OLD SANDSTONES NEW HORIZONS

10 Middle East Well Evaluation Review

OLD SANDSTONES NEW HORIZONS

Paleozoic sandstones are now the most

exciting oil exploration target in Arabia. Oil

and gas have recently been discovered in

large quantities in these ancient rocks in Saudi

Arabia, Oman the United Arab Emirates. Slightly

younger Mesozoic sands are also important

targets in Yemen, Syria and Egypt.

Roy Nurmi examines the recent discoveries in

the Paleozoic and explains why these sandstones

hold great promise for the future. Eric Standen

also shows how the high-resolution Formation

MicroImager (FMI*) tool helps in understanding

these sandstones. Moujahed I Husseini of Saudi

Aramco describes in detail the geology of the

Permian marine sandstones around Riyadh in

which oil was first discovered two years ago.

Contributions by Don Hadley, United States Geological

Survey, Al Ain, UAE and Jaap Focke, Shell, Rijswijk, The

Netherlands. Many of the photographs in this article were

kindly supplied by Don Hadley.

11Issue 11, 1991.


The search for hydrocarbons in the

deeper and older Paleozoic rocks

intensified as the number of super-

giant discoveries in Cretaceous and Juras-

sic carbonates declined. The first step was

to drill into the uppermost Paleozoic car-

bonate Khuff Formation - a move that led

to the discovery of a wealth of gas

reserves. More recently, drilling around

the margins of Arabia’s prolific oil

provinces has enabled explorationists to

find oil in rocks older than the Khuff.

In the past few years, Petroleum

Development Oman (PDO) and ELF

have found oil in Oman’s Paleozoic sand-

stones. However, the heavy grade oil

and the complexity of some fields make

development of these reserves a techno-

logical challenge.

Major gas discoveries have been

made in the Paleozoic Haima

sandstones under central Oman’s pre-

existing and shallower carbonate reser-

voirs. Hydrocarbons have been located

in the Saih Nihayda and Saih Rawl fields

at depths greater than 14,000ft and

16,000ft, respectively.

The discovery of oil in Saudi Ara-

bia’s Paleozoic sandstone by the Al

Hawtah 1 wildcat well in June 1989 was

the first firm indication of possible oil

reserves. Subsequent large oil finds in

these Permian marine sandstones

southeast of Riyadh, Saudi Arabia, have

prompted further evaluation of the Pale-

ozoic system.

Fig. 2.1: DRILLING ON THE EDGE: A

PDO drilling rig at the edge of a vast

desert sand sheet at Rub al-Khali.

Mik

e B

row

n

0 500 km

15Number 11, 1991.

Fig. 2.2 (Above): The red curve shows the latitudinal position of the City of Muscat, Oman during

geologic time (calculated by M. W. Hughes Clark of PDO). During much of Paleozoic time, the

Arabian Peninsula lay south of the equator. Little carbonate sediment would have accumulated,

thus explaining the predominance of quartz sandstone reservoirs. Later in geologic time when the

Arabian Plate approached its present position, carbonates and carbonate reservoirs became

dominant.

Fig. 2.3 (Right): Location of Paleozoic sandstones in Arabia (green dots). The red dot shows the

site of the Nabatean tomb (on the opening pages of this article) which is cut into Siq sandstone at

Mada in Salih, Saudi Arabia. This is the oldest Paleozoic sandstone in Arabia.

Preca

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ian

Cambr

ian

Ordov

ician

Siluria

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Devon

ian

Carbo

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Perm

ian

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30°S

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ands

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199

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cien

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hoto

Lib

rary

Khuff Formation

Unayzah Formation

Qusaiba Shale

Lower Paleozoic clastics

Oil

Precambrian basement

Shedgum area of Ghawar complex

Hawtah Field

Wadi Birk

Abu Jifan Field


The search in Saudi Arabia

by Moujahed I Husseini

Recent discoveries of several billion bar-

rels of Paleozoic oil in the previously

unexplored sandstones of Central

Province of Saudi Arabia highlight the

prospective nature of the Paleozoic

rocks on the Arabian Peninsula. The oil

is of very high quality (Arabian Super-

light) and is sulphur free with gravity

exceeding 43° API.

The Unayzah Formation (see figure

2.4) is the main sandstone reservoir and

this lies immediately underneath the

youngest Paleozoic rock unit - the car-

bonate Khuff Formation - which is also of

Permian age. The Unayzah Formation

comprises two sandstone reservoirs with

highly variable porosities which average

about 20%. However, permeabilities of

several darcies are relatively common.

Older Paleozoic sandstone reservoirs

are present to the northeast of Riyadh

and the lateral continuity of these clastic

sequences, combined with the good

reservoir quality, suggests that these

sandstones will be a prime exploration

target in central Arabia for many years.

Fig. 2.5: The Upper Permian Khuff

Formation outcrops next to the

Arabian Shield to the west of Riyadh

and is over 20,000ft deep in the

Arabian Gulf. The structure contour

lines are in feet and show the top of

the Paleozoic rocks.

Fig. 2.4: The Paleozoic structural traps

of Saudi Arabia are typified by

Hawtah Field where the Unayzah

sandstone traps oil, sourced by the

Qusaiba Shale below the Khuff cap

rock. At Abu Jifan, a combination trap

juxtaposes the source rocks of

Qusaiba Shale against Lower

Paleozoic clastic sandstones. At

Shedgum, the Devonian reservoir is

stratigraphically trapped between the

Khuff cap rock and the Qusaiba Shale

source rock. The location of this cross-

section is shown on the map to the

right.

Dilam

Raghib

Hawtah

Ghinah

Hilwah Nuayyim

Hazmiyah

Khursaniyah

Berri

Abu Safah

Oatif

Shedgum

Tinat

Gha

war

com

plex

Red Sea Arabian Shield

8000

1600

0

0

16000

Hawtah Field

Abu Jifan Field

Shedgum area of Ghawar complex Riyadh

Khuff outcrop

0 400km

Wadi Birk

Abqaiq

17Number 11, 1991.

Paleozoic - the prime target

The uppermost Paleozoic carbonate

Khuff Formation was producing gas in

Saudi Arabia years before the discovery

of oil in the underlying sandstones. The

oil discovery at Qirdi-2 well in 1979

marked the first significant find in a

Saudi Arabian Paleozoic sandstone.

Later that year, sweet gas was found in

even older Paleozoic (Devonian) sand-

stones in the Shedgum area of the

supergiant Ghawar complex.

A far more prolific discovery was

drilled shortly later, in 1982, at Abu Jifan

Field, which flowed more than

8,000BOPD of sweet, high-gravity oil

from Lower Paleozoic sandstones at

13,500ft. In 1982, the deepest discoveries

of oil and gas were made in Paleozoic

sandstones in Saudi Arabia (oil in Tinat

Field at 14,800ft and gas in Abu Safah

Field at 14,500ft).

A petroleum potential for Paleozoic

sandstones across central Saudi Arabia

was indicated by these encouraging

results in eastern Saudi Arabia’s deep

wells together with hydrocarbon shows

encountered, in 1984, in Paleozoic sand-

stones in water wells west of Riyadh at

Wadi Birk.

Shortly afterwards, in late 1986, the

Saudi Arabian Oil Company (Saudi

Aramco) began exploring central Saudi

Arabia and has since discovered seven

fields south of Riyadh, including the

giant Hawtah Field. Other Paleozoic

sandstone reservoirs have also been

discovered to the northeast of Riyadh

where wells were drilled deeper to test

the sandstone beneath the established

fields.

Fig. 2.6 (Above): The Paleozoic section

outcropping along the edge of the

Precambrian Shield in Saudi Arabia - a

challenge to explorationists?

Fig. 2.7 (Right): Very permeable sand from an

excellent reservoir section of the Unayzah

Formation in the discovery well of the

Hawtah Field. The rock has 18.4% porosity

and 4,260mD permeability.

Don Hadley


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Let’s talk traps

In Saudi Arabia, the Paleozoic, Khuff

and underlying sandstone traps are pre-

dominantly structural and vary in size

and complexity. Some have a broad,

low-relief, overlying deeper fault blocks

while others are complex, highly fault-

ed, rift-type blocks.

The typical Unayzah structural trap

is moderate in relief due to basement

faulting. The vertical closure of the

Unayzah structures is generally less

than 100m.

The structural traps grew during sev-

eral tectonic pulses in the Cretaceous

and Triassic. But the movements were

most significant in the Late Paleozoic

Hercynian event. A variety of possible

stratigraphic traps have also been recog-

nized and successfully tested.

The general cross-section (figure

2.4 on page 14) of the Paleozoic in cen-

tral Arabia shows two stratigraphic traps

- at Shedgum and Wadi Birk. At

Shedgum, the Devonian sandstone

pinches out on the flank of the structure

forming a trap between the Khuff cap

rock and the Silurian source rock. In the

discovery well, sulphur- free gas and

condensate f lowed at a rate of

26.8MMSCFD and 760BCPD at 2,300psi

from a 3/4in choke from a depth of

13,563ft.

At Wadi Birk, a sandstone pinchout

between a tight basal Khuff interval and

Precambrian basement was defined by

water wells. Both these pinchout obser-

vations are important as they provide a

guide to other exploration targets that

may exist along the flanks of large struc-

tures where the Paleozoic sandstones

pinchout. This occurs at the Khurais,

Ain Dar and Abqaiq fields.

The combined structural/stratigraph-

ic trap at Abu Jifan Field can also be

seen on the generalized cross-section

across central Arabia. These stratigraph-

ic traps prove that reservoirs older than

the Permian-Carboniferous Unayzah

and Khuff reservoirs should be targets

in Paleozoic exploration. For all the

traps discovered to date, the regional

seals are the tight Upper Permian Khuff.

Fig. 2.8: The Paleozoic stratigraphic column is modified after Powers (1968); El Khayal and

Wagner (1985); al Laboun (1987); Miessner et al (1988); Vaslet (1990); Mahmoud et al (in

press); McGillivray and Husseini (in press).

19Number 11, 1991.

Sweet and sour in Saudi

Two types of Paleozoic hydrocarbons

are found in Saudi Arabia. The first is

located in central Saudi Arabia’s Paleo-

zoic sandstones and is a sweet, high-

gravity oil and gas. The second, in con-

trast, is sour gas found in large Khuff

reservoirs to the northeast.

The source rock of the sour Khuff

gas has not been firmly established but

the two most likely candidates are the

Qusaiba Shale or Precambrian carbon-

ates which are buried deep beneath

eastern Arabia, onshore and offshore.

These appear to be the only sequences

with sufficient thickness to provide the

prolific quantities of Khuff gas. This is

particularly true of large gas accumula-

tions such as the Qatar Dome which

has estimated reserves of about 100Tcf.

Fig. 2.9 (Below and right):

Red siltstone from flood plain

deposits of the Unayzah

Formation. The core is from

the Unayzah Formation in the

Hawtah-1 well.


Source rocks: all eyes on Qusaiba Shale

There are strong indications that the Sil-

urian Qusaiba Shale is the source rock

of the hydrocarbons in Paleozoic sand-

stones. The main indications are the iso-

tope and biomarker distributions

between oils from the Paleozoic sand-

stone and bitumen extracted from sever-

al possible source rocks.

In addition, the Qusaiba Shale is the

richest of all the possible source rocks

with a maximum total organic content of

6.15%. It is followed by the Jauf Forma-

tion with 3.7%, the Unayzah Formation

with 2.1%, the Khuff Formation with

1.34%, and the Hanadir and Ra’an Shales

with 1.13% and 0.68%, respectively.

The Qusaiba Shale is relatively thick

and is present throughout most of cen-

tral Saudi Arabia’s basins. Today, the

Qusaiba Shale is buried at depths of

10,000ft to 15,000ft throughout Saudi Ara-

bia and this implies that it is in the

hydrocarbon-generating window. In

northwestern Saudi Arabia, the Silurian

Shale is thought to have been ‘frozen’ at

depths conducive to oil generation since

the Late Paleozoic. At this time, the Her-

cynian orogeny uplifted the area and

effectively reset the subsidence curve of

these rocks back to shallow depths.

Moreover, the early entry of oil into the

pore systems of Paleozoic sandstones

appears to have helped to preserve the

reservoir character.

Fig. 2.10 (Above and

left): Cross-bedded

fluvial sandstones of

the Unayzah

Formation

(Permo-

Carboniferous

age) from the

Hawtah Field,

south of Riyadh.

111Number 11, 1991.

Focus on the Unayzah Formation

The Unayzah Formation in central Arabia

comprises a succession of siliclastic fluvial,

coastal, deltaic and shallow marine sedi-

ments. In southern Arabia, these range in

age from Late Carboniferous to Late Permi-

an and are the equivalent of the Al-Khlata

Formation glacial clastics in Oman.

Younger marine sediments from the same

sequence also correlate well with Oman’s

Gharif Formation which is Early Permian in

age.

Sedimentological and biostratigraphical

evidence shows that there is a regional

marine incursion which sits above fluvial

sediments of the lower depositional

sequence. This incursion, which is marked

by transgressive lag deposits, indicates

that a large sea once entered the region

from east and northeast, covering central

and southeast Arabia.

A regionally extensive progradational

succession lies above these transgressive

lag deposits and this can usually be divid-

ed into three depositional zones - a basal

red siltstone, a middle sandstone and an

upper layer of interbedded mudstones and

sandstones. This rock sequence shows that

the Permian Sea covered Saudi Arabia

before the major transgression which

deposited the Khuff Formation.

The middle sandstone is fine-to-medium

grained, moderately well sorted, laterally

continuous and, in places, of good reser-

voir quality. In some areas, it is over 150ft

thick. This is the reservoir rock for the

newly-discovered Hawtah, Dilam and

Raghib fields and was deposited in a vari-

ety of paralic environments - shoreface,

foreshore and delta channel.

The top unit of the three was deposited

in a coastal or delta plain environment and

is made up of channel and splay sand-

stones. It also contains variegated mud-

stones and some soil profiles. This rock

sequence shows that the Permian Sea over

Saudi Arabia receded before the major

transgression which resulted in the deposi-

tion of the Khuff carbonates.

There are also other Paleozoic sand-

stone sequences on the Arabian Peninsula

which have exploration potential and in

Oman, some of these have already been

found to house oil. But to understand

these exploration targets, it is necessary to

review the entire Paleozoic System.

Fig 2.11: Cross-

laminated current

ripple structures

and thin lenses of

shale from a

shallow marine

sequence of the

Unayzah

Formation.

Core photographs

of the Unayzah

Formation in this

article were kindly

supplied by G. S.

Ferguson and J.

Hajhoj of Saudi

Aramco.

Fig 2.12: Outcrop of the Qusaiba Shale which is the probable

source rock for the new discoveries

in the Unayzah

Formation

sandstones west

of Riyadh.

Pho

to: M

D M

ahm

oud

Further reading on Saudi Arabia

The Paleozoic Petroleum Geology of Central Arabia, by J. G. McGillivray and M. Husseini, AAPG

Bull. v. 74, 1991; also SPE paper 244, 1991 SPE Middle East Oil Show.

The Lower Silurian Qalibah Formation of Saudi Arabia, by M. D. Mahmoud, D. Vaslet and M. I.

Husseini, AAPG Bull., in press.


The Paleozoic in a nutshell

During the early part of the Paleozoic,

in Cambrian and Ordovician times, cen-

tral Arabia formed part of a stable shelf

at the edge of the Gondwana land mass

in which the Saq and Qasim clastics

were deposited. Later, during the Late

Ordovician and Early Silurian periods,

polar glaciers and related fluvio-marine

systems deposited the Zarqa and Sarah

formations unconformably above older

rocks, including the Precambrian base-

ment. As these glaciers melted, the sea

rose rapidly and this led to the deposi-

tion of the upward-coarsening Qalibah

Formation.

At the end of the Silurian, the sea

level dropped and the Tawil Formation

sandstones were deposited. During the

Middle Devonian, Hercynian mountain-

building movements uplifted and tilted

central Arabia towards the east, expos-

ing older rocks to erosion. In the Late

Carboniferous, glacio-fluvial conditions

led to the deposition of the Unayzah

Formation clastics in central Arabia

which now contain oil.

By the Late Permian and Early Trias-

sic the Arabian Peninsula, which had

been eroded to a peneplane, was flood-

ed by a warm-water regression and this

led to the deposition of the Khuff For-

mation carbonates.

These Paleozoic deposits were

altered extensively by later tectonic

forces, changing areas where oil could

be trapped and creating new migration

pathways. During the Triassic, rift tec-

tonics along the Zagros mountain belt

transmitted extensional stresses across

Arabia. These produced localized fold-

ed and faulted areas such as those seen

in the Hawtah Field. Faulting produced

at this time may have provided a con-

duit for oil migration from the Qalibah

Formation to the Unayzah Formation

sandstones. The Triassic faults rarely

disturb the Khuff Formation which is

the regional seal for the Unayzah reser-

voirs.

More recently, during Tertiary and

younger times, central Arabia was again

uplifted and tilted eastwards. This led to

rapid and widespread erosion of both

Mesozoic and Cenozoic rocks. However,

unlike eastern Saudi Arabia, these cen-

tral Arabian structures do not appear to

have been structurally reactivated dur-

ing the Cretaceous.

Focus on grabens

The discoveries in central Arabia have

led to the evaluation of the hydrocar-

bon potential of sandstones in other

scarcely investigated Paleozoic basins

of Saudi Arabia. Seismic reconnais-

sance surveys and regional studies indi-

cate that many of these basins contain

the same petroliferous source rocks

and sandstone reservoirs of central

Saudi Arabia. In fact, some of these

areas also exhibit significantly greater

structural development than central

Saudi Arabia.

A major Precambrian half-graben

system can be seen in regional surface

seismic sections across the Western

Rub al-Khali in Saudi Arabia. This half-

graben was formed at the same time as

those in southern Oman and the Najd

Rift when extensional forces dominated

the Arabian Plate.

The Western Rub al-Khali grabens

contain both NS-SE trending faults and

basins in addition to the more common

N-S trending horsts and grabens. Seis-

mic data revealed a rock sequence simi-

lar to that found in Oman’s Huqf Group

carbonates which are thought to be the

source rocks for the Ghaba and South

Oman Infra-Cambrian salt basins. Future

exploration wells will test the possibili-

ties of these basins.

Fig. 2.13: Fossil

traces indicating a

marine shelf origin

for this sandstone

within the Qasim

(Tabuk)

Formation in

Saudi Arabia.

Fig. 2.14: The

Paleozoic sandstones

are deeply buried

along the eastern end

of The Gulf. Abdul

Raman Ali and Samir

Silwadi of Abu Dhabi

National Oil

Company reported at

the 1989 SPE Middle

East Oil Show that

these sandstones

only have fair to low

porosity with low

ranges of

permeability. Shown

left is a Scanning

Electron Microscope

micro photograph of

the pore system in

one of the Abu Dhabi

gas discoveries (Hair

Dalma-3; 15,532ft). It

reveals little

intergranular space

as a result of quartz

overgrowths.

Dr. Talaat Hassan, ADMA.

21Number 11, 1991.

Oman’s decade of discovery

Exploration in Oman has almost doubled

the country’s reserves since 1980. In just

over a decade, the reserves have

increased from 2,484Mb to 4,250Mb with

a steady increase of about 50Mb each

year over the past five years. Gas

reserves have also increased significant-

ly since gas exploration began in 1985.

More recently, drilling deeper beneath

known oilfields has yielded a number of

significant gas finds including those at

Saih Nihayad, Saih Rawl and Barik.

Oman’s total gas reserves are now esti-

mated to be greater than 9.91Tcf.

This gas search has yielded some

surprising light oil finds. These include

the Khuff discovery in 1985 under Yibal

Field and a more recent find in a deeply

buried Precambrian carbonate reser-

voir rock in Al-Noor Field. The amazing-

ly high porosity found at a depth of

4,800m now opens up several deep car-

bonate prospects in South Oman.

Production from a thick interval of

‘shales’ in the same well has further

alerted explorationists to be aware of all

possibilities. In October 1991, PDO

began drilling its first offshore well -

only the fourth well to be spudded in

the Arabian Sea portion of the Indian

Ocean. The well is located in the south,

some 63km offshore at a depth of 105m.

PDO is concerned about the hard

limestone on the sea bed in this area

and the well may have difficulty reach-

ing its target. However, almost all the oil

produced by PDO from Paleozoic sand-

stones of South Oman, in addition to

that from younger overlying carbonates,

appears to have come from the Precam-

brian source rocks. Geochemical and

geological evidence indicates that the

source rock was the carbonate/evapor-

ites of the Cambrian Huqf Group. At the

Middle East session of the 1991 AAPG

International Meeting in London in

October, Geert Konert presented PDO’s

evidence for an elaborate history of

early maturation and migration in Huqf

carbonate reservoir zones such as in

the Al-Noor Field.

Over much of South Oman, the dis-

solution of Huqf salt units has resulted

in the upwards migration of this Pre-

cambrian oil into the overlying Paleo-

zoic sandstones and Cretaceous carbon-

ates. The Precambrian oil is classi-

fied as either Huqf or ‘Q’ crude depend-

ing on its geochemical characteristics. A

plot of the sterenes contained in Oman

Huqf and ‘Q’ crude oils shows them to

be markedly different from the Saudi

Aramco Paleozoic crude oil. The Khuff

oil of Yibal Field appears to be of the

same origin whereas Saudi Arabia’s

Paleozoic oil seems to have its source in

0 500 km

Absent (sediment source)

Continental to littoral

Marine (sandstone, shale)

Aden

Riyadh

Dubai

San’a

Muscat

?

?

Indian Ocean

Huqf oil

Area of Huqf oilfields

Oil from Silurian Shale

Gas from Silurian Shale

PDO 1st offshore exploration

a Silurian Shale northeast of Hawtah

Field. The organic-rich Silurian shales of

the Safiq Formation are also a source

rock for the Haima Group sandstones

which are found in the Elf Sahmah Field

near the Saudi/Oman border and the

pre-Khuff sandstones of the UAE.

Fig. 2.15 (Above):Depositional environments

of Silurian Shale with the marine area having

source rock potential for Paleozoic

reservoirs in Saudi, the Safq area of Oman

and the UAE.

Fig. 2.16 (Right): The geochemistry of

crudes from Saudi and Oman

Paleozoic fields sourced by

Silurian Shale are both similar to

each other and to extracts

from the Silurian Shale. The

Oman Huqf and ‘Q’ crudes

sourced from Precambrian age

rocks are quite

different (modified from

P. J. Grantham et

al, 1990).

90 80 70 60 50 40 30 20 10 C 29

C 27

C 28

90

80

70

60

50

40

30

20

10 90

80

70

60

50

40

30

20

10

Silurian Shale

Saudi Paleozoic-crudes

Safiq area crudes

Huqf crudes "Q” crudes

Mod

ified

from

Bey

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, 199

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Saudi Arabia

Canoxy Sunah-1 discovery

Red Sea

Yepco Alf

Jamblyya 1

Oil field

Gas field

Gulf of Aden

150 miles

300 km 0

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Yemen

As'ad al Kamill

Russian discoveries

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Saq Saq

Siq

Siq

J’bala Gp Abia Gp

Fatima Gp V V

V

V V

V

V V

V V

V

V

V V V

V

V

V V V

V

V V

V V V

V V

V

V V

V

V V

? ?

?

? ?

Wajid

Wajid

Wajid Akbara

Sabatayn Amran Shuora Madbi Shuqra

Naifa

Kohlan Kohlan Hanifa Dhruma

Mafraq

Jilh Sudair

Khuff

Haushi Gp

Safiq Shale

Haima Gp

Ara

Ghabar Gp

Huqf Gp El Hota Ain Sarit

?

?

Subsurface in northern part

�� @�À�@�À�@�À�@�À �Sandstone, Quartzite Siltstone

Limestone

Dolomite

Shale

Conglomerate

Salt

Anhydrite

Volcanics V V V

Garif Fm Al Khlata Fm

? ?

Fig. 2.17: Location of Yemen discoveries.

Yemen - the new frontier

Yemen has been a focus of attention

since Ian Maycock led Hunt Exploration

Company to the discovery of oil in the

sandstones of the Maarib/ Jawf graben

basin. This find was a surprise because

explorationists believed that the targets

were Jurassic carbonate reservoirs

which had source rocks and evaporite

seals reminiscent of the prolific Arab For-

mation to the north.

Since its first discovery, Yemen Hunt

Oil Company has found 11 oil and gas

fields, all of which have been found in

Jurassic sandstones. About 200,000b/d of

sweet crude are currently being pro-

duced. Three recently discovered gas

condensate fields - Al-Raja, Dostour Al-

Wihdan and Al-Saidah - will add to the

gas production from the Asaad Al-Kamil

Field.

These sandstones range from fluvio-

deltaic in the Alif Formation to deeper

marine, possibly turbidite, in the Lam

Formation (Upper Amran Group). The

Alif sandstones usually exhibit good

reservoir qualities with porosities in the

range of 20 - 25% and permeabilities

exceeding one darcy. The reservoir seal

is the Sabatayan (Safer) Formation which

includes a number of thick Jurassic salt

units (Upper Tithonian age).

Oil was later found in Jurassic Amran

carbonates (Amal, East and West Ayan

fields), southwest of the Yemen Hunt Oil

Company concession, by a Soviet-led

effort. Dolomitization, vugs and fractures

have enhanced the reservoir character

of some of the shallower water carbonate

deposits and there is evidence to suggest

depositional variation.

Canadian Occidental Petroleum Ltd is

leading explorationists still further east

in Yemen. Five of six wells have been

successful and as a result Canadian Oxy

has more than tripled its planned spend-

ing in this relatively unexplored country

- drilling 13 wells this year in their 6.8M

acre block.

The first discovery well for Canadian

Oxy, the Sunah-1, flowed 3,767b/d 36

degree gravity oil for 12 hours through a

Fig. 2.18: The

Paleozoic and

Lower Mesozoic

stratigraphic

column for Saudi

Arabia, Yemen and

Oman (modified

after Beydoun,

1991).

7/8in choke with 378psi flowing tubing

pressure. It has been suggested that the

Sunah Field may be nearly as large as

the 500Mb Alif Field although more wells

are needed to determine the field’s

potential. A well in another structure,

Camaal, in the same block recently pro-

duced 4,900b/d during an extended pro-

duction test.

23Number 11, 1991.

0

0.01

0.02

0.03

0 60 120 180 240 300 360 Azimuth

Fre

quen

cy

Modelled

Observed River flow to north

Active only at flood stage

In-channel and bar top deposits

Vertical accretion

Silurian Qusaiba Shale

Qasim Formation

Saq Sandstone

Zarga and Sarah formations

Dipping into fluvial channels

Channel deposits and their geometry

can often be interpreted on outcrops by

analysing the cross-bedding within the

sandstone. Occasionally, dipmeters pro-

vide enough dip information for this

analysis but often the size of the cross

bedding is too small to be analyzed

using dipmeter data alone. However, the

detailed images provided by Formation

MicroScanner* (FMS) images allow the

analysis of even the smallest cross-beds

within fluvial channels.

Schlumberger-Doll Research (Luthi et

al 1990) has demonstrated that it is possi-

ble to analyze and model the fluvial

cross-bedding within Upper Paleozoic

and Lower Mesozoic sandstones of the

Western Desert of Egypt using FMS

images. The FMS images detected around

100 individual cross-beds over a vertical

interval of 120ft, with an average cross-

bed thickness of less than one foot.

The cross-bed distribution exhibits an

azimuth scatter well in excess of 180

degrees. A large number of models were

tested (350 combined models) with the

best result being obtained for semi-circu-

lar bedforms in a sinuous river channel

with a slight oblique migration of 5° to

account for the data’s asymmetry.

According to the interpretation, the

mean channel flow is towards NNW

which is similar to the modern flow of

the Nile River. As there are virtually no

floodplain deposits over the entire inter-

val, it is further interpreted that a wide

braided-river system of sinous channels

existed in which crescentic (lunate) sub-

aqueous dunes prevailed.

Fig. 2.19 (Above left): Fluvial cross-bedding data from FMS imagery which indicated from modelling that the ancient river flow was N to NE in

direction with a channel migration of 5° and a low (S=0.5) sinuosity.

Fig. 2.20 (Above right): Probable appearance of the Late Paleozoic-Jurassic river system in the Western Desert of Egypt.

Fig. 2.21 (Top): Sketch of the Early Paleozoic (Ordovician) channels associated with glaciation in

central Arabia (from Denis Vaslet, 1990).

Fig. 2.22 (Above): Late Paleozoic channel in the Gharif Formation outcrop in southern Oman.

Jaap

Foc

ke


Fig. 2.23:

Erosional

channels in a

submarine

channel system

are filled with

clay in this

offshore well

from the Cauvery

basin in India.

The white

(resistive)

material in the

clay fill is

predominantly

oyster shell

fragments. The

depth scales are

in metres.

At the 1991 Annual AAPG Meeting,

the Jules Braunstein Memorial Award

was presented to a team of geologists,

which included Prof. Paul Potter, Mike

Grace and Dr. Gordon Pirie, who used

images as well as dips to analyze chan-

nel reservoirs in the USA.

Channels occur in nearly every type

of Middle East sandstone reservoir and

are present in most depositional settings.

They are common in the reservoirs asso-

ciated with glacial deposition in both

Oman and Saudi Arabia.

Large erosional valleys, up to 8km

wide, and channels associated with

glaciation which occurred during the

Lower Paleozoic of Saudi Arabia, can be

seen on outcrops and in subsurface

seismic data acquired by Saudi Aramco

in northwest Saudi Arabia. The chan-

nels are cut into the Qasim Formation

and are overlain by the source rock, Sil-

urian Quasiba Shale. Perry Dobson of

Texaco Research suggested that gas is

being produced from the equivalent

sands in Jordan’s Risha structure.

Outcrops of similar channels associ-

ated with a later glaciation (Carbonifer-

ous) can be seen in Gharif Formation

outcrops in southern Oman (figure

2.22). Subsurface channels in this sand-

stone were identified using FMS

imagery from a well in the Thuleilat

Field in Oman. The geometry and orien-

tation of the channels could be directly

interpreted from the images. Also identi-

fied in the FMS image were caliche soil

horizons and calcite cemented fractures

in a porous sandstone.

Display scale 1/50

3492.0

3494.0

3496.0

3498.0

3500.0

Display scale 1/50.00 GR 100.00

3498.8

3499.0

3499.2

3499.4

3499.6

25Number 11, 1991.

Filling in channel data

In addition to using cross-bedding for

analysing channels, it is often possible

to see other features which may reveal,

even more directly, the shape and orien-

tation of the channel system. In

analysing a submarine shelf deposit,

cores and FMS images indicated that cut

and fill troughs, which are very common

in channels, can be seen in the images.

Channel fills associated with subma-

rine shelf and slope deposits along

India’s southeast coast have also been

analyzed using FMS images. These

revealed the orientation of some of the

clay-filled troughs within the deposits.

Examination of the borehole images

indicated that the sands and shale with-

in the lower part of the interval (3,497m

to 3,507m) were deposited as channel

fills. The trough-like shape of the thicker

shale bodies suggests that they were the

final fill after erosion and sand deposi-

tion. The texture of some of the fill is

sufficiently coarse to be directly visible

in the images. The particles range in size

from pebbles to small cobbles.

Similar coarse pebbles were

observed during the later examination

of the core and can be seen in the core

photographs of figure 2.23. Note that the

scale of the photograph is approximate-

ly twice the size of the accompanying

FMS image. Most of the associated shale

is discontinuous and may include shale

clasts carried by the channel currents.

The shale wedge-like geometry, inter-

preted as channel fills using FMS

imagery, were matched with the core.

Although the detrital clasts within these

channel-like troughs were found to be

oyster fragments, it is thought that these

fragments were carried into this deep-

water setting. This is supported by the

presence of deep-water fossils in the

non-channel deposits.

The orientation of the axes suggests

that the channels were affected by N-S

oriented basement ridges. The channels

were buried by either prodelta shales or

part of a submarine fan.

Thin shale beds in the sequence

above the channel deposits indicate a

very different depositional geometry. The

interval contains eight individual shale

beds which are thin, relatively parallel

beds of high conductivity (seen as black

in FMS imagery). These are less well

Fig. 2.24: Thin

gently dipping

shale beds of a

prograding fan-

like deposit are

clearly visible in

sequence

overlying the

submarine

channels in figure

2.23. The depth

scale is in metres.

detected by the Gamma Ray shown to the

right which has poorer vertical resolution.

The information on shale geometry,

depicted so clearly by the FMS, indicates

that these shales extend over a large area

away from this well and could possibly be

correlated with a nearby well.

Display scale 1/50 0.00 GR 100.00

3482.0

3484.0

3480.0

3486.0

3488.0


0-40°

Wind direction

Areal view of Barchan Dune

High-angle dips (10.1-40) Medium-angle dips (5.1-10) Low-angle dips (0-5)

Cro

ss�-

sec

tion

NE

S

W

SW

Riding on the storm

With enough bedding information, it is

often possible to understand the direc-

tion of current movement, whether

water or air, so that the geometry of the

reservoir body can be determined. In

the past, the geometrical limitations of

the dipmeter made it difficult to reliably

define the cross-bed types and geome-

try as they are often too small to be cor-

related from button-to-button or pad-to-

pad. FMS images now present a wealth

of bedding information much like a core

but with the added advantage of having

the dip of the well, the formation, and

continuous orientation of the bedding

features.

Early research work on the dipmeter

analysis of bedding demonstrated that it

was possible to determine the dune

type origin for the ancient dune

deposits in the Nugget reservoirs in

Wyoming. The dip and azimuth distribu-

tion of the cross-bedding and the rela-

tionship of azimuth to the dip angle was

used to determine the dominant dune

types. It was found that dune geometry

is revealed by separating the dip data

into different dip magnitude: low-angle

(0°-5°), medium-angle (5°-10°) and the

high-angle (10°- 30°) dips. The change in

dip magnitude with changes of azimuth

revealed the crescent shape of the dune

slip faces.

It was interpreted that barchan-like

(crescent-shaped slip faces) were domi-

nant, but also present were some longi-

tudinal-type dune features. The dip

within these longitudinal features is to

the SE and NW. However, the vector

average of these two modes is to the

SW, which is the same as the crescent-

shaped dune faces.

Reference

Eolian Sandstone Reservoirs: Bedding Facies and

Production Geology Modelling, by Roy Nurmi, SPE

paper 14172 (1985).

10-40° 0-5° 5-10°

Fig. 2. 25: Dip angle and direction of

bedding in an eolian sandstone reveals the

original dune types in an ancient desert.

The high-angle dips (red) provide the wind

direction. The azimuthal spread of the

medium-angle (blue) and low-angle

(green) dips on both sides of the high-angle

dips reveal the form of the dune steep

slope (shown below in figure 2.26).

Whereas, the dips with a southeast and

northwest direction (see left in yellow)

reveal that they were also longitudinal

ridges (shown in figure 2.27).

Fig. 2.26: A crescent-

shaped steep dune type

is revealed by the

eolian cross bedding of

figure 2.25.

Fig. 2.27: The direction of the dominant wind which makes

dunes migrate can be determined by analysing the internal

bedding of layers - in the same way as analysing cross-

bedding in river or marine deposits when

determining the direction of ancient currents.

When the dominant wind is blowing in a

desert, the sand moves up the gentle

windward side of the dunes and

accumulates on the steep leeward

side. This sand is added as

layers (laminae), with

layer upon layer added as the wind continues to

blow. The geometry, dip angle and dip

direction of these layers is the same as that

of the steep leeward slope on which they are accumulated. The

various dune types and shapes can be determined for

subsurface dune deposits by using either oriented core or borehole imagery. A group of

longitudinal dunes which are elongated in the direction of the dominant wind direction are shown

in this figure.

0-5°

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