmaniyah, and Abu Safah fields. Log correlations of Jauf penetrations in the EasternProvince of Saudi Arabia show excellent reservoir development and continuity.

The Jauf Reservoir ranges in thickness from 292 to 474 ft in the Ghawar Area. The type log for the JaufFormation is shown in Figure 8. The Jauf can be crudely divided into two units based on log character.The lower unit is characterized by “cleaner” looking sands on the gamma-ray, with few shale-siltstoneintercalations. The upper unit is characterized by “dirtier” looking sands (higher gamma-ray), withseveral shale intercalations. Both fining- and coarsening-upward sequences are evident in the upperunit. Reservoir quality is best developed in the upper unit at Ghawar.

As noted previously, the Jauf Reservoir lacks the silica cementation that is so prevalent in other pre-Khuff siliciclastic units. The reservoir sands are weakly-cemented with authigenic illite clay, whichoccurs as grain coatings, as well as pore lining and bridging filaments (Cocker and Al-Shahab, 1995,unpublished report). The illite grain coatings have inhibited quartz cementation, and is primarilyresponsible for the preservation of high porosity in these deeply-buried sandstones.

The abundance of dispersed illite in the reservoir sands has a significant effect on wireline log responses,and, if not accounted for, can lead to pessimistic formation evaluations (Figure 8). Among these potential“pitfalls’” is the lowering of resistivity values due to the excess bound water and high cation exchangecapacity of the illites. This can lead to pessimistic water saturation calculations and potentially bypassedlow-resistivity pay zones. Sands with resistivities as low as 0.35 ohm-meters and Archie-derivedwater saturations as high as 65% have produced water-free hydrocarbons in the Jauf Reservoir.


285


Figure 8: Jauf Formation type log showing the development of the Jauf Reservoir. Note the lowerresistivities displayed in the Jauf Reservoir sands due to the presence of authigenic illite clays.

GAMMA RAY (API)

0.0

2.0 3.0NEUPORC (%)

0.20 2,000.0 45.0 -15.0200.0

LITHOLOGYWELL

CUTTINGS

RESISTIVITYDENSITY-POROSITY

RESDILD

DENSITY (grams/cm)

(OHM-METER)

Pre-KhuffUnconformity

KhuffFormation

Pre-UnayzahUnconformity

Jauf

Res

ervo

ir

TawilFormation

SharawraMember

100

Fee

t

UnayzahFormation

JaufFormation


286

Wender et al.

Figu

re 9

: S

tru

ctu

ral c

ross

-sec

tion

X-X

’ in

the

Nor

ther

n G

haw

ar A

rea.

Th

e lo

cati

on o

f cr

oss-

sect

ion

is s

how

n in

Fig

ure

s 2

and

5.

X'

Jauf

Taw

il

Min

jur

Jilh

Sud

air

Khu

ff

Sha

raw

ra

Qus

aiba

Qas

im

Saq

Pre

cam

bri

anB

asem

ent

05

Km

Dhr

uma

Mar

rat

Min

jur

Jilh

Sud

air

Khu

ff

Juba

h

Jauf

Taw

il

Sha

raw

ra

Qus

aiba

Qas

im

Saq

X

1,00

0 ft

Pre

-Una

yzah

Unc

onfo

rmity

Dhr

uma

Mar

rat

Min

jur

Jilh

Sud

air

Khu

ff

Juba

h

Jauf

Taw

il

Sha

raw

ra

Qus

aiba

Qas

im

SaqPre

-Una

yzah

Unc

onfo

rmityW

ell-A

Wel

l-BW

ell-C

Wel

l-DW

ell-E

Wel

l-F


287


Additionally, the presence of illite in the sands increases the gamma-ray response due to the potassium,which makes the sands look “dirtier”. An additional log evaluation “pitfall” caused by the presenceof illite is the suppression of the “gas effect” noted on neutron-density logs. This suppression of theneutron-density curve crossover is caused by the higher apparent neutron porosity due to the abundantbound water and correspondingly high hydrogen index associated with the illite.

The Jauf Reservoir is host to the recently discovered giant gas-condensate field at Hawiyah, and isalso the productive pre-Khuff gas reservoir at Shedgum, ‘Uthmaniyah, and Abu Safah. Porosities upto 30% are observed in the Jauf Reservoir, which is extremely high considering the present-day depthsof approximately 14,000 ft. As noted earlier, the preservation of these high porosities is due to thepresence of illite grain coatings.

Seal Rock

The major regional seal for the pre-Khuff petroleum system corresponds to the transgressive shalesand carbonates of the basal Khuff Formation. These shales and tight carbonates were deposited overa widespread area during the Late Permian sea level rise. The basal Khuff forms the top seal to theUnayzah Reservoir in four-way structural closures in the Southern Ghawar Area. The basal Khuffalso forms the top seal to the Jauf Reservoir in the combination structural-unconformity type traps atShedgum, Hawiyah, and ‘Uthmaniyah. In the unconformity-related traps, the basal Khuffunconformably overlies a truncated Jauf Reservoir section, which was eroded during the HercynianOrogeny.

The Jauf Reservoir may also be sealed by the informally named “D-3D Zone” (Figure 3). This shaleyinterval is a widely correlatable transgressive unit directly overlying the Jauf Reservoir sandstones.As noted previously, this unit is progressively better-developed to the northeast, where it forms theseal for the Jauf gas accumulation at the offshore Abu Safah field. To the southwest, this unit getssandier, resulting in a progressive loss of sealing capacity.

Lateral seals associated with Hercynian faults are also important in controlling hydrocarbonaccumulations in the Ghawar Area. These pre-Khuff faults have juxtaposed the down-dropped JaufReservoir sands against shales, siltstones, and tight sandstones of the pre-Jauf section on the upthrownfault block. These faults, in turn, are overlain and sealed by the basal Khuff Formation; thoughreactivated faults cutting the Khuff are known to exist at Ghawar. It is also considered possible thatthe fault planes, with low calculated smear-gouge ratios, could act as effective seals.

Trap Formation

Structural development and trap formation in the Ghawar Area took place in four distinct stages: theCarboniferous stage associated with the Hercynian Orogeny; the Early Triassic stage associated withthe rifting along the Zagros suture and the opening of the Neo-Tethys sea; the Late Cretaceous stageassociated with the collisional event that led to the emplacement of the Semail Ophiolite Complex inOman (First Alpine Event); and the Middle to Late Tertiary stage associated with the opening of theRed Sea and collision of Arabia with Eurasia (Second Alpine Event). Three structural cross-sectionsillustrating the trapping configurations for pre-Khuff hydrocarbons in the Ghawar Area are shown inFigures 9 to 11. The locations of these cross-sections are shown on the PUU subcrop map (Figure 5).

Interestingly, the eight structures studied in the Ghawar Area (Figure 2) show different growth histories.Figure 12 is a summary chart of the individual structures in the Ghawar Area showing the differinggrowth histories as well as the presence of associated gravity anomalies and hydrocarbons.

To evaluate these growth histories, a series of isochron, isopach and time structure maps of criticalhorizons were generated using most of the available higher fold seismic data. The maps selected toillustrate the four main growth periods include: Base Khuff to Base Qusaiba Isopach (Carboniferous


288

Wender et al.

Figu

re 1

0: S

tru

ctu

ral c

ross

-sec

tion

Y-Y

’ in

the

Cen

tral

Gh

awar

Are

a. N

ote

the

fau

lt/t

run

cati

on tr

ap d

evel

oped

in th

e Ja

uf

For m

atio

n o

n th

e ea

st f

lan

k o

fth

e G

haw

ar h

orst

blo

ck.

Wel

l B

rep

rese

nts

th

e d

isco

very

wel

l of

th

e H

awiy

ah g

as-c

ond

ensa

te f

ield

in

th

e D

evon

ian

Jau

f Fo

rmat

ion

. Th

e lo

cati

on o

fcr

oss-

sect

ion

is s

how

n in

Fig

ure

s 2

and

5.

YY

'

Pre

cam

bri

anB

asem

ent

Dhr

uma

Mar

rat

Jilh

Sud

air

Khu

ff

Una

yzah

Juba

h

Jauf

Taw

il

Sha

raw

ra

Qus

aiba

Qas

im

Saq

Dhr

uma

Mar

rat

Min

jur

Jilh

Sud

air

Khu

ff

Juba

hJa

uf

Taw

il

Sha

raw

ra

Qus

aiba

Qas

im

Saq

05

Km

1,00

0 ft

Pre

-Una

yzah

Unc

onfo

rmity

Pre

-Una

yzah

Unc

onfo

rmity

Min

jur

Wel

l-AW

ell-B

Wel

l-CW

ell-D


289


Figu

re 1

1: S

tru

ctu

ral

cros

s-se

ctio

n Z

-Z’ i

n t

he

Sou

ther

n G

haw

ar A

rea.

Not

e th

e d

evel

opm

ent

of t

wo

sub

sid

iary

an

ticl

inal

tre

nd

s ea

st a

nd

wes

t of

th

em

ain

Gh

awar

hor

st b

lock

. B

oth

of

thes

e st

ruct

ure

s (T

inat

an

d W

aqr)

are

pro

du

ctiv

e in

the

Per

mia

n U

nay

zah

For

mat

ion

. Th

e lo

cati

on o

f cr

oss-

sect

ion

issh

own

in F

igu

res

2 an

d 5

.

ZZ

'

Pre

cam

bri

anB

asem

ent

05

Km

1,00

0 ft

Wel

l-AW

ell-B

Wel

l-CW

ell-D

Dhr

uma

Jilh

Sud

air

Khu

ff

Una

yzah

Juba

h

Jauf

Taw

ilS

hara

wra

Qus

aiba

Qas

im

Saq

Dhr

uma

Jilh

Sud

air

Khu

ff

Una

yzah

Min

jur

Qus

aiba

Qas

im

Saq

Pre

-Una

yzah

Unc

onfo

rmity

Min

jur

Sha

raw

ra


290

Wender et al.

Hercynian Event); Jilh Dolomite/Mid-Jilh Reflector to Top Khuff/Base Khuff Isochron (Triassic ZagrosRifting Event); pre-Aruma Unconformity to Shu’aiba Isochron (Late Cretaceous First Alpine Event);and Top Aruma Time Structure (Tertiary Second Alpine Event). The later time structure map wasselected because a consistent regional seismic pick on the pre-Neogene Unconformity (PNU) couldnot be made. It is reasoned that the top of the Aruma Formation would have been deposited as arelatively flat surface, and that significant relief of that horizon is attributable to the Middle-Late Tertiarygrowth event.

Carboniferous Growth Event (Hercynian)The Hercynian Growth Event is evident on the Base Khuff to Base Qusaiba Isopach, which is shown inFigure 13. The isopach thinning indicated on this map is due to erosion associated with the HercynianOrogeny of probable Carboniferous age. Flattening of seismic sections on the Base Khuff also illustratesthis growth event, as shown in Figure 14. Note that on the isopach map significant Hercynian growthis evident at Ghawar, Tinat, Sahba, and Waqr. Only minor thinning occurs at Harmaliyah. No thinningoccurs at Abqaiq and Udaynan, indicating these structures to be non-Hercynian. Note that the Abqaiqstructure, as well as several other “late” structures in the area such as the Dammam and Awali Domes,most probably originated by post-Paleozoic movements of Infracambrian salt. These structures werenot affected by Hercynian movements, and have preserved Upper Paleozoic sections on the crest.

Hercynian structures in the Ghawar Area are more economically attractive, in that the Paleozoicreservoirs contain higher BTU gases with significant condensate yields. These structures also tend tobe more fully-filled to structural spill point.

Figure 12: Table of individual structures in the Ghawar Study Area (see Figure 2) showing thediffering growth histories and associated positive gravity anomalies and hydrocarbon occurrences.

STRUCTURE

ABQAIQ

GHAWAR

HARMALIYAH

NIBAN

SAHBA

TINAT

UDAYNAN

WAQR

CARBONI-FEROUS(HercynianOrogeny)

EARLYTRIASSIC

(Zagros Rifting)

LATE CRE-TACEOUS(1st AlpineOrogeny)

TERTIARY(2nd AlpineOrogeny)

GRAVITYANOMALY

PRE-KHUFFHYDRO-

CARBONS

None

Strong

STRONG

Weak

WEAK

Strong

Moderate

MODERATE

Moderate

None

Moderate

Shows

Gas/Condensate

Gas

NotDrilled

Gas

Oil

Shows

Gas/Condensate


291


Figure 13: Base Khuff to Base Qusaiba Isopach Map illustrating thinning due to theHercynian Orogeny (red = thinnest; blue = thickest; white = eroded).

GHAWAR

A

A'

0 20km

ABQAIQ

NIBAN

UDAYNAN

WAQR

SAHBA

TINAT

HARADH

HARMALIYAH

SHEDGUM

HAWIYAH

'AIN DAR

'UTHMANIYAH

FAZRAN


292

Wender et al.

Early Triassic Event (Zagros Rifting)The next major growth event occurred in the Early Triassic in response to rifting along the ZagrosSuture and the opening of the Neo-Tethys Sea. During this extensional event, earlier Hercynianstructures were reactivated, further enhancing these structures. This growth event is illustrated bythe Jilh Dolomite (or Mid-Jilh Reflector) to Top Khuff (or Base Khuff) Isochron Map (Figure 15). Notethat Early Triassic growth is only associated with Hercynian structures (i.e., Ghawar, Sahba, Tinat, andWaqr). No structure in the Ghawar Area can be attributable to Triassic growth only.

Late Cretaceous Event (First Alpine Orogeny)A major growth phase in the development of structures in the Eastern Province occurred in the LateCretaceous. Uplift and erosion associated with Late Cretaceous tectonic activity created the widespreaderosion surface of the pre-Aruma Unconformity (PAU). All structures studied in the Ghawar Area,

Figure 14: Seismic section flattened on the Base Khuff Event illustrating thinning of the BaseKhuff to Base Qusaiba interval due to erosion associated with the Hercynian Orogeny. The locationof the line is shown in Figures 2 and 13.

MIDDLEJILH

BASEKHUFF

BASEQUSAIBA

HARADH TINAT

0 10

Km

A0.

5 se

cond

(TW

T)

A'


293


Figure 15: Jilh to Khuff Isochron Map illustrating thinning due to Triassic growth(Zagros Rifting Event) (red = thinnest; blue = thickest).

MID JILH-BASEKHUFF

JILH DOL-TOPKHUFF

GHAWAR

UDAYNAN

WAQR

TINAT

NIBAN

SAHBA

HARADH

HARMALIYAH

FAZRAN

ABQAIQ

SHEDGUM

'AIN DAR

'UTHMANIYAH

HAWIYAH

0 20km


294

Wender et al.

Figure 16: Pre-Aruma Unconformity to Shu’aiba Isochron Map illustrating thinning due toLate Cretaceous growth (First Alpine Orogeny) (red = thinnest; blue = thickest).

GHAWAR

C

C'

0 20km

'AIN DAR

UDAYNAN

WAQR

TINAT

NIBAN

SAHBA

HARADH

HARMALIYAH

FAZRAN

ABQAIQ

SHEDGUM

'UTHMANIYAH

HAWIYAH


295


with the exception of Tinat, have been affected by this growth period. The Late Cretaceous growthevent is represented by the PAU-Shu’aiba Isochron Map (Figure 16). A flattened seismic section on thePAU is shown in Figure 17.

Note the significant growth that took place at Ghawar and Abqaiq in the Late Cretaceous. The youngerAbqaiq structure originated at this time and continued to grow in the Tertiary. At Ghawar, whichoriginated in the Hercynian, the variable thinning noted indicates that the component structural blocksmaking up the giant structure, grew at differing rates.

Weak to moderate structural growth occurred at Sahba, Waqr, Niban, and Harmaliyah during thistime, while Tinat experienced very little to no Late Cretaceous growth. The Udaynan structure alsoformed at this time, but had an unusual development that is unique among the Ghawar Area structures.The Udaynan structure originated as a result of the reversal of regional dip from the west to the east inLate Cretaceous time. Prior to the Late Cretaceous, the Udaynan Area was a “hinge zone” where thesediments thickened westward into the basin. In Late Cretaceous time, the basin shifted to the east,reversing the dip at Udaynan and creating the structure. The eastward tilting continued in the Tertiary,further enhancing the east limb of the Udaynan structure.

Mid-Late Tertiary Event (Second Alpine Orogeny)The final episode of structural development in the Ghawar Area occurred in the Middle-Late Tertiary.This growth event is shown by the Aruma Time Structure Map (Figure 18). Note that there isconsiderable relief at the top Aruma level at Ghawar and Abqaiq, indicating significant post-Arumagrowth of these structures. This relief is also shown on the seismic section of the ‘Ain Dar, Shedgum,and Abqaiq structures (Figure 19). It appears the Tertiary growth has simply enhanced the previouslyexisting structural configurations at Ghawar and Abqaiq, with no new structural trends developing.

Figure 17: Seismic section flattened on the pre-Aruma Unconformity event (PAU) illustratingthinning of the PAU to Shu’aiba interval due to erosion associated with Late Cretaceous growth(First Alpine Orogeny). The location of the line is shown in Figures 2 and 16.

BASE KHUFF

BASE JAUF

TOP KHUFF

HADRIYA

JILH DOLOMITE

ARAB-D

SHU'AIBA

PRE-ARUMAUNCONFORMITY

ARUMA

C C'HAWIYAH HARMALIYAH

0 10

Km

0.5

seco

nd(T

WT

)


296

Wender et al.

Figure 18: Aruma Time Structure Map illustrating growth in the Tertiary(Second Alpine Orogeny) (red = high; blue = low).

GHAWAR

D

D'

0 20km

'UTHMANIYAH

HAWIYAH

TINAT

SAHBA

WAQR

NIBAN

UDAYNAN

HARADH

'AIN DAR

HARMALIYAH

SHEDGUM

ABQAIQ

FAZRAN


297


Figure 19: Seismic section of the Northern Ghawar Area showing the ‘Ain Dar, Shedgum, andAbqaiq structural culminations. The location of the line is shown in Figures 2 and 18.

Other structures in the Ghawar Area show weak to moderate growth during this episode, and wereprimarily affected by the continued northeast tilting of the entire region during this time. This tiltinghas had a negative effect on lower relief structures in the area, effectively “opening up” the previouslyexisting west closure of several horizons.

Generation and Migration

As previously discussed, the primary source rock for the Paleozoic hydrocarbons in the Ghawar Areais the “hot shale” of the Lower Silurian Qusaiba Member. Figure 20 is a burial history diagram of theUdaynan well (Figure 5), which shows the maturation history of the Base Qusaiba. The Udaynanburial history closely approximates that of the basinal areas immediately east and west of Ghawar,which are the most proximal “kitchens” for hydrocarbon generation in the Ghawar Area.

The burial history diagram indicates that the Base Qusaiba entered the oil window at approximately230 Ma, in the Middle to Late Triassic. The Base Qusaiba progressed through the oil window andentered the wet gas/condensate window at approximately 125 Ma, in the Early Cretaceous. The BaseQusaiba then proceeded through the wet gas window and entered the dry gas window at approximately60 Ma, in the Early Tertiary. The Base Qusaiba remains in the dry gas window at present, though thegenerative potential may be spent in the deepest portions of the basins.

The relationship of the source rock maturation and migration to trap development was previouslyshown in Figure 6. The exploration ramifications of this petroleum system model are significant. Themodel indicates that oil generation and migration occurred after Carboniferous (Hercynian) and Early

0 10

Km

ARUMA

PRE-ARUMAUNCONFORMITY

SHU'AIBA

ARAB-D

HADRIYA

JILH DOLOMITE

TOP KHUFF

BASE KHUFF

BASE JAUF

0.5

seco

nd(T

WT

)

D D''AIN DAR SHEDGUM ABQAIQ


298

Wender et al.

Figure 20: Udaynan burial history diagram showing the maturity level of the Base Qusaiba sourcerock. The location of the Udaynan structure is shown in the southwest corner of Figure 5.

Triassic growth, but before the development of Late Cretaceous and Tertiary structures. Therefore,only earlier-formed Hercynian and Early Triassic structures in the area can be considered prospectivefor oil. Late structures, such as Abqaiq and Udaynan, would only be prospective for gas, with laterstructural development favoring drier gases. The model further indicates that earlier-formed oilaccumulations would be displaced by later-arriving gas, providing adequate migration pathwaysexisted.

The above scenario could explain two anomalies that occur in the pre-Khuff of the Ghawar Area: theUnayzah oil accumulation at Tinat, and the prominent oil staining of the Jauf gas-condensate reservoirat Shedgum and Hawiyah. The Tinat oil occurrence is anomalous because, at 15,000 ft, it is deeperthan the gas/condensate accumulations at Hawiyah and Shedgum (14,000-14,500 ft). The source rockis considered the same (Qusaiba), and the present-day geothermal gradients are very similar (about1.40°F/100 ft). However, the growth histories of the two structures are different, with Tinat experiencingvery little of the late growth that affected Ghawar (Figure 10).

Both Ghawar and Tinat originated in the Hercynian, which would have allowed oil generated in theMesozoic to migrate to these structures. The late growth that occurred at Ghawar is thought to have

Oil Window

Base Qusaiba

Wet GasConden-

sate

DryGas

AGE (Million Years)0100200300400500600

DE

PT

H (

feet

)

0

5,000

10,000

15,000

20,000

25,000

HP

Maturity Level

Immature

Dry Gas Only

Wet Gas/Condensate

Peak Oil/Generationand Expulsion

Oil Preservation

PMOEPKJTPUCLCDSOC


299


reactivated the Hercynian faults, allowing gas that was being generated at the time to migrate to thestructure and displace the previously existing oil accumulation. This would explain the oil stainingthat is observed in the Jauf Reservoir at Ghawar. The Tinat structure, with minor late growth and faultreactivation, may not have received the late-arriving gas to displace the oil. The high Gas/Oil Rationoted at Tinat (2,100) may be due to partial migration of gas into the structure, or in-situ cracking ofthe oil to gas.

CONCLUSIONS

Current exploration efforts to increase the non-associated gas reserves in the Kingdom are focusing onthe pre-Khuff siliciclastics. The main exploration objectives are sandstones of the Middle Carboniferous(?) to Lower Permian Unayzah Formation and Lower to Middle Devonian Jauf Formation.

Pre-Khuff hydrocarbon traps are found in simple anticlinal closures as well as more complexcombination structural-stratigraphic traps. The structural-stratigraphic traps involve faulting and/ortruncation of the Jauf Reservoir on the flanks of Hercynian structures. The recent Hawiyah gas-condensate discovery on the eastern flank of Ghawar is an example of this type of trap. The major topseal for the pre-Khuff accumulations in the Ghawar Area are shales and tight carbonates of thetransgressive basal Khuff Formation.

Trap formation and modification are attributed to four distinct growth periods: the Carboniferous(Hercynian Orogeny), Early Triassic (Zagros Rifting), Late Cretaceous (First Alpine Orogeny), andTertiary (Second Alpine Orogeny). Structures in the Ghawar Area show differences in growth histories,which have impacted the amount and type of hydrocarbon occurrences found in the structures.

The primary source rock for the pre-Khuff hydrocarbons is the basal “hot shale” of the Lower SilurianQusaiba Member of the Qalibah Formation. The basal Qusaiba shales began oil generation in theTriassic and proceeded through the wet gas-condensate window in the Cretaceous. The Qusaiba nextentered the dry gas generation phase in the Early Tertiary, and remains in the dry gas window atpresent.

ACKNOWLEDGMENTS

The authors wish to thank the Ministry of Petroleum and Mineral Resources and the management ofSaudi Aramco for permission to publish this paper. We also thank Dr. M.I. Al-Husseini and twoanonymous referees for their reviews of this manuscript, which resulted in an improved version.

REFERENCES

Al-Hajri, S.A., L.E. Wender, J. Filatoff and A.K. Norton 1996. The Occurrence of Devonian Sediments inthe Eastern Ghawar Area and their Exploratory and Stratigraphic Implications. Presented at the SecondMiddle East Geosciences Conference, GEO’96. GeoArabia: Middle East Petroleum Geosciences(Abstract), v. 1, no. 1, p. 103.

Al-Laboun, A.A. 1987. Unayzah Formation: A New Permian-Carboniferous Unit in Saudi Arabia. AmericanAssociation of Petroleum Geologists Bulletin, v. 71, p. 29-38.

Alsharhan, A.S. and C.G.St.C. Kendall 1986. Precambrian to Jurassic Rocks of Arabian Gulf and AdjacentAreas: Their Facies, Depositional Setting and Hydrocarbon Habitat. American Association of PetroleumGeologists Bulletin, v. 70, p. 977-1002.

Beydoun Z.R. 1991. Arabian Plate Hydrocarbon Geology and Potential - A Plate Tectonic Approach. AmericanAssociation of Petroleum Geologists, Studies in Geology, no. 33, p. 77.


300

Wender et al.

Cole, G.A., M.A. Abu-Ali, S.M. Aoudeh, W.J. Carrigan, H.H. Chen, E.L. Colling, W.J. Gwathney, A.A.Al-Hajji, H.I. Halpern, P.J. Jones, S.H. Al-Sharidi and M.H. Tobey 1994. Organic Geochemistry ofthe Paleozoic Petroleum System of Saudi Arabia. Energy and Fuels, v. 8, p. 1425-1442.

Glennie, K.W. 1995. The Geology of the Oman Mountains: An Outline of their Origin. Scientific Press Ltd.,UK.

Grabowski Jr., G.J. and I.O. Norton 1995. Tectonic Controls on the Stratigraphic Architecture and HydrocarbonSystems of the Arabian Plate. In M.I. Al-Husseini (Ed.), Middle East Petroleum Geosciences, GEO‘94.Gulf PetroLink, Bahrain, v. 1, p. 413-430.

Hughes-Clarke, M.W. 1988. Stratigraphy and Rock Unit Nomenclature in the Oil Producing Area of InteriorOman. Journal of Petroleum Geology, v. 11, p. 5-60.

Levell, B.K., J.H. Braakman and K.W. Rutten 1988. Oil-Bearing Sediments of Gondwana Glaciation inOman. American Association of Petroleum Geologists Bulletin, v. 72, p. 775-796.

Husseini, M.I. and S.I. Husseini 1990. Origin of the Infracambrian Salt Basins of the Middle East. In J.Brooks (Ed.), Classic Petroleum Provinces. Geological Society Special Publication no. 50, p. 279-292.

Loosveld, R.J.H., A. Bell and J.J.M. Terken 1996. The Tectonic Evolution of Interior Oman. GeoArabia:Middle East Petroleum Geosciences, v. 1, no. 1, p. 28-51.

Mahmoud, M.D., D. Vaslet and M.I. Husseini 1992. The Lower Silurian Qalibah Formation of Saudi Arabia:An Important Hydrocarbon Source Rock. American Association of Petroleum Geologists Bulletin, v.76, p. 1491-1506.

McClure, H.A. 1978. Early Paleozoic Glaciation in Arabia. Palaeogeography, Palaeoclimatology,Palaeoecology, v. 25, p. 315-326.

McClure, H.A. 1980. Permian-Carboniferous Glaciation in the Arabian Peninsula. Geological Society ofAmerica Bulletin, v. 91, part 1, p. 707-712.

McClure, H.A., E. Hussey and I. Kaill 1988. Permian-Carboniferous Glacial Deposits in Southern SaudiArabia. Geologisches Jahrbuch, Reihe B, v. 68, p. 3-31.

McGillivray, J.G. and M.I. Husseini 1992. The Paleozoic Petroleum Geology of Central Arabia. AmericanAssociation of Petroleum Geologists Bulletin, v. 76, p. 1473-1490.

Murris, R.J. 1980. Middle East: Stratigraphic Evolution and Oil Habitat. American Association of PetroleumGeologists Bulletin, v. 64, p. 597-618.

Senalp, M. and A. Al-Laboun 1996. Stratigraphy and Age of the Glacial Deposits in Qasim Region, CentralArabia. Presented at the Second Middle East Geosciences Conference, GEO‘96. GeoArabia: MiddleEast Petroleum Geosciences (Abstract), v. 1, no. 1, p. 192-193.

Sengör, A.M.C., D. Altiner, A. Cin, T. Ustaumer and K.J. Hsu 1988. Origin and Assembly of the TethysideOrogenic Collage at the Expense of Gondwana Land. In M.G. Audley-Charles and A. Hallam (Eds.),Gondwana and Tethys. Geological Society Special Publication no. 37, p. 119-181.

Stern, R.J. 1985. The Najd Fault System, Saudi Arabia and Egypt: A Late Precambrian Rift-related TransformSystem? Tectonics, v. 4, p. 497-511.

Vaslet, D. 1987. Early Paleozoic Glacial Deposits in Saudi Arabia; A Lithostratigraphic Revision. SaudiArabian Directorate General for Mineral Resources, Technical Record BRGM-TR-07-1, 24 p.


301


ABOUT THE AUTHORS

Vaslet, D. 1989. Late Ordovician Glacial Deposits in Saudi Arabia; A Lithostratigraphic Revision of the EarlyPaleozoic Succession. Saudi Arabian Directorate for Mineral Resources, v. 3, p. 13-44.

Williams, P.L., D. Vaslet, P.R. Johnson, A. Berthiaux, P. LeStrat and J. Fourniguet 1986. Geologic Map ofthe Jabal Habashi Quadrangle, Sheet 26F, Kingdom of Saudi Arabia. Saudi Arabian Deputy Ministryfor Mineral Resources. Geoscience Map GM-98A.

Lawrence E. Wender was a Geological Consultant with theArea Exploration Department of Saudi Aramco, having beeninvolved in the study and exploration of the Paleozoic of SaudiArabia. He recently rejoined Mobil Oil Corporation and hasnearly 20 years of oil industry experience. Lawrence holds aMSc degree in Geology from the University of Utah.

Martin F. Dickens is a Geophysical Specialist with SaudiAramco, having joined the company in 1991. He has 20 yearsof international experience. Since 1993 he has been exploringfor hydrocarbons in the Eastern province of Saudi Arabia.Martin holds a BSc in Geophysics from the University ofSouthampton.

Jeffrey W. Bryant joined Saudi Aramco in 1990 as aGeologist working frontier exploration of the northwesternregion of Saudi Arabia. Since 1993 he had been exploring forPaleozoic deep gas reserves in the Eastern Province of theKingdom. Jeffrey has nearly 20 years of oil industryexperience, including Alaska exploration and US Gulf Coastdevelopment with Exxon, and Gulf Coast of Mexicoexploration with Agip. Jeffrey holds a MSc in Geology fromthe University of Arizona. He is currently with CMS Energy.


302

Wender et al.

Manuscript Received 15 January, 1998

Revised 18 February, 1998

Accepted 21 February, 1998

Allen S. Neville is a Geophysical Specialist with SaudiAramco. He was previously with Gulf Oil Corporation andhas over 20 years of oil industry experience. Since joiningSaudi Aramco in 1987, he has been involved in the study andexploration of the Paleozoic in Saudi Arabia. Allen holds aMSc in Geology from Wright State University.

Abdulrahman M. Al-Moqbel is a Geophysicist with SaudiAramco. He graduated in 1995 from the University of Pacific(California) with a BSc in Geophysics. He has worked in theGeophysical Data Processing Division and the Area ExplorationDivision. Abdulrahman has been involved in the study andexploration of the Paleozoic hydrocarbons in the EasternProvince of Saudi Arabia. He is a member of the AAPG.

Paper presented at the 3rd Middle East Geosciences Conference and Exhibition,GEO’98, Bahrain, 20-22 April, 1998.


paleozoic (pre-khuff) hydrocarbon geology of the ghawar

Documents