Surface expressions of the Ghawar structure, Saudi Arabia

Salih Saner*, Khattab Al-Hinai, Dogan Perincek

The Research Institute, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia

Received 27 May 2004; received in revised form 5 December 2004; accepted 10 December 2004

Abstract

The Ghawar anticline is approximately 225-km long and 25-km wide in the subsurface, but the surface structural expression is not obvious.

Identifying subtle structural imprints in the young Mio-Pliocene sedimentary cover is therefore of great importance in developing a structural

growth model for the Ghawar field.

The Ghawar area, between the Jafurah sand desert to the east and the Rubayda (Dahna) sand deserts to the west, is characterized by a

rougher topography when compared to the surrounding, rather smooth, flat areas. This rough geomorphology of the structure can also be

noticed on the satellite images. A geomorphologic elevation map of the area and a subsurface structural contour map of the top Arab-D

(Upper Jurassic) reservoir reveal very similar geometric shapes.

Calcareous sandstone deposits of the youngest Hofuf formation (Mio-Pliocene) cap the structural highs along the axis of the anticline and

spectacular fractures, caves, mesas, and monumental geomorphologic features have developed in the escarpments. Fractures in the Dam

formation (Middle Miocene) are not as conspicuous as in the Hofuf formation, but indigenous fractures are clearly recognized in this

formation. A match between the directions of some topographic lineaments and projected surface traces of subsurface faults from seismic

cross sections can be observed. However, at the field locations, faults have yet to be defined on the surface.

Surface indications suggest that the structure has been active until the present day. Elevations of a bedding plane on the Shedgum plateau

reveal a 0.258 dip angle, which cannot be distinguished on the surface by the human eye. The movements that occurred during the 4 million

years since the Pliocene period reveal an average tilting of 0.068 per one million years.

q 2005 Elsevier Ltd. All rights reserved.

1. Introduction

The Ghawar field, the largest oil field in the world, is a

structural trap in the Eastern Province of Saudi Arabia

(Fig. 1). Approximately 25 km wide and 225 km long

Ghawar anticline forms the southern part of the 400 km long

En Nala uplift axis which was first identified by Steineke

and Kock in 1935 during surface mapping in the area

(Arabian American Oil Company Staff, 1959). The En Nala

and some other parallel arches are believed to be basement

uplifts reactivated since the Precambrian (Ayres et al.,

1982), whereas some oilfield structures are of salt doming

origin (Edgell, 1991). Basement uplifts and salt domes in

this region are differentiated with positive and negative

gravity anomalies, respectively (Edgell, 1992). Regardless

of their origins, some structures may exhibit surface

0264-8172/$ - see front matter q 2005 Elsevier Ltd. All rights reserved.

doi:10.1016/j.marpetgeo.2004.12.006

* Corresponding author. Tel.: C966 3 860 3872; fax: C966 3 860 3685.

E-mail address: ssaner@kfupm.edu.sa (S. Saner).

manifestations. The N–S trending Kuwait Arch is a

basement horst with about 100 m of topographic expression

(Carman, 1996). The Dammam structure, the first oil

discovery of Saudi Arabia, is a salt dome where distinctive

circular Eocene age rocks are cropping out through the

surrounding Quaternary desert sand and sabkha deposits

(Tleel, 1973; Weijermars, 1999).

Previous studies suggested that the Ghawar structure

was initiated in the Jurassic and remained active during

the deposition of Mesozoic and Tertiary sediments

(Arabian American Oil Company Staff, 1959; Ibrahim et

al., 1981). Plate movements and related tectonic stresses

prevalent during Hercynian and Alpine orogenesis stages

appear to be responsible for structural developments

(Billo, 1983; Akbar and Sapru, 1994; Grabowski and

Norton, 1995; Hooper et al., 1995; Marzouk and El Sattar,

1995). Wender et al. (1998) highlighted four stages of trap

development in the Ghawar field: (1) Carboniferous

(Hercynian), (2) Early Triassic (Zagros Rifting), (3) Late

Cretaceous (Early Alpine Orogeny), and (4) Tertiary (Late

Alpine Orogeny).

Marine and Petroleum Geology 22 (2005) 657–670

www.elsevier.com/locate/marpetgeo

Riyadh

Hofuf(Al Hassa)

Dammam

Jubail

Qatif

0 50 100 km

N

Fadhili

Khuraish GHAWAR

Abqaiq

BAHRAIN

Basra

ARABIANGULF

Haradh

ShedgumPlateau

R u b a y d a D e s e r t J a f u r a h D e s e r t

En

Nal

a A

xis

IRAN

KUWAIT

QATAR

Kuwait

IRAQ

Shiraz

ZAGROS CRUSH ZONE

Abu Dhabi

Doha

Z a g r o sF o r e l a n d

UNITED ARABEMIRATES

SAUDIARABIA

Fig. 1. Map showing the location of the Ghawar oil field and Zagros tectonic belt in the Arabian Gulf area.

S. Saner et al. / Marine and Petroleum Geology 22 (2005) 657–670658

Ghawar structure is recognizable from satellite images

(Halbouty, 1980). This is the evidence of Late Tertiary

tectonic activity deforming the young sedimentary cover.

Therefore, structural traces are expected to be apparent in

the surface formations of Mio-Pliocene age. Study of

surface structural elements provides important information

for interpreting the subsurface structural model and

development. Extrapolation of outcrop fracture data to

subsurface was applied successfully in some oil fields

elsewhere (Mercadier and Makel, 1991).

The aim of the surface studies in the Ghawar area is to

investigate structural features cropping out in the field, to

establish a correlation between the surface and subsurface

structural features, and to produce information helpful in

developing new structural models of the multi-reservoir

zones for the field.

2. Stratigraphy

The Ghawar field and adjacent areas are covered by

mainly Miocene and Pliocene age sedimentary rocks

(Fig. 2). Dam and Hofuf are the only formations cropping

out in the area. A lithological columnar section for these two

formations is shown in Fig. 3.

2.1. Dam formation (Middle Miocene)

The Dam formation crops out in the eastern and southern

areas of the Ghawar field where relatively deep erosion has

taken place. Well exposed sections are seen in quarries which

provide material for the manufacture of cement in the area

(Fig. 4A). The Dam formation mainly consists of light gray

limestone and marl approximately 90 m thick. A marly upper

DESCRIPTIONAGE FORMATION

TH

ICK

NE

SS

(m

)

LIT

HO

LOG

Y

KHARJ

HOFUF

DAM

HADRUKH

DAMMAM

QUATERNARY

MIO-PLIOCENE

EOCENE

28

95

91

84

33

Sand, silt, and gravel

Lacustrine limestone, gypsum, gravel

Calcareous massive sandstone, marl

Limestone and marl

Sandstone, limestone

Limestone, dolomite, marl

Fig. 2. Generalized stratigraphic column of the Mio-Pliocene sequence in the Ghawar area (after Powers et al., 1966).

S. Saner et al. / Marine and Petroleum Geology 22 (2005) 657–670 659

interval forms a peculiar hummocky geomorphology, which

resembles marl heaps covering the area (Fig. 4B). Commonly

these heaps are capped by a thin duricrust. Stromatolitic algal

growths, molluscan, pelletoidal and foraminiferal calcar-

enites and argillaceous limestones are the principal carbonate

facies identified within the formation.

The Dam formation conformably overlies the early

Miocene age Hadrukh Sandstone. In the absence of the

Hadruk, it lies unconformably on Eocene age Rus and

Dammam formations (Al-Sayari and Zotl, 1978). The Dam

formation represents a marine transgression extending

120 km inland from the present coastline to the northern

end of the Ghawar structure depositing pinnacle reefs on the

higher parts, calcarenites in surrounding high-energy areas

and muds in quiet lagoons. To the west of this paleo-

coastline, a continental sequence was deposited equivalent

of the Dam and Hofuf formations (Powers et al., 1966).

2.2. Hofuf formation (Mio-Pliocene)

The Hofuf formation (approximately 95 m thick) forms

the plateaus and mesas to the west-northwest of Al-Hassa

oasis. Outcrops along the western side of the Abqaiq-

Haradh road are well exposed. Loose sandy and muddy

lithologies are generally capped by duricrusts, which slow

down erosion. In the lower 19 m interval, a conglomerate

contains quartz, various igneous rock fragments, and

metamorphic materials originated from the basement

complex of the Arabian Shield, and limestone gravels

from Paleozoic—Mesozoic formations (Fig. 4C). Today

gravel-covered plains are mostly formed as a result of desert

erosion of the Hofuf formation, where the heavier

constituents remained as residual or lag gravels. Above

the conglomerate is an 18-m thick yellowish, calcareous,

massive sandstone which forms prominent ledges, cliffs,

monumental features, and caves (Fig. 4D). The yellowish

massive sandstone interval is succeeded by a 49-m thick

fluvial sequence. Alternating light gray meandering river

channel sandstone and red mudstone (flood plain deposits)

are recognized in the fluvial sequence (Fig. 4E). The

uppermost 9 m interval of the Hofuf formation is blocky and

nodular, heterogeneous sandy limestone (Fig. 4F). This

nodular zone was described as a conglomerate sequence in

previous studies, and based on Lymneae and Chara fossils,

Fig. 3. Lithostratigraphic sequence of the Hofuf and Dam formations (modified after Powers et al., 1966; Al-Sayari and Zotl, 1978).

S. Saner et al. / Marine and Petroleum Geology 22 (2005) 657–670660

either a Late Miocene or Pliocene age was given (Powers et

al., 1966).

A gradual transition from the Dam formation to the

Hofuf formation can be observed locally (Fig. 5A).

However, in some localities the Hofuf formation starts

with incised channel sandstones on the Dam formation

(Fig. 5B). These contact relations indicate a contemporary

deposition of carbonates and deltaic clastics within the

basin. Highly calcareous (50%) sandstone is further

evidence of a predominantly calcareous basin.

3. Geomorphology

The Ghawar oil field is characterized by a rough

topography when compared to the surrounding rather

smooth flat areas called the Jafurah sand desert to the east,

and the Rubayda (Dahna) sand deserts to the west. In the

northern and northeastern part of the Ghawar field,

escarpments, caves, mesas, and plateaus form beautiful

natural scenery. To the east of this field, one of the largest

oases in the world, the famous Al-Hassa oasis, extends from

the Ghawar highlands eastward to the Al Jafurah sand desert

where karst springs give life to the oasis. The elevation of

this oasis is around 130–160 m above sea level rising to

290 m on the Shedgum Plateau.

An escarpment extends for 150 km between Abqaiq and

Haradh, where small isolated wadis eroded into the scarp

(less than 500 m long) have no connection with larger wadi

systems. These features are recorded as genuine evidence of

the marine origin of the escarpment by Al-Sayari and Zotl

(1978). A marine transgression during the Plio-Pleistocene

reached to this region and played an important role in

formation of scarps and caves in the area.

The extent of the marine transgression during the Plio-

Pleistocene coincides with that of the Middle Miocene

transgression which is marked by the marine sediments of the

Dam formation (Powers et al., 1966). The locality of these

two coastlines seems to be connected with the position of the

Ghawar anticline, which runs parallel to the escarpment.

Fig. 4. Lithological properties of the Dam and Hofuf formations: (A) a fresh outcrop of the Dam limestone in a quarry and a crossing mineralized natural

fracture; (B) typical hummocky morphology of the Dam Formation is resulted by alteration and erosion of the upper marly interval; (C) a fining upward

lenticular conglomerate forms the lowermost interval of the Hofuf formation; (D) yellowish calcareous massive sandstone interval of the Hofuf formation is

prominent with ledges, scarps and monumental geomorphologic features; (E) red mudstone—sandstone interval of the Hofuf formation contains meandering

river channels, red flood plain siltstones and some mud cracks, which are evidences for fluvial deposition; (F) the top-most limestone interval of the Hofuf

formation has blocky or nodular appearance that obliterates textural properties.

S. Saner et al. / Marine and Petroleum Geology 22 (2005) 657–670 661

The uplift of the anticline probably formed a barrier to the

transgression front along this line.

NW or NNW trending joint systems at Jabal Al Qarah

have been exploited by wave-cut gorges. The caves of

marine origin were later modified by fluviatile erosion

(Al-Sayari and Zotl, 1978). The cave, Ghar An Nasab, at the

eastern side of Jabal Al-Qarah is more than 1.5 km long, has

developed along mainly rectangular crossing joints and

shows many branches. Hussain et al. (2001) studied general

geological aspects in the vicinity of Jabal Al-Qarah caves.

Erosion of the horizontally bedded Hofuf formation created

extensive geomorphologic features in the area (Fig. 5).

By contrast, the flat lying Shedgum plateau, shows a

distinct absence of well-defined stream channels over its

interior. The presence of broad, shallow sinkholes and the

permeability of the underlying rocks suggest that alteration

due to infiltration is far more important than run-off erosion.

Capillary water in a prevailing warm and semiarid climate

Fig. 5. Geomorphic features in the Ghawar area: (A) mesa hills represent uneroded Hofuf formation sitting on the Dam formation with a gradual contact; (B) a

steep scarp displaying a channel sandstone in the Hofuf formation, which is incised in the Dam formation; (C) monumental geomorphic feature formed by

uneroded Hofuf formation horizontal beds; (D) mushroom shaped Hofuf outcrop topped by a duricrust and display radial fractures on sides; (E) a monumental

block standing upright after a severe wind erosion in the desert conditions; (F) a cave developed along a fracture in the Hofuf formation.

S. Saner et al. / Marine and Petroleum Geology 22 (2005) 657–670662

developed a massive carbonate-enriched duricrust that

retards the surface erosion.

4. Comparison of topographic and structural maps

Today an almost horizontal Mio-Pliocene Hofuf for-

mation exists on the structural highs, but it is eroded in the

structural low areas. This reveals a pattern which is contrary

to conventional folds where the youngest formations are

normally eroded on anticlines but preserved in the

synclines.

Work by the Arabian American Oil Company Staff

(1959) showed a predominance of carbonate rocks in the

Mio-Pliocene over the En Nala axis, and deposition of

softer marly rocks on the flanks. This has resulted in

topographically low features such as Wadi Faruq on the

west, and gravel plains on the east, which are assumed to

have resulted from more rapid erosion of the softer rocks

in the synclinal areas. However, here in this study we

have observed that the carbonate rocks on structural highs

and softer rocks on flanks, do not belong to the same

time-stratigraphic unit. Soft rocks to the east are mainly

Middle Miocene age (Dam formation), whereas hard rocks

AIN DAR

SHEDGUM

UTHMANIYAH

HAWIYAH

HARADH

Fig. 6. A topographic map (left) and a top Arab-D structure map (right) of the Ghawar field (after Arabian American Oil Co. Staff, 1959) show a marked

resemblance, which indicates active growth during post-Pliocene times.

S. Saner et al. / Marine and Petroleum Geology 22 (2005) 657–670 663

on structural highs are Mio-Pliocene age (Hofuf

formation).

The Hofuf formation forms prominent mesa hills about

25 km to the east of the Ghawar oil field, such as in

the town of Hofuf. Erosion is more extensive in the

structurally lower areas than the higher areas along the

crest of the anticline. The observation of more extensive

erosion in synclines than anticlines may conflicts with the

general expectation for geographic locations where water-

runoff erosion is of concern. However, wind erosion is the

dominant erosion mechanism in the Ghawar desert

conditions and it is more effective in the areas where

rock weathering is widespread. Rainwater infiltrating in

the porous and permeable Hofuf formation starts flowing

laterally towards synclines, when reaches to underlying

impermeable marls of the Dam formation. Seasonal water

accumulation is probably the cause of weathering in

synclines and wind transport of disintegrated grains is the

cause of erosion. Rocks at higher elevations remained un-

eroded, because of less weathering effect of infiltrating

water.

More effective erosion in synclines than anticlines formed

a surface topography, which resembles the subsurface

structural maps of the field. High-resolution elevation data

measured during 3-D seismic survey was used to construct a

detailed topographic contour map of the area. This map and a

subsurface structural contour map of the Arab-D reservoir

are shown in Fig. 6. A comparison of the two maps provides

an evidence that the structure is active until the present day.

5. Shedgum surface anticline

The Shedgum plateau is surrounded by escarpments of

the Hofuf formation except in the north, where the slope is

gradual. The uppermost limestone unit of the Hofuf form-

ation forms the topographic surface of the plateau

(Fig. 7). This resistant unit resembles a turtle back and

protects the area from erosion. The bedding plane and the

topographic surface both appear horizontal. However,

elevations on the plateau indicate a water-divide extending

NNE-SSW, which agrees with the subsurface structural axis

Fig. 7. An East–West schematic cross-section across Shedgum plateau showing structure, lithology, and geomorphologic relations of the Hofuf formation.

S. Saner et al. / Marine and Petroleum Geology 22 (2005) 657–670664

(Fig. 8). Elevations on E–W cross sections reveal a 0.258

slope, which cannot be distinguished by human eye on the

surface. This is further evidence of structural growth

continuing until the present day.

A 0.258 dip angle of the Pliocene bedding reveals

approximately 0.068 tilting for each one million year.

Based on this estimation, Jurassic beds (150 million years)

are expected to have 98 dip in the subsurface. In fact, the slope

of the structural flanks at Arab-D level is about 5–88, reaching

a maximum of 108 in some places. In this case, a constant

uplift rate has been assumed. However, varying rates of

structural growth are suggested by truncation of some

sediments, which indicate episodes of more rapid uplifting.

6. Surface fractures

Anticlines contain a complex pattern of joints, but

usually there are two dominant directions of extensional

jointing. One set is nearly parallel to the axis of the

structure (axial joints); the other set is at right angles to the

axis (cross-axial joints). These orthogonal joints are present

in all anticlines, even those with very low amplitude. Unlike

drag folds, compressional folds usually contain conjugate

shear joints, especially on the flanks of the structure.

General geometrical fracture characteristics in the Middle

Eastern oil fields were defined by Nurmi et al. (1993). In our

study, orientations and other deterministic features of the

surface fractures in the Ghawar field were measured to

interpret their origin and development mechanisms.

Spectacular fractures and monumental geomorphologic

features have been developed in the Hofuf escarpments. For

example, the famous Jabel al Qarah caves, which are

attractive tourist locations in the area, are developed along

such NE and NW trending joints and fractures. Geologists

find these caves ideal for fracture distribution analysis to

interpret tectonic stress directions. Al-Sayari and Zotl, 1978

recorded that the most prominent joint set strikes northwest,

and another less prominent set strikes northeast. Halsey, 1980

also measured two sets offractures in the Hofufformation and

related them with the SW trending Zagros stress.

Halsey also incorporated lineament orientations in his

interpretations, and he pointed out that the Haradh-

Dammam area shows strong peaks in the NNW sector.

6.1. Hofuf formation fractures

Since fractures in the Hofuf formation are particularly

well developed, a fracture survey in the area was undertaken

to determine fracture orientations with respect to the

Ghawar structural trend and contribute additional data to

the structural model of the Ghawar field.

In the escarpments of the massive yellowish calcareous

sandstone interval of the Hofuf formation, about 10–20 m

high fractures are spaced at 15 m intervals. Subsequent

weathering and erosion caused large openings and even

cavings (Fig. 5F). Al-Sayari and Zotl (1978) explained the

enlargements and cave developments as a result of sea-wave

erosion and salt water weathering during the Plio-Pleistocene

transgression. Vertically less continuous layer-dependent

fractures are also seen where layering is apparent. Usually

these secondary layers are 0.7–1 m thick, and joint spacing

transecting these beds is around 1 m. Fracture orientations

usually indicate the existence of two sets of joints. Some

highlights regarding the Hofuf fractures are as follows:

†

Fractures are not seen in the western escarpments of

the Shedgum plateau because erosion is not deep

enough to reach to the massive calcareous sandstone

interval (Fig. 7).

†

Fracture directions in the Hofuf formation outcrops are

inconsistent, and not proper for establishing a meaningful

fracture pattern. Although two sets of joints are dis-

tinguishable in each outcrop in the Ghawar field, no

agreement was observed even between orientations of

joints measured at proximal outcrops.

†

Most fractures perpendicular to the outcrop surface

penetrate the outcrop 5–10 m deep, but do not continue

further. Fracture opening decreases with depth. Weath-

ering, drying and shrinkage are possible causes of these

fractures. Around a mesa hill in Fig. 9A, fracture

directions impart a radial scatter in a rose diagram.

Fig. 8. Topographic map of the Shedgum plateau. Plateau surface corresponds to a bedding plane, and elevation contours on the plateau also represent the

stratum contours of a structural map.

S. Saner et al. / Marine and Petroleum Geology 22 (2005) 657–670 665

†

There are many cubic blocks along scarps moved away

from the main outcrop body on a slippery material of the

upper Dam formation. Fractures limiting these blocks are

extensional due to gravity sliding (Fig. 9B), and difficult to

differentiate from those formed structurally.

†

Erosion of material along fractures formed tall upright

standing pillars or mushroom shaped remnants.

Radial extensional fractures are very common in pillars

due to uniaxial load of the overlying beds (Fig. 5D).

In general, fractures observed in the Hofuf formation are

planar or slightly irregular, but at the same time many of

them are very irregular due to erosion and caving. No block

displacement along the fractures was observed. Their

orientations usually indicate the existence of two sets of

joints. Although they are mostly open, occasional calcite or

gypsum-filled fractures were also encountered. Indigenous

tectonic fractures measured at a fresh road-cut indicated two

main sets, one trending N98E and the other showing a

N778W direction (Fig. 10).

6.2. Dam formation fractures

Fractures observed in the Dam formation are less

conspicuous than those developed in the Hofuf formation.

Fig. 9. Nontectonic fractures: (A) fractures developed perpendicular to outcrop surface in Hofuf formation are apparent at the ledges and diminish inward, they

change direction as scarp direction changes; (B) fracturing due to gravity sliding on marly surface in the Hofuf formation; (C) radial fractures on a marl mass

caused by a heave effect of swelling clayey material underneath the duricrust surface; (D) temperature origin multidirectional fractures.

S. Saner et al. / Marine and Petroleum Geology 22 (2005) 657–670666

Compared to Hofuf formation fractures, the Dam formation

fractures are indigenous joints, which can be seen in

the quarries. Dominant fractures, in a quarry close to

Shedgum anticline, strike N58W (Fig. 11A). In many places

well-developed fracture traces can be observed on clean

horizontal bedding surfaces exposed among marl masses

(Fig. 11B). A plan view of this outcrop helped to

Fig. 10. Indigenous 33 tectonic fractures measured at a fresh outcrop in the Hofuf

differentiate and to analyze dominant, crossing, and oblique

fracture directions.

No fractures exist in the marly upper interval of the Dam

formation. Usually marl masses are capped by hard

duricrusts, which are cut by surficial non-tectonic local

fractures. Some radial duricrust fractures occurred as a result

of clay swelling underneath the marl masses (Fig. 9C).

formation indicate two main sets, one N98E and the other N778W direction.

Fig. 11. Deep seated Dam formation fractures: (A) fractures in a quarry close to Shedgum anticlinal axis and rose diagram of 39 measurements showing

dominant natural fractures strike N58W direction; (B) clean bedding surface showing the dominant fractures parallel to the long axis of the photo, crossing

fractures perpendicular and an oblique fracture in the lower right corner.

N

W E

0%

20.00 16.00 12.00 8.00 4.00 4.00 8.00 12.00 16.00 20.00

N

W E

0%

20.00 16.00 12.00 8.00 4.00 4.00 8.00 12.00 16.00 20.00

Landsat-image lineaments

Topographic lineaments

Fig. 12. Landsat image showing major lineaments in the Ghawar and surrounding areas, rose diagram of 413 Landsat-image lineaments, and rose diagram of

504 lineaments measured from topographic maps.

S. Saner et al. / Marine and Petroleum Geology 22 (2005) 657–670 667

S. Saner et al. / Marine and Petroleum Geology 22 (2005) 657–670668

Some outcrops of either Hofuf or Dam formations

display polygonal fractures on the surface, which are

probably due to temperature variations in desert conditions

(Fig. 9D). These fractures also do not reveal any orientation.

7. Lineament interpretation

Geomorphic lineaments are mapped and statistically

analyzed to interpret the geological phenomena that created

them. Lineaments in the Ghawar field were studied from

two data sets (Fig. 12): (1) satellite images and (2)

topographic maps. A small-scale Landsat image in Fig. 12

Fig. 13. Some topographic lineaments match with the surface traces of deep seated

(A) match the N358W trending high-angle normal fault trace detected in the seis

shows compilation of major lineaments identified in large

scale images. The satellite-image lineaments indicated

N558W and N358E direction major trends. Lineaments on

topographic elevation maps yielded a very strong direction

peak between N308W and N408W.

The investigation of the relationship between surface

lineaments and structure was the main concern in lineament

survey. To the northeastern edge of the Shedgum plateau,

elongated hills trending N308W (Fig. 13A) match with

deep-seated faults observed in seismic sections, which cut

the entire sediments up to surface (Fig. 13B). Subsurface

vertical faults in seismic sections show vanishing offsets

towards the surface. The match between topographic

faults. Parallel elongate ridges in the Eastern edge of the Shedgum plateau

mic section (B).

S. Saner et al. / Marine and Petroleum Geology 22 (2005) 657–670 669

lineaments and the trace of the fault on the surface infers

that some topographic lineaments are projections of deep

seated faults.

Applications of Landsat imagery in lineament studies

and structural interpretation were shown by several authors

(Merin and Moore, 1986; Prost, 1994; Berger, 1994; Sabins,

1997). Lineaments set striking N358E direction, measured

from satellite images in the Ghawar area is parallel to the

Zagros tectonic stress direction. These lineaments corre-

spond to extensional fractures; however, the trend of

Ghawar anticline does not concur with the Zagros stress

direction, and this complicates the interpretation of fracture,

structure axis, and Zagros stress directions interrelations.

8. Surface-subsurface structure correlation

The En Nala axis shows a strong positive gravity but

magnetic maps are essentially featureless. Gravity and

structural maps resemble each other (Edgell, 1992). The

strong gravity anomaly requires the assumption of actual

basement displacements beneath the En Nala axis. The form

of the anomaly suggests horst type uplift through a pile of

sediments.

Arab-D is the most important oil reservoir in the Ghawar

field. Structure map of the top Arab-D shows slightly curved

flanks with dip angles generally varying between 5 and 88,

but in places reaching 108.

The significance of changes in thickness of the strati-

graphic units is taken as a possible clue to the history of the

growth of the Ghawar structure. According to Arabian

American Oil Company Staff (1959), a paleo structure map

of the Arab-D member on a late Wasia datum shows a low

but reasonably clear beginning for the Ghawar structure.

According to this publication, by Wasia-Aruma unconfor-

mity time the structure was well developed. Some further

growth was indicated in middle Eocene time and during the

Eocene-Miocene unconformity, and the growth of Ghawar

was completed by Miocene time. These authors interpreted

very minor dips found in the Miocene-Pliocene rocks either

as initial or as the result of solution collapse. However, in

our study, slightly dipping flanks have been proven to be

structural, suggesting a continuing structural growth during

post Mio-Pliocene time. Young uplifting is consistent with

the Neogene-Holocene anticlinal growth of the neighboring

Harmaliyah oil field, located about 25 km to the East of the

Ghawar structure (Ibrahim et al., 1981).

9. Conclusions

Young sedimentary cover is concealing the Ghawar

structure and its recognition is difficult at the field.

However, this study revealed visible geomorphologic traces

that help in the recognition of the structure on the surface,

and provides useful information for elucidating the growth

history. Dips of the flanks of the structure at the surface

Mio-Pliocene and Pleistocene outcrops are calculated to be

0.258. This structure can be recognized on the surface if

special attention is given to indications.

The youngest beds folded are of Mio-Pliocene Hofuf

formation. Crestal areas of the anticline are represented by

topographic high elevations. Geomorphologic elevation

map and subsurface structural map of the top Arab-D

reservoir are very similar in shape. This excellent match

indicates the forces that formed the Ghawar structure were

very similar during Mio-Pliocene time

So far no fault planes have been observed in the field.

However, surface projections of some faults, seen in seismic

cross sections, match with topographic lineaments.

Most of the fractures observed in the Hofuf formation are

not related to the structural growth of the Ghawar anticline.

They are generally formed at the edge of deeply eroded

escarpments as a result of gravity and atmospheric

conditions, and hard to differentiate from structural

fractures. More reliable structural fractures are observed

in quarries opened in Dam formation.

Growth of the Ghawar structure was active in the

Pleistocene and probably even in the Quaternary. The

average growth rate, indicated by tilting of the flanks, is

0.068 per one million years.

Acknowledgements

The authors acknowledge the supports of managements

of the Research Institute of King Fahd University of

Petroleum and Minerals, and Saudi Aramco for this study

under KFUPM/RI Project No. 23078. Acknowledgements

are also extended to Dr Andrew Mann of Robertson

Research International Ltd. and to Dr Nicolas M. Herrera

of KFUPM/RI, who read the manuscript and suggested

improvements; and Prof David G. Roberts and Mr John N.

Diggens, who reviewed the manuscript for the Marine and

Petroleum Geology Journal.

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