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Saudi Sandstone Correlations

Large volumes of oil have recently beendiscovered in the Palaeozoic Unayzah

sandstones of central Saudi Arabia. While themain reservoirs in the Permian Unayzah arewell-understood by Saudi Aramco, the pre-

Silurian section has proved to be morecomplex. However, detailed geological studiesand thorough integration of all data, includingwireline logs, electrical borehole imagery and

core led geologists to some startlingconclusions and significant discoveries about

the nature of the sedimentary sequence incentral Saudi Arabia. These new correlations

will have a major influence on future oil and gasexploration in the region.

Tom Connally, of Saudi Aramco, and ElliottWiltse outline the efforts which were required to

decipher the depositional history of the oil-bearing sandstones and discuss the wider

implications of this discovery.

Abu Jifan

Dilam

Raghib

Hilwah

Ulayyah

Nuayyim

Ghinah

HazmiyahHawtah

Riyadh

Ghawar

Qirdi

Qatar

Khurais

Bahrain

HRDH

ST

Ain Dar

0 50 100

Km

Seismic line

28 Middle East Well Evaluation Review

The recent discovery of large vol-

umes of super light oil in the

Unayzah sandstones in central

Saudi Arabia (figure 2.1) has pleased

explorationists from the Saudi Arabian

Oil Company (see Middle East Well

Evaluation Review, Number 11, 1991).

Since these discoveries were made,

Saudi Aramco's geologists and engineers

have been trying to decipher the deposi-

tional history of the deeper pre-Silurian

sandstones. Initially, they were thought

to be equivalent to the Saq Formation

and the Hanadir member of the Qasim

Group (figure 2.2), which lies beneath

the major oil-bearing zones to the east.

There were good reasons to assume this.

The new sandstone units were remark-

ably similar to the more familiar rocks

found in the east, they occurred at simi-

lar depths, had approximately the same

thickness (about 3000 ft) and lay below a

major Silurian shale source rock.

By the late 1980s, wireline logs

recorded in the new fields of Dilam,

Hawtah, Hilwah, Nuayyim and Raghib

were being compared with logs in the

more easterly Ghawar Field. The logs

were significantly different in the deeper

part of the wells, below the main oil-bear-

ing intervals. Characteristic marker

shales from the deep Saq and Qasim for-

mations, which were obvious in the

Ghawar logs, could not be found in logs

from the new wells. The absence of dis-

tinctive ‘marker beds’ made it difficult to

divide the new sandstone sequence into

separate stratigraphic units. Lacking

clear lithostratigraphic information, and

without fossil evidence to help identify

and date the rock layers, geologists faced

a tricky correlation.

Surface seismic surveys taken across

the central region did not make the pic-

ture any clearer. The resolution of the

deep seismic data was poor in the

deeper parts of the basin, partly due to

the overlying Khuff carbonate layer

which absorbed much of the seismic

energy. A conventional wireline log cor-

relation between wells was attempted

but this did not help to identify the units.

Though the wells had been extensively

cored it was impossible to produce a reli-

able correlation of the sandstones

between wells. Age dating of fossils

which were found in minor shales within

the sandstones indicated that the

sequence may have been younger than

the familiar Saq Formation with which a

correlation was being sought, but in the

light of the overwhelming lithostrati-

graphic similarity of the units, this sce-

nario was considered very unlikely.

Fig. 2.1: Location map

of the area of interest in

central Saudi Arabia.

29Number 16, 1996.

440 million years young

Drilling in the region continued on the

assumption that the sediments in the

area had similar depositional environ-

ments to the Saq and Qasim formations.

However, in the late 1980s, fossil evi-

dence from newly-drilled wells indicated

that the formations were of Late Ordovi-

cian/Early Silurian age - somewhat

younger than the Cambro-Ordovician

Saq sandstones (figure 2.2). On the basis

of this information, Saudi Aramco geolo-

gists decided to have a closer look at the

deep sandstone formations.

New techniques were used to gather

evidence that would reveal the deposi-

tional history of these deep formations.

Saudi Aramco, determined to unravel

the complexity of this correlation prob-

lem, carried out fresh biostratigraphic

studies and embarked on a between-well

borehole imaging study. A total of

11,000 ft of Formation MicroScanner*

(FMS) tool images were taken in eight

wells in central Saudi Arabia.

This new information helped to

unravel some of the anomalies in the

data. The core-like images from the FMS

allowed geologists to examine the sand-

stones’ detailed bedding characteristics

and textures such as cross-bedding, ero-

sional truncation, soft-sediment deforma-

tion and vertical grain size trends.

Diagenetic (alteration) and other post-

depositional features (fractures, uncon-

formities, cement fabrics, etc.) were also

recognized on the images. For the first

time in wells where core was not avail-

able, Saudi Aramco geologists were able

to identify several different depositional

phases produced by aeolian, fluvial,

deltaic and tidal environments.

Fig. 2.2: Fossil evidence disproved

the initial correlation of the new

sandstone units with the older Saq

Formation and Qasim Group.

D e v o n i a n

S i l

O r d o v i c i a n

C a m b r i a n

Jubah Fm.

Jauf Fm.

Tawil Fm.

Subject of the paper

Qasim Gp.

U

M

L

U

M

L

U

U

M

L

L

Sharawra Mbr.

Qusaiba Mbr.

Ra'an Mbr.

Kahfah Mbr.

Hanadir Mbr.

Burj Fm.

Siq / Yatib Fm.

Z a r q a

S a r a h

Quwarah

Mbr.

Unit 4 Unit 3 Unit 2 Unit 1

Unit: Lithology

Sarah

Marginal marine shoreface

Marine tidal complex

Marine deltaic/fluvial

Non-marine alluvial/ marginal marine deltaic

Depositional environment

Zarqa

Aeolian

Base- ment

Qasim/ Saq

XXX

Unit 1

Unit 2

Unit 3

Unit 4

Unit 5 Fm.

S I L U R I A N

O R D O V I C I A N

GlaciogenicFA - FR

GV - EF

EM - GD

Ashgill

Cardoc

Llandeilo

Llanvirn

Arenig

Tremadoc

Pridoli - Ludlow

Wenlock

Llandovery

Saq Fm.

Unit 5


Fig. 2.3a: A continental

alluvial fan - the probable

depositional environment for

the sediments which comprise

Unit 2.

Basin basics

Detailed analysis of sedimentology and

palaeocurrent directions in the five

Upper Ordovician formations (Units 1 to

5) has helped in the understanding of

basin development during the Middle

and Upper Ordovician. It has also

explained a great deal about the difficul-

ties geologists have experienced in the

past.

Unit 1 is a well-sorted fine-grained

quartzarenite which reaches thicknesses

of 400 ft (122 m) and has a conglomerate

layer at its base. On logs, the formation

appears as a ‘clean’ blocky low gamma-

ray interval with high porosities. The

sandstone contains steeply dipping

cross-beds with a consistent trend. It is

not bioturbated and there is no evidence

of marine influence. Unit 1 was deposited

on land and is the result of wind action in

a semi-desert or desert environment.

Unit 2 was deposited in an alluvial fan

environment (figure 2.3a) with sediments

sourced from the west and north. The

unit, which can be up to 600 ft (183 m)

thick, is composed of various sediments

ranging from siltstone to coarse conglom-

erates and pebbly sandstones (figure

2.3b).

Several features identify this unit.

• There are low-angle tabular and trough

cross-beds.

• The unit displays a moderately high

gamma-ray response with a serrated

appearance.

• There are variable dips which trend

east to south.

• Bioturbation (sediment disturbance

caused by burrowing organisms) is rare.

• There is moderate diagenetic alter-

ation with patchy cementation.

• Average grain size increases toward

the western margin of the depositional

basin. In general, grain size increases as

we approach the sediment source.

1ft

Fanhead trenches

Proximal fan

Mid-fan

Distal fan

Streamflow

Fault line

Mountains

Fig. 2.3b: Excellent

pebble conglomerate in

Ordovician Unit 2. This

example is taken from

a typical Ulayyah well.

31Number 16, 1996.

Fig. 2.4a: Tidal bundles from a typical Hawtah

well showing the increasing marine influence

through Unit 3.

Fig. 2.5b: The low-

angle cross-beds in this

FMS image from a

typical Hawtah well

are characteristic of the

fluvial-deltaic facies.

Fig. 2.4b: Alluvial fan

and fan delta, depositional

environments - lower part of Unit 3.

Fig. 2.5a: Fluvial-deltaic

depositional environment

- upper part of Unit 3.

The Unit 3 sandstone contains a mix-

ture of quartz and lithic fragments. The

average grain size of this interval

increases from the base to the top. In

one section, the unit is more than 1300 ft

(397 m) thick. Cores and FMS imagery

indicate that both the upper and lower

Unit 3 contacts are erosional unconformi-

ties. Identifying and understanding the

nature of gaps within and between each

unit is essential for accurate correlation

across the region.

The main distinguishing characteris-

tics of Unit 3 are as follows.

• There are low-angle tabular and trough

cross-beds (less than 10°).

• A moderately high gamma ray

response with a serrated appearance.

• Dips trend east to south.

• Bioturbation, mostly vertical Skolithostype trace fossils, is common.

• Diagenetic alteration is moderate with

patchy and vein-fill cementation preva-

lent especially in the coarse-grained sec-

tions in the western wells.

• The average grain size trend becomes

coarser toward the western margin of the

depositional basin.

The lower part of Unit 3 is an alluvial

fan and fan delta (figure 2.4a and b), but

the upper part shows a greater marine

influence, with low-angle cross-beds

characteristic of fluvial-deltaic environ-

ments (figure 2.5a and b).

1ft

1ft

Sea

N

High-angle fault

Debris flow fansConglomeratic alluvium Proximal

alluviumDistal

alluvium

Flood plain Sea

Fig. 2.6a: Marine tidal

complex - Ordovician Unit 4.

�� RR T££ ¥Tidal channel

River channel

Tidal currents

ORD-3

ORD-2

Coastal barrier

sand

Swamp


Fig. 2.6b: Coarse

cobble conglomerate in

Unit 4. This layer

represents an intra-

formation 4

unconformity. The high

gamma-ray reading

from this interval led to

its misidentification as

a shale.

Unit 4 comprises 300 ft to 400 ft (92 m

to 122 m) of compositionally and textu-

rally ‘mature’ quartz sandstone. In sim-

ple terms, this means that the sandstone

has a high quartz content and that indi-

vidual quartz grains are well rounded.

This ‘maturity’ indicates that the grains

have come from erosion of older sedi-

ments rather than igneous or metamor-

phic rocks.

The sands of Unit 4 were probably

derived from reworking of Saq and older

Ordovician formations which were pre-

sent in western Saudi Arabia. Variations

in the quantity of siltstone and coarse

clastic components are seen in the Unit 4

subcrop. Formation density decreases

up the rock sequence from Unit 2 to

Unit 4. This reflects the greater depth of

burial and associated compaction which

have affected the older formations.

Two distinct sediment types are

found in Unit 4:

• intensely bioturbated, massive sands

which retain some relict, tabular, cross-

stratification; and

• sands with high-angle cross-beds

which lack bioturbation.

Almost all of the bioturbation in

Unit 4 is Skolithos burrows. Both the

bioturbated and tabular cross-stratified

sands originated as very well sorted,

texturally and chemically mature sedi-

ments. Clay ripup clasts are rare, while

clay drapes are common, implying an

alternation of low- and high-energy

depositional environments.

The relative proportions of these two

sand types vary across the subsurface of

central and southern Arabia. The

intensely bioturbated Unit 4 sands are

common in eastern Saudi Arabia, but fur-

ther west, the tabular sands become

more common and a shallow-water,

higher energy, coarse clastic facies pre-

dominates. This variable sedimentology

reflects the variety of environments in a

marine tidal complex (figure 2.6a).

The identifying characteristics of

Unit 4 are as follows.

• There are tabular and trough cross -

beds with moderate dips (between 10°

and 20°).

• Bi-directional or herringbone cross-

bedding is evident.

• There is a clean blocky gamma-ray

response, due to compositional maturity.

• Dips trend east or southeast.

• Widespread bioturbation is found, pri-

marily vertical Skolithos traces.

• There is moderate diagenetic alter-

ation, with zones of patchy and vein-fill

cementation which occur in discrete

intervals.

The unconformity which occurs in the

middle of Unit 4 is marked by 50 ft to 65 ft

(15 m to 20 m) of coarse cobble and peb-

ble conglomerate. The interval was mis-

takenly correlated to the Hanadir Shale

before the borehole images were exam-

ined (figure 2.6b). The unconformity was

noticed on tadpole plots. The spiky

response on the gamma-ray log was

caused by the high proportion of igneous

rock fragments in the conglomerate.

The increase in grain size and regres-

sive nature of this event, (sediments

change from marine to continental) is

evidence for a tectonic event during the

deposition of Unit 4.

Unit 5 is a feldspathic sandstone

about 340 ft (104 m) in thickness. The

Unit 5 sands were eroded from older

Saq/Qasim units in western Arabia. Like

Unit 4, it is texturally and composition-

ally mature. A characteristic log marker,

composed of siltstone interbedded with

varying thicknesses of silty, poorly

sorted sandstone, occurs at the base.

These basal siltstones can be up to 60 ft

(18 m) thick. The Unit 5 sandstones

which overlie the siltstones are very well

sorted, sometimes bioturbated and have

abundant shallow water sedimentary

structures. Current laminations are com-

mon. Distinguishing characteristics are

as follows.

• The fabric is diagenetic. Unit 5 is

directly overlain by the Sarah/Zarqa

glaciogenic deposits, which acted as a

conduit for the pore waters which cause

mineral changes. The diagenetic fabric

makes conventional tadpole plot inter-

pretation confusing, since many high-

angle dips are present in this

diagenetically contorted sandstone.

• Over-steepened and contorted

sequences are also common in Unit 5

(figure 2.7a). This is due to loading

caused by rapid deposition in the glacio-

genic Sarah/Zarqa, and by ice masses.

Unit 5 was deposited in a marine tidal

environment (figure 2.7b). It is overlain

by the glacially-derived diamictites of the

Zarqa Formation (figure 2.8a). In the

Hawtah well, the upper Unit 5 sands are

massive and show highly variable sort-

ing. The massive appearance is due to

intense diagenetic alteration (figure 2.8b).

1ft

Fig. 2.8a: Facies tract

across Unit 5 shelf, with Zarqa

proglacial streams advancing from the

southwest.

Fig. 2.7b:

A marine tidal

depositional

environment - Unit 5.

��

��

��

Precambrian

Precambrian

N

O R D - 4

HRDH

ABJF

DILM

ORD-2

ORD-1

ORD-3

Saq

Qasim

HRDH PROJ

RGHB

ABJF PROJ

NYYM PROJ

HWTHHLWH

ULYH

V-well

Intertidal

Tidal channels

Subtidal upper shelf

Zarqa braided plain

33Number 16, 1996.

Fig. 2.7a: This FMS image from a typical Hawtah well shows soft sediment

deformation in Unit 5. This type of deformation is common in Unit 5 and

may be the result of ice loading during the end-Ordovician glaciation.

Fig. 2.8b: This FMS image from a Hawtah well

shows the base of the Zarqa diamictites lying

unconformably on diagenetically altered Unit 5

sediments.

Coastal p

lain

Bay Ebbtidal Delta

Waves

Barrier

Unit 5

��

yyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyyy

1ft

1ft


Sarah/Zarqa formations

At the end of the Ordovician, most of the

continents were located in the southern

hemisphere, in or around the landmass

which geologists refer to as Gondwana.

The African landmass occupied the

South Polar position and was affected by

direct glacial processes. The effects of

glaciation - ice sheets, glacial till deposits

and intense glacial erosion - are

recorded in Africa, South America and

the Middle East (figures 2.9 and 2.10).

In central Arabia the Zarqa Formation

provides clear evidence of glaciation.

The Zarqa is a heterolithic unit which

infilled glacial palaeovalleys. Around

Hawtah and Nuayyim, it is composed of

interbedded diamictites, trough to tabu-

lar cross-bedded sandstones and rare

siltstones. The diamictites, which lie

unconformably above Unit 5 (figure

2.11), consist mainly of quartzite with

lesser amounts of chert. Granite,

metavolcanic, metasedimentary and sed-

imentary rock fragments are also present

in significant amounts.

South Pole

Ice flow directions

Glacial deposits

Fig. 2.10: At the end of the Ordovician, most of the continents were clustered around the South Pole

and experiencing the effects of a short-lived but intense glaciation. Ice sheets covered much of Africa

and reached South America and the Middle East. The diamictites of the Zarqa Formation are typical

of the glacial sediments deposited at this time.

Fig. 2.9: Various

sedimentary mechanisms

associated with the end-

Ordovician glaciation.

Glacier snout

Englacial debris

Basal debris

Lodgement tillMarine sediment

Subaquatic flow till

35Number 16, 1996.

The supporting matrix is poorly

sorted, fine-grained siltstone and sand-

stone. Using borehole imagery, we can

identify several distinguishing character-

istics for the Zarqa Formation.

• There are random, high-angle dips in

the glaciogenic sediments (the scatter of

values on a dip plot is often referred to

as a ‘bag of nails’).

• Rounded to elongated clasts are pre-

sent, which range in size from granules

to cobbles.

• The bedding seen in cores and FMS

images of the diamictites has often been

over-steepened by ice loading and rapid

sedimentation (figure 2.12).

• Massive bedding, cut-and-fill scour fea-

tures, micro-faulting and soft sediment

slumping are also common.

• Sandstone bedforms range from paral-

lel to tabular cross-bedded.

The Sarah Formation is known, from

subsurface records, to extend from

southeastern Saudi Arabia to southern

Jordan. Despite this large geographical

extent, the unit is usually less than 300 ft

(92 m) thick.

In west-central Arabia, the Sarah

Formation consists of pebbly, poorly

sorted, parallel to tabular cross-bedded

sandstones (figure 2.13). The pebbles in

these sandstones are composed primar-

ily of quartzite and chert. Metavolcanic

and igneous clasts are present, but rare.

Rare marine microfossils and bioturba-

tion structures have been recorded. On

borehole images and on logs, the distin-

guishing characteristics for the Sarah

Formation are as follows.

• There is a blocky, moderately high

gamma-ray response. The Sarah Forma-

tion is invariably cleaner than the Zarqa

Formation.

• Log porosity is higher than in the

Zarqa Formation.

• A finer average grain size, more sand-

stone and more uni-directional lower

angle bedforms are found in the Sarah

Formation than in the Zarqa Formation.

Fig. 2.11: FMS from a Raghib well, showing the

unconformable base of the Zarqa.Fig. 2.12: FMS from a Hawtah well

showing Zarqa Formation diamictites

with steeply dipping cross-beds.

Fig. 2.13: FMS from

a typical Nuayyim

well showing the

base of Sarah

Formation sands

above the

unconformity, and

diamictites below it.

1ft 1ft

1ft


Marking boundaries

Having gathered and examined a wealth

of sedimentological data, Saudi Aramco’s

geoscientists turned their attention to the

cross-well correlation which had previ-

ously eluded them. Sequence bound-

aries at each formation contact were

identified by examining:

• distinct changes in induration (identi-

fied as colour changes on the FMS);

• intra-formational conglomerates;

• ‘hardground’ fabrics at the sequence

boundary indicating periods of non-

deposition and cementation;

• bedding thickness changes; and

• changing dip azimuth and magnitude in

cross-bedded sandstones at boundaries.

Mineral analysis of data gathered by

the GLT* (Geochemical Logging Tool)

indicated that the composition of the

sandstones in the central region was sig-

nificantly different from those in the Saq

Formation (figure 2.14).

Examination of dip and bedding data

showed that the dominant palaeocurrent

direction for the Upper Ordovician sand-

stones was towards the southeast -

almost opposite to that in outcrops of the

Saq Formation where palaeocurrents

trend north or northeast (figures 2.15 and

2.16).

This conclusive evidence prompted

Saudi Aramco to re-examine the correla-

tions between sand sequences found in

well intervals below the giant Ghawar

Field. Many of the sequences which had

previously been diagnosed as Cambro-

Ordovician proved to be younger

Middle to Late Ordovician in age.

Armed with clear proof that the sand-

stones were younger than the neigh-

bouring Saq Formation, Saudi Aramco

explorationists embarked on a more

detailed study of the images. An exami-

nation of the FMS data using the Image

Examiner Workstation immediately

proved that the same vertical sequence

occurred within each of the eight wells.

The re-interpreted conglomerate layer

proved an excellent marker, particularly

in the Raghib Field.

One of the study’s most startling reve-

lations was that the Saq and overlying

Qasim formations had been eroded in

the central region and replaced by

younger rocks.

Fig. 2.14: Comparison of the ‘older’ section in

the Ain Dar well and the ‘younger’ Ordovician

section in the Hawtah well. Mineralogical data

from the GLT* tool in the Hawtah well is

compared with X-Ray diffraction (XRD)

analysis of cuttings from the Ain Dar well.

X5000

X5200

Mid

dle

Shale

Arkose

Sub-arkose/arkose

Quartzarenite

Quartzarenite/sublithic arenite

Arkose/sub-arkose

Quartzarenite

Qusaiba

Sarah/Zarqa

Unit 5

Unit 4

Unit 3

Unit 2

Unit 1

X4200

X4400

X4600

X4800

X5400

X5600

X5800

X6000

Quwarah

Ra’an

Kahfah

Handir

A

B

C

Lower

Mid

dle

Upp

erLo

wer

Upp

er

LowerSarah

QASIM

SAQ

Cam

bria

nO

rdov

icia

n

Silurian

Shaly sand/sub-graywacke

Shale

Siltstone/graywacke

Shale

Graywacke

Quartzarenite

Arkose

ANDRHAWTAH

Igneous

Low

erU

pper

Mid

dle

Precambrian

Ord

ovic

ian

Silu

rian

X3600

X3800

X4000

X6200

X6400

37Number 16, 1996.

Fig. 2.15: Stratigraphic cross-section using Unit 4

as the datum, based on raw data. Transport

directions are predominantly to the southeast

and east. This is clearly different to the transport

directions recorded in the Saq Formation which

are to the north and northeast, in the area north

of Riyadh.

Fig. 2.16: Bedding and cross-

bedding picked in the Saq section

from a well in the far north,

showing transport directions to

the north and northeast.

X550

T/03

Sarah/Zarqa glacial valley

Base pebbly lower Zurqa

T/04T/04T/04 Datum: T/04

Intra-04 unconformity

Basal 04 unconformityT/03

Intra-04 conglomerate

X650

X750

X850

X950

X750

X850

X950

X050

X150

X550

X650

X750

X850

X950

X050

X350

X450

X550

X650

X750

X550

X650

X750

X850

X950

HAWTAH HILWAH RAGHIB DILAM ABJFNS

N

W E

S1 ft


KRSN

ABJF

DILMULYH

V

VNYMHWTH

HLWH RGHB

HRDH

ANDRSDGM

UTMN UTMNUTMNUTMN

MKSR

200

0

0

400

600

800 10

00

1000

800200

Upper Ordovician thickness

Basement0 50 100

km

Isopach contoursin feet

300 200

0

600800

200

400

600

800

0

0 50 100

km


Isopach contours in feet

KRSN

DILMULYH

V

V

NYMHWTH

HLWHRGHB

HRDH

ANDR

SDGMUTMN UTMN

UTMNUTMN

MKSR

ABJF

Basement

Fig. 2.17a: Palaeocurrent directions and isopach contours for the Ordovician section, central Arabia,

with thicknesses in feet. The black arrows indicate major transport directions. The blue lines indicate

minor transport directions. Upper Ordovician Unit 2 is an alluvial fan deposit with variable dips to

south and east.

Fig. 2.17b: Unit 3 records the transition from alluvial fan to marine environment with alluvial

palaeocurrents trending south, while those in the centre of the basin trend east.

Unit 2

Unit 3

Showing the flow

Palaeocurrent indicators, such as sedi-

mentary cross-bedding, are very impor-

tant in the reconstruction of basin

morphology and changing depositional

environments through time. The ability

to identify areas of high relief (sediment

sources) and low relief (areas of deposi-

tion) within the region means geologists

can chart changing basin morphology

through time. The data collected from

central Arabia shows a sedimentary

sequence which has been strongly influ-

enced by underlying structures, primar-

ily the Central Arabian Arch.

The sediments which make up Unit 2

are typical of alluvial fan environments.

The overall basin configuration (figure

2.17a) reflects the influence of the

Central Arabian Arch which was an area

of high relief (and, therefore, sediment

source) at this time. The Ulayyah and

Hilwah wells contain sandy conglomer-

ates which were deposited in a proximal

(close to source) alluvial fan environ-

ment. Palaeocurrent indicators show that

this sediment was transported predomi-

nantly to the south.

The distal (further from source) fan

deposits in the Dilam, Hawtah and

Nuayyim wells reflect a facies change to

finer grained, better sorted and sandy

rocks. Wells located towards the centre

of the basin have dips which trend to the

east rather than to the south.

Wells at Haradh, Khursaniyah and

Abu Jifan fields - the centre of the Unit 2

basin - reflect the fact that the axial

trough of the basin was dipping towards

the north.

Ordovician Unit 2 was deposited in an

alluvial fan depositional environment.

Toward the top of the unit there is a

greater marine influence - producing fan

delta and marginal marine sediments.

In Ordovician Unit 3 (figure 2.17b) we

see a gradual change from alluvial fan

(terrestrial) environments to a setting

with greater marine influence. The

Ulayyah and Hilwah wells contain sandy

conglomerates which were deposited in

a distal alluvial fan environment. Current

features at the base of this unit trend

south.

39Number 16, 1996.

Fig. 2.17c : The abundance of long shore drift sedimentary dips in Unit 4 (c) indicates a marine

environment.

The appearance of deltaic/shoreface

sediments reflects a change to finer

grained, better sorted, more sandy facies

in wells at Dilam, Hawtah, Nuayyim and

Abu Jifan. Overall, the direction of trans-

port in the basin is towards the east.

Marine environment tidal and wave-

generated currents dominate the upper

part of this unit. Tidal bundles and bi-

directional dips (which indicate wave

motion) become more abundant. Dip

magnitude and dip azimuth variation are

common in the fully marine Upper

Ordovician Unit 3. Sediment samples

from Haradh mark a shallow eastern

shelf of the basin.

The basin morphology for Unit 4 (fig-

ure 2.17c) also reflects the influence of

the Central Arabian Arch. The directional

features in the northwest wells - at Dilam

and Raghib - trend more to the south

than in the basinward wells of Hawtah,

Nuayyim, and Hilwah. Wells located in

the centre of the Unit 4 basin, at Abu

Jifan, Haradh and Khursaniyah, reflect

the fact that the axial trough of the basin

was dipping to the north. Eastern wells of

the Unit 4 basin do not indicate the pres-

ence of an eastern shelf. Long shore drift

sedimentary dips are more common (in

wells at Dilam and Raghib) underlining

the marine nature of this tidal deposit.

The Central Arabian Arch also played

a significant role in the regional basin

configuration during the deposition of

Unit 5 (figure 2.17d). Palaeocurrent fea-

tures in the northwestern wells (at Dilam

and Raghib) are directed predominantly

to the south, in contrast to the more bas-

inward wells located at Hawtah,

Nuayyim and Hilwah.

The end of the Ordovician was

marked by a global glaciation, centred

on North Africa. We might expect that

the facies and basin configuration

encountered in the Upper Ordovician

would be very different from those in the

Cambrian or Lower Ordovician. In the

absence of high-quality deep seismic

data, (quality being limited by the thick

Khuff Formation) we must understand

basin morphology and facies distribution

in order to predict the geographical and

stratigraphic position of reservoirs.

KRSN

ABJF

DILMULYH

V

VNYMHWTH

HLWH RGHB

HRDH

ANDRSDGM

UTMN UTMNUTMNUTMN

MKSR20

030

040

0

400

300

200

0


Basement0 50 100

km


KRSN

ABJF

DILMULYH

V

VNYMHWTH

HLWH RGHB

HRDH

ANDRSDGM

UTMN UTMNUTMNUTMN

MKSR

300

200

HRDHHRDH

200

0

300

0


Basement0 50 100

km


Fig. 2.17d: Unit 5 is marked by the influence of the Central Arabian Arch, dividing the shallow

marine environment into two distinct sub-basins.

Unit 4

Unit 5


Tectonic ups and downs

During the Early Ordovician, compres-

sional forces caused a period of uplift in

central Arabia. The Central Arabian

bulge formed a topographic high which

was probably subjected to faulting dur-

ing the Middle Ordovician.

The valleys filled with Middle and

Upper Ordovician sediments - including

Units 1 to 5 which, although younger

than the rocks in the horsts, occur at

similar depths. This process, coupled

with the lithostratigraphic similarity of

the older and younger units and a lack of

fossil material for age identification,

made the initial interpretation difficult.

At present, Saudi Aramco explo-

rationists are investigating the tectonic

movements which led to the initial uplift,

associated erosion and subsequent

down-faulting of these central sand-

stones. These movements were followed

by a major depositional period lasting

approximately 30 M years which led to

the creation of the Upper Ordovician

Units 1 to 5 and Sarah/Zarqa sandstone

units.

removed Unit 5 by erosion. The diamic-

tites are poorly-sorted, clastic rocks

which contain a wide range of particle

sizes.

This new understanding of basin

development has improved the interpre-

tation of other formations in Arabia. In

addition, the re-processing of seismic

data, guided by FMS images, is helping

explorationists to find tectonic deforma-

tions in southern and central Arabia.

Saudi Aramco geologists are currently

reinterpreting old data to help redefine

the seismic sequence boundaries and

determine unconformities.

Each sandstone was deposited in a

different environment. The oldest unit

is an (as yet undated) aeolian sand-

stone, produced in a desert-like envi-

ronment. This is a clean and well sorted

sand with well rounded grains and a

consistent cross-bedding direction - all

characteristic features of aeolian

deposits. Above this are the thicker

sediments which make up Unit 2 and

Unit 3. These were deposited in deltaic

or distal alluvial environments which

produced fine-grained rocks with low -

angle cross-bedded sands and conglom-

erates.

Unit 4, which overlies Unit 3, com-

prises a tidal/marginal marine sandstone.

Unit 4 also contains the distinctive con-

glomerate layer which was used as a

marker in all of the wells. Above the con-

glomerate horizon, Unit 4 has the texture

of a bioturbated marine sand.

Unit 4 is overlain by Unit 5. The

Sarah/Zarqa Formation, composed of

glacially-derived diamictites has locally

Fig. 2.18: This new seismic line shows the Qasim/Saq sequence and the Ordovician deposits which lie above the unconformity.

Careful analysis of good seismic data can show the physical and structural relationships between units, but age and

stratigraphical correlation required geochemical analysis, borehole imagery and fossil evidence.

41Number 16, 1996.

More than core

The combination of core analysis and

FMS borehole imagery helped to solve

the correlation problems which Saudi

Aramco faced in central Arabia.

Having established that Units 1 to 5

were of Upper Ordovician age, geolo-

gists had to deduce the depositional his-

tory of these previously unknown

formations. The transition from aeolian

sandstones in Unit 1, through deltaic and

marine sediments to the glacially-

derived rocks of the Zarqa and Sarah

formations, is recorded with great detail

in the FMS borehole images.

In addition, the images have allowed

geologists to separate diagenetic effects

from syndepositional features.

New efforts are being made to define

seismic sequence boundaries (figure

2.18). Re-interpretation of older seismic

data and new acquisition have been

stimulated and guided by the identifica-

tion of unconformities which were

recorded using cores (figure 2.19) and

borehole imagery.

Fig. 2.19: The lower Unit 2 sediments (a) were deposited in an alluvial environment. This example is

from a Raghib well. The Unit 3 core (b) is from a well at Abu Jifan and (c) shows typical Zarqa

Formation sediments.

(a)

(c)

(b)

The authors would like to thank themanagement of Saudi Aramco and theSaudi Arabian Ministry of Petroleum for

permission to publish this article.

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