looking for yemen's hidden treasure - schlumberger

CYAN MAGENTA YELLOW BLACK

LOOKING FOR

YEMEN’sHIDDEN TREASURE

The discovery of major oil reserves in

Yemen's Marib-Shabwa graben has forced

explorationists to rethink their ideas about

this region's petroleum geology. It has also begged

the question: Are there any more oil-rich areas still to

be discovered? Recent work has shown that

Yemen's promising sedimentary basins owe their

existence to the break-up of the Gondwana super-

continent which started about 150 million years ago -

at a time when Arabia was still connected to Africa.

Geologists from Yemen Hunt Oil Company

(YHOC) give an exclusive overview of the deposition

of the Marib sub-basin while the Yemen Ministry of

Oil and Schlumberger Middle East trace the tectonic

events that led to the creation of all the country's oil

and gas regions.

Authors:

Yemen Ministry of Oil: Asker Ali Taheri.

Yemen-Hunt Oil Company Geology Team:

Mark Sturgess, Dr. Ian Maycock and Gary Mitchell.

Texaco E & P Technology: Dr. Alfredo Prelat.

Schlumberger: Dr. Roy Nurmi and Mario Petricola.

Contributions: Dr. Ziad Beydoun of MarathonInternational Petroleum (G.B.) Ltd., Phil Magor ofCrescent Petroleum Company, Mamdouh Nagati of theInternational Petroleum Company.

We are extremely grateful to British Petroleum RemoteSensing Division for supplying the satellite photographsfor the magazine cover and for figure 2.5. The satellitephotographs on these two pages were kindly providedby Texaco E & P.


14 Middle East Well Evaluation Review

Fig. 2.1: BREAKING UP IS HARD TO DO:

The Gondwana super-continent began to split up during the

Jurassic. The first signs of the break-up were characterized by

Y-shaped cracks in the earth’s crust - so-called triple junctions. The oil-rich Marib-Shabwa

graben, which is longer than the Gulf of Suez, was formed in one of the failed arms of a

triple junction (shaded blue).

The break-up of the Gondwana

super-continent started about 150

million years ago when Arabia

was still connected to Africa. During

this time, the Africa-Arabian plate sepa-

rated from India, Australia, South Amer-

ica and Antarctica. It was a further 100M

years before the Gulf of Aden was creat-

ed and isolated Arabia from the African

continent (figure 2.1).

The first sign of the break-up of the

Gondwana land mass during the Juras-

sic was a characteristic Y-shaped crack

in the earth’s crust. This appeared at so-

called triple junctions where the pull-

apart forces interact (figure 2.1). At a

typical triple junction separation, two of

the arms of the ‘Y’ tend to separate and

graben depressions develop which fill

with water. Over millions of years the

land masses drift further apart, becom-

ing separated by an ocean with a half

graben on either side (Middle East WellEvaluation Review, Number 8, 1990).

A graben is also formed as the third

arm of the triple junction separates. But

this separation is often short- lived, leav-

ing an elongated depression or rift in

the earth’s crust which eventually fills

with sediments. These sediments can

include organic-rich source rocks,

shales, porous sandstones and carbon-

ates. If they are subsequently capped

by sealing evaporites, hydrocarbons

may be trapped within reservoirs. This

sedimentary cocktail is seen in many

places around the world, from Egypt’s

Gulf of Suez to West Africa’s Niger Delta

and India’s Cambay Graben. It is also

seen in Yemen.

The Marib-Shabwa graben - Yemen’s

major oil-producing region - is thought

to be a failed arm of the Y-shaped

crustal separation which occurred as

Africa

India

Australia

Arabia South

America

North America

Europe

Antarctica

Tethys Sea

through time can give explorationists

essential clues to the location of possi-

ble oil reserves. For example, about 10

years ago, major gas deposits were

located in sediments of Gondwana age

offshore of northwest Australia. Conti-

nental drift data has shown that these

deposits were laid down when north-

west Australia lay adjacent to northeast

India. As a result, oil and gas have

recently been found in this area of the

subcontinent.

Oil and gas deposits have also been

found in sediments of Gondwana age in

the Red Sea and the Gulf of Aden,

although none of these is of commercial

size. Studies of plate tectonics in this

region are underway and the results

could guide explorationists to new

reserves. However, the tectonic details

around the Horn of Africa are made

more complex by the separation of the

Arabian land mass from Africa during

the Oligocene. Two arms of the Y-

shaped plate separation formed the Red

Sea and Gulf of Aden grabens while the

third, failed arm, extended southwards

into the Ethiopian land mass. This

depression, now called the Afar Trian-

gle, has since been filled with volcanics,

clastics and evaporites.

Gondwana fragmented during the Juras-

sic. Plate tectonic studies have shown

that the Gulf of Aden was prised open

by an advancing Indian Ocean ridge

during the Early Oligocene (35M years

ago) but it took until the Late Oligocene

(25M years ago) for the continents to

separate sufficiently to connect the

southern Red Sea with the Indian

Ocean.

Microfossil evidence, obtained by

the International Deep Sea Drilling Pro-

ject, indicates that the northern Red Sea

and the Gulf of Suez did not link with

the Mediterranean Sea until the

Miocene. Around this time, the Red Sea

became connected to the Mediter-

ranean via the Gulf of Suez. Figure 2.2

shows the position of the oceanic ridges

and the direction of sea-floor spreading

which has occurred since the break-up

of the Gondwana land mass.

Convection currents in the earth’s

molten mantle are thought to be the

driving force behind these crustal sepa-

rations. Some of the plates are now sep-

arated by thousands of miles. When the

crustal plates separate, new ocean floor

rocks are created by magma spewing

up from the earth’s molten mantle. As

these rocks cool, magnetic minerals in

the rocks align themselves with the

earth’s magnetic field. Periodic rever-

sals in the the direction of the earth’s

magnetic field are therefore recorded in

the ribbons of relatively recent sea-floor

rocks which run parallel to the ocean

ridges. This, in effect, creates a huge

magnetic bar code that encapsulates

the earth’s magnetic history since the

land masses separated. From this, geol-

ogists have been able to map the age of

the earth’s crust.

Information about plate movements

15Number 12, 1992.

Fig. 2.2: As Gondwana separated, the rift in

the Indian Ocean floor eventually penetrated

the Middle East/Africa area resulting in the

creation of the Gulf of Aden. The Red Sea

crust is split and separating as the Arabian

Plate moves northeast away from Africa.

Ocean crust is still being added in the centre

of both the Gulf of Aden and the Red Sea.

(Map reproduced courtesy of UNESCO and

kindly supplied by Geopubs, UK.

Tel: 44 582 580978).

Fig. 2.3: Solidified magma (lava) extruding

like toothpaste from the Red Sea floor

separation and rifting.

(Photo: Courtesy Dr. David A. Ross, Woods

Hole Oceanographic Institute).

Indian Ocean Ridge

Mid-A

tlantic Ridge

50mm



Shear recyclings

Fault analysis in the Marib-Shabwa

region shows that the direction of the

failed arm, which led to the creation of

the Marib-Shabwa Basin around 160M

years ago, was dictated by large-scale

NW- SE and NE - SW shear faults that

began a billion years earlier, during the

Precambrian. The NE-SW lines of weak-

ness in the earth’s crust are termed the

Najd fault zones and can be seen cross-

ing Arabia and Africa. Unlike human

bones, these faults do not strengthen on

healing. They remain planes of weak-

ness in the earth’s crust.

When Gondwana split apart, the

crust in Yemen responded by stretch-

ing, fracturing and block-faulting along

one of these ancient fault zones. The

size of the graben that developed origi-

nally extended throughout Yemen and

may have reached as far as present-day

Somalia - more than twice the length of

the Gulf of Suez graben.

At this time, Yemen and Somalia

were connected and, for this reason,

there is no evidence of rifting in the pre-

sent-day Gulf of Aden. Exploration for

hydrocarbons in the Somalian portion

of the graben has only just begun.

Since oil was first discovered in

Jurassic sandstones at Alif Field in 1984,

11 commercial discoveries have been

made in Yemen in the Marib-Shabwa

graben by YHOC and others. The explo-

ration drilling has revealed that each of

the sub-basins defined by geophysical

means (gravity, magnetics and seismic)

was filled by various types and

sequences of sediments, resulting in dif-

ferent reservoir types. The Marib sub-

basin is a complex mixture of

interfingering sandstones, evaporites

and shales with the sandstone reser-

voirs being dominant. In contrast, the

Iyad sub-basin is dominated by carbon-

ate reservoir zones. There are only

very minor sandstones within the rift

sequence and a thin Kolhan Formation

sandstone reservoir of a pre-rift origin.

The Shabwa sub-basin has one

major sandstone reservoir unit and has

shown little carbonate reservoir poten-

tial. The southernmost sub-basin, the

Borlaf, is slightly younger than the

Marib-Shabwa graben as are the

grabens which developed further east.

They have no Jurassic salt but are filled

by Cretaceous clastics with minor car-

bonate intervals. The south-eastern-

most sub-basin has a prospective

Jurassic section including thick salts

but no commercial oil has yet been

found.

Fig. 2.4: Testing the Alif 1 well. (Photo: YHOC).

Fig. 2.5: MARIB MAGNIFIED: Most of Yemen’s treasure trove of oil has been found in the Marib-

Shabwa Graben. This LANDSAT photograph shows the location of the major fields.The Alif Field

reserves are estimated at 500 million barrels. Alif 1 discovery well flowed from two separate

zones at rates of 3,669B/D (40.4° API) and 4,162B/D (39.8°API) with 55 million ft3/day of natural

gas. Asa'ad Al Kamil Field, which was discovered in 1988, is the second largest sandstone

producer in Yemen and lies some 15km northeast of Alif Field. The field has recoverable reserves

of about 140 million barrels of oil and 2.7 trillion ft3 of gas and covers an area of 60km2. Azal

Field is a subsidiary structure of the Alif Field. It was declared commercial on April 15th 1987 after

the discovery well produced 5,400B/D (39°API). The field is 9km long and 5km wide. YHOC has

also discovered many other sandstone fields in the Marib Basin including Saif, Jabal Nuqum,

Raydan, Al Wihdah, Al Shura, Al-Raja and Dostur Al-Wihdah.

17Number 12, 1992.

(Photograph kindly supplied by BP Remote Sensing Division, UK.)



Treasure hunt

When Yemen Hunt's Alif 1 well was

tested at a combined rate of 7831 BOPD

(40.4°API) in July 1984 it sparked off

intense industry interest in a previous-

ly ignored part of the Arabian Peninsu-

la. As development of the Alif Field and

exploration continued, it became clear

that a new stratigraphic framework was

required.

The sedimentary section within the

basin is, for the most part, unlike any-

thing else seen in the Jurassic of the

Arabian Peninsula, and much of the

basin-fill is not seen in outcrop. YHOC

and it’s partners, Exxon and Yukong,

have now drilled over 300 wells within

the basin, and these have provided the

information and control for the con-

struction of a stratigraphic nomencla-

ture. Many of the formations are

present only in the subsurface and thus

are named after wells, whereas others

are named for surface features or out-

crops.

The term Amran Group has been

applied by previous authors (Beydoun

1964) to describe all formations within

the Upper Jurassic, however in the

YHOC nomenclature it is confined to

the carbonate-dominated Saba and

Arwa Formations (figure 2.6). The

regional transgression of the Tethyan

ocean reached the area of the Marib-

Jawf basin during the Oxfordian †.

Initially, this resulted in reworking

the arkosic sandstones of the Kohlan

Formation (Triassic-Lower Jurassic)

which were redeposited in a progres-

sively carbonate dominated near-shore

environment. A broad shallow shelf

developed leading to the deposition of

low to moderately high-energy lime-

stones, which YHOC has termed the

Saba Formation.

Dolomitization and the creation of

vuggy porosity have been important in

creating reservoir potential within this

unit. In the early Kimmeridgian† a rapid

drowning of much of this shelf carbon-

ate occurred, as the result of pre-rift

‘sag’; however, carbonate deposition

returned and appeared to be able to

keep pace with subsidence. Occasional

larger scale movements on the deep-

seated basement faults controlling this

subsidence, led to the introduction of

siliciclastics which occasionally halted

carbonate deposition. This sequence of

massive limestones (mudstones and

wackestones), interbedded with thin

shales and minor sandstones, has been

termed the Arwa Formation.

Active rifting/graben formation then

followed and the low-energy shelf car-

bonates of the Arwa Formation were

‘drowned’. Rapidly deposited basin-

margin fault-scarp submarine fan com-

plexes developed on the southern and

northern sides of the basin, and these

have been called the Henneye and

Ayban formations, respectively. Con-

glomerates (figures 2.7 and 2.8) and

medium to coarse-grained sandstones

make up the bulk of these formations,

however organic rich claystones, are

common within the Henneye Forma-

tion. In the remainder of the basin a

thick sequence of non-calcareous

shales, the Meem Formation, was

deposited. The age of these sediments

is believed to be uppermost Kimmerid-

gian to lowermost Tithonian††. The

shales in the basin centre are very poor

sources for gas, but as the southern

† Two benthonic foraminifera, Pseudocyclammina

jaccardi and Kurnubia palestiniensis, occur together in the

Saba Formation suggesting an Oxfordian or younger age.

†† The dinocyst, Subtilisphaera paeminosa, has its first

downhole occurrence midway through the Meem. Forma-

tion. This is thought to be equivalent to the top of the Autis-

siodorensis ammonite zone in Europe ie. the Tithonian-

Kimmeridgian boundary.

Fig. 2.6: A sequence

of Jurassic deposits

fills the northwest

portion of the

Marib-Shabwa

graben. The

conglomerate along

the basin margins

confirms that some

of the graben

subsidence

occurred at the

same time as

deposition. The

stratigraphic names

were introduced by

YHOC because the

existing outcrop

terminology was

inadequate to

describe these

deposits.

YHOC

Raydan

Amla’ah Group

Lam Formation

Harib Formation

Henneye

Formation

Saba Formation

Saba FormationArwa Formation

Meem Formation

South NorthStrat "A1" test Al - Tawilah # 1 Meem # 1

Jebal Ayban # 1Alif Field

Formation

Ayban Formation

Basement

Marib Group

Amran Group

Sea level

Distal turbidite

Shelf carbonate

Basinal shales

Mid-fan sands

Upper-fan conglomerates

19Number 12, 1992.

margin is approached they became

excellent sources for oil and gas. This

probably reflects the presence of an

oxygen minimum layer, associated with

worldwide oceanic anoxia typical of

this time, which impinges upon the rela-

tively shallower side of the half-graben.

Subsidence rates began to decrease,

but input of sediment from the basin

margin continued in a similar fashion.

This is represented by the coarse clas-

tics of the upper Ayban Formation to

the north, and the Harib Formation to

the South. The latter contains very few

shales, unlike the underlying Henneye

Formation. In the basin plain environ-

ment, distal turbidites and hemipelagic

claystones and limestones were

deposited. This creates a very distinc-

tive and remarkably correlatable log

response over an area greater than

2,000km2, and this has been named the

Lam Formation (figures 2.6 and 2.7).

This is the major source for the oil and

gas discovered in the Marib-Jawf Basin,

but in a similar fashion to the Meem

Formation, is leaner in the Jurassic

depocenters. Meanwhile to the west of

the present YHOC contract area, a large

deltaic system began to prograde east-

ward.

During the deposition of the Marib-

Jawf Group, as described above, there

appear to have been only minor fluctua-

tions in sea-level, but a major regression

took place towards the end of the Titho-

nian. This led to the abandonment of

both the delta to the west, and the

Fig. 2.7: Wadis

flowing into the

Marib-Shabwa

graben formed sub-

aerial fans during

low-water levels and

submarine fans at

high-water levels.

(Harms and Fowler,

1987).

Fig. 2.8: ALL MIXED UP:

A conglomerate from a

fan sequence cored in the

Jebal Ayban No. 1 well.

The rock contains

pebbles of porphyry,

micro-granite, schist and

vein quartz.

30mm


The evaporites are represented by

thin anhydrites toward the edge of the

post-rift sag, which thicken and become

massive clean halites towards the

depocentre. The end of Amla’ah Group

sedimentation in the Marib-Jawf basin is

represented by a final clastic sequence,

which is overlain by Lower Cretaceous

limestones and shales.

West East

Al-Shura # 1 Al-Wihdah # 1 Alif Field

Ma'een # 1 Sean # 1 Yah # 1 Al-Tahreer

Safer 1 Formation

Safer 2 Formation

Safer 3 Formation Safer 4 Formation

Safer 5 Formation

Alif Formation Sean Formation

Yah Formation Pro-delta shales

Lam Formation

basin-margin fan complexes. Erosion of

the former, tectonic, and/or climatic

events, and the none restricted nature

of the basin led to the deposition of a

cyclic sequence of a basal evaporites,

pro-delta claystones, polycyclic sand-

stones, and upper thin organic-rich

deep-marine shales. These sediments

infilled the topographic lows around the

delta front and basin-margin fans and

are termed the Amla’ah Group (figure

2.6).

Although contrary to normal proce-

dure in the defining of formations, we

felt that each formation within the group

should consist of the vertical sequence

outlined above (figure 2.9).

The sandstone members of these for-

mations are the reservoir horizons for

all the accumulations in the Marib-Jawf

concession area. They were deposited

in braided stream, deltaic, and turbiditic

environments (figures 2.10 and 2.11),

and are excellent reservoirs unless

affected by halite cementation or authi-

genic clays.

References

ZR Beydoun, 1964: The Stratigraphy andStructure of the Eastern Aden Protectorate,Overseas Geol. Min. Res. Supp. Series,Bull. no. 5, London, Her Majesty’s Stationery Office.

M Septfontaine, 1981: Les ForaminiferesImperfores des Milieux de Plate-forme alMesozoique: Determination Pratique,Interpretation Phyllogenetique, et UtilisationBiostratigraphique, Revue deMicropaleontologie, v. 23, no. 3/4, p. 169-203.

I Maycock, 1986: Oil Exploration andDevelopment in Marib/Al Jawf Basin, Yemen(abstract); AAPG Bull. v. 70/7, p. 930.

P Lucas et al, 1988: Sedimentological Study ofthe Alif Formation for the Alif Field, Azal Field,and Wildcat Wells, Ma’een-1 and Sean-Ba-1,Robertson Research International Ltd, Reportno. D-038.

M Sturgess, JG Mitchell and I Maycock,1992:Proposed Jurassic Lithostratigraphy for theEastern Marib-Jawf Basin, Yemen Hunt Oil Copublication.

Fig. 2.9: This stratigraphic cross section shows the cyclical nature of the Jurassic evaporites

(purple), shales, and reservoir sandstones (yellow) comprising the Amla'ah Group in the YHOC

concession area.

YHOC

Middle East Well Evaluation Review20

Channel mouth bars and associated sediments

Lower shoreface

Extensive high-energy braid plain

Mud diapir

Nearshore mass-flows

Marine mudstones

Pro-delta mudstones (Faces AB1) with thin high-density turbidite sandstones

Offshore barrier bar

Lagoonal mudstones

Rapidly prograding braided system

Mud diapir

Fig. 2.10 (Left): THE BIRTH OF ALIF:

The paleogeography of the Jurassic at the time the Alif

reservoir sands were deposited (150M years ago). The

blue line marks the edge of the YHOC concession. The

shore line would have fluctuated with changes in

subsidence and/or sea level.

Fig. 2.11 (Below): Typical depositional facies for the

Alif coastal sands and deltaic settings which occurred

during the Jurassic. (Lucas et al, 1988).

Fig. 2.12: DIRECTION

FINDING: The bedding

geometry and facies of the

Alif Formation can be defined

using imagery and/or oriented

cores. Information about

bedding orientation, derived

from a Formation

MicroScanner* survey,

enables geologists to analyze

the paleocurrent direction of

the sandstone reservoir facies.

It also allows structural dips

to be assessed which helps

with the computation of

bedding geometry. The

analysis of fault type and

orientation was also carried

out using imagery.

1ft

Delta top/ coastal plain

0 40km

Braided fluvial

Deep hypersaline marine

Lower prodelta slope

Delta front

N

21Number 12, 1992.



Six gems in the Jurassic

Six carbonate fields have been discov-

ered in the Iyad sub-basin of the Marib-

Shabwa graben which lies southeast of

the YHOC sandstone discoveries. The

fields are West, East and Central Iyad,

Amal, Magraf and Al-Gor. Four other

structures were drilled by Technoex-

port but they were found to be dry. Its

concession block was later acquired by

Nimr Petroleum Company†††.

The fields are producing from a num-

ber of Jurassic carbonate intervals with

minor zones found in thin sandy zones

and the fractured basement. Technoex-

port estimates the fields contain more

than 3 billion barrels. Most of the crude

is 41-43°API with low sulphur content.

The remainder is 36°API with higher

sulphur content (in a separate horizon)

which may be associated with a gas

cap. Reserves of natural gas are estimat-

ed at about 15 trillion cubic feet.

Whether this much oil and gas will be

recovered is now in doubt as the reser-

voir pressures have declined rapidly.

Faulting and the lenticular nature of

some of the carbonate and sandy zones

has created small compartments in the

reservoirs. The rapid decrease in reser-

voir pressure in some zones is probably

caused by faulting or zoning .

The lithology of the reservoir zones

includes both limestones and dolomites

with a wide variety of pore types

including interparticle, intercrystalline,

vugs, moulds and fractures.

The depositional origin of the car-

bonate reservoirs includes reefal mate-

rial and grainstones. However, much of

the sequence appears to be of deep-

water origin and was probably deposit-

ed after the drowning of the basin.

Shoals and banks appear to have devel-

oped on the apex of the underlying

structure.

††† Arco Shabwa Inc, a local subsidiary of ARCO

International Oil and Gas Company, is supplying

Nimr Petroleum Company with exploration and pro-

duction services on the 1,930km2

Block 4 which con-

tains the Amal and Iyad fields.

Fig. 2.14 (Above): Amran limestone forms the foundation for the ancient Marib dam. The

sluice gates of the dam, which have been rebuilt after collapsing during ancient times, are

made of blocks of Amran limestone. Inset is a core of Amran dolomite showing

heterogeneous textures and also leached fossil moulds. (Photo: YHOC).

Pho

to: J

oach

im C

hwas

zcza

1"

23Number 12, 1992.

��

��

? ? ?Naifa Fm

Am

ran

Gro

up

Upper Amran Group

?

Amala'ah Group

Lower Amran Group

Kohlan Fm

Basement (fractured)

SW NE

Shelf margin (outcropping) Iyad sub-basinUnder Old Marib dam

Jurassic shallow-water fossiliferous

and shoaling oolitic limestones are

found outcropping within the northwest

region of the graben basin. Further

southeast, the outcropping facies are

basinal shales. Unfortunately, a major

fault between the two outcrop areas

casts some doubt whether these were

deposited at the same time.

Some of the characteristics of these

carbonates in the Iyad sub-basin sug-

gest a turbiditic depositional environ-

ment. However, the dolomitization and

leaching of another main carbonate unit

suggests there may have been an evap-

orative phase with a shallower basin.

The main Jurassic salt units overlie

these carbonate reservoir zones.

An increase in carbonate within the

sediments of the Jurassic basin is

found in the Iyad and Amal field areas

of the Marib-Shabwa graben. A signifi-

cant sandstone reservoir is found to the

east in the Shabwa sub-basin, although

the multiple sandstones of the Marib

are absent.

The basin topography was taking

shape during the deposition of the

Amran Group as the limestone facies

outside of the graben basin are shallow

water sediments with an abundance of

fauna and floral typical of shallow

water. A clear change into more restrict-

ed facies is observed along the edge of

the basin in this area. Further evidence

that the graben depression was already

present during the deposition of the car-

bonate comes from the syndepositional

faulting with conglomerate units which

are well exposed along the northwest

margin of the graben.

The Jurassic salt units in the Marib-

Shabwa graben play both a positive and

negative role in petroleum exploration

and development. The location of some

of the most excellent dolomite reservoir

zones appears to be associated with the

position of the depositional limits and

facies changes with the evaporite units.

Fig. 2.13: Carbonates, shales and evaporites are the dominant Jurassic fill in the Iyad sub-basin which lies in the central part of the

Marib-Shabwa graben. The reservoirs in the Iyad and Amal fields are primarily in the dolomite and limestone intervals which are

found both above and below the Jurassic salt sequence (Amla’ah Group).

The thick salt unit appears to be the

major seal in the Marib and Iyad sub-

basins because in places where it is

absent there are no hydrocarbons in

the underlying Jurassic sequence. The

plastic nature of the thick salt units also

prevents the hydrocarbons from being

lost through the extensive faulting in the

graben. However, the presence of a

thick salt presents drilling problems,

especially because the underlying

Jurassic reservoir rocks are over-pres-

sured. The salt also absorbs much of

the acoustic energy of seismic surveys

complicates the seismic evaluation of

these reservoirs.

Out

crop

mod

ified

from

BR

GM

unpu

blis

hed

stud

ies.



Pickled pores

This salt precipitation in Alif sandstone

reservoirs was probably due to the

reduction in pressure and temperature

during production. Such problems were

encountered in the Azal Field where

they were thought to be associated with

coning of supersaturated aquifer waters

below the thin oil column.

Core analysis shows that 50%-70% of

the reservoir pores, or 10%-15% of the

bulk volume, are filled with halite. In

some cases, the salt cementation is

patchy but, in others, discrete layers

can be seen. Some of the salt contained

within the sandstones was present

shortly after deposition. However, some

of the halite precipitated much later,

after the sand had compacted but

before the oil was in place. The halite,

removed by methanol flushing, restored

the porosity of the oil zone to 16%-18%.

The amount of halite salt cement in

the sandstone can be determined from

Thermal Decay Time (TDT*) tool mea-

surements. Figure 2.18 shows a TDT log

from an interval of Alif sandstones. This

was integrated with other openhole

logs, using the Elemental Analysis

(ELAN*) approach to formation analy-

sis, to deduce the amount of salt in the

rock pores.

The chlorine atoms react to the neu-

trons emitted by the TDT tool and give

off gamma rays which in turn are mea-

sured by the sensors in the TDT tool.

Although normally logged in cased

hole, for this application the TDT is run

in open hole.

The effect of halite on the standard

logging measurements suggests a low-

density mineral. However, it is not pos-

sible to evaluate its volume

quantitatively. A scanning electron

microscope (SEM) view of a core sam-

ple in which halite cement fills much of

the pore spaces is shown in figure 2.18.

The 15pu-20pu of halite in the reser-

voir sandstone is shown in the ELAN

presentation (figure 2.18). When the for-

mation capture cross section is high,

such as in a halite layer, the TDT sigma

value is unrepresentative of the true for-

mation sigma. But in halite-cemented

sands, the measurement is reliable

because the sigma formation value

remains below the borehole sigma

value.

In the Alif sandstones, a theoretical

sigma value of 750cu has been used for

halite interpretation. Using this

approach, the original porosity, which

equals the measured porosity plus the

volume of halite in the rock, remains

fairly constant between 15pu and 20pu

in the clean section of top sand. This

suggests that the halite was introduced

into the sandstone before burial com-

paction and other diagenesis had

occurred. In essence, the reservoir was

pickled.

The Jurassic salt appears at the sur-

face in seven salt domes (three in the

Mintaq area at the southern end of the

basin; three in the Shabwa central area

and one at Safer).

The exploration potential of the

entire basin is confirmed by the

petroleum occurrences at each of these

salt domes. The basin continues

towards the southeast and has a thicker

but younger sedimentary sequence

near the Balhaf area, close to the Gulf of

Aden.

Fig. 2.16: Central portion of an exposed salt dome with Amla’ah Group salt which outcrops

within the YHOC concession area.

Fig. 2.17: The Jurassic to

modern volcanics found

within the Marib-Shabwa

graben were once considered

a negative factor for oil

exploration in this basin.

YH

OC

Joac

him

Chw

aszc

za

25Number 12, 1992.

Fig. 2.18: Sodium and chlorine atoms

combine to make a very simple cubic

molecular structure which is the

mineral halite, more commonly

called ‘table salt’’. It is the chlorine

which absorbs the neutron particles

emitted by a TDT tool. They in turn

give off gamma-ray radiation which is

measured by the TDT tool. The

volume of chlorine defined is used in

the determination of the halite

volume shown in the Elemental

Analysis (ELAN*) log. The SEM

photograph (top) shows halite

crystals which have grown within the

pore space between the sand grains.

Neutron burst from TDT tool excites halitesin rock. Gamma rays are given off.

SE

M fr

om C

hevr

on/A

AP

G

500x magnification

Chlorine atoms in the salt molecule giveoff gamma rays when excited by theTDT tool’s neutron burst.

One of the halite crystals is shown inmauve to indicate that it has beenexcited and is giving off gamma rays.

100f

t



Searching for clues

Other grabens, which have a similar

trend, may extend from Saudi Arabia

into Yemen. The direction of the Qamar

trough, which lies in southeast Yemen,

may have been influenced by the

regional shear faults and, as yet, the oil

potential of this trough has not been

adequately tested.

Connie Andre of the Smithsonian

Institute has shown that the Najd fault

system was reactivated and extended

southeast during the Mesozoic and

Cenozoic†. It is possible that more

grabens formed, producing similar sedi-

mentary basins to the Marib Basin.

This is also supported by remote

sensing data (spot stereoscopic, The-

matic Mapper (TM) multispectral

images, Thermal Infra-red images and

Advanced Very High Resolution

Radiometer data). These have revealed

a major shear zone to the southeast of

the Najd fault zone in Arabia. The total

length of the visible fault system is over

1,300km but it is possible that a younger

extension of the Najd system extends

beneath the sands of the Empty Quar-

ter, across the Rub-al-Khali desert. Seis-

mic data also indicates that the Najd

fault complex extends under the sands

of the Empty Quarter. This may even

connect with similar faults we can see

in southwest Yemen. The recent Elf

well in northeast Yemen may shed

more light on the details of this relative-

ly unexplored region.

The direction of movement along

these faults is left (sinistral) lateral and

offsets up to 2.5km are visible. If the

faults extend beneath the desert sands,

more reservoirs may be found. En eche-

lon folds and secondary faulting may

have occurred, creating hydrocarbon

traps in the Paleozoic sandstones or

Jurassic sandstones and carbonates.

The tectonic movements associated

with the Najd extensions have probably

led to the juxtaposition of Infracambrian

Ghaber Group source rocks with

younger, more porous, reservoir hori-

zons. The Ghaber Group also includes

limestones, dolostones and sandstones

which have been shown by Ziad Bey-

doun to correlate with the oil-produc-

tive Huqf Group lying to the east, in

Oman’s Dhofar region. Yemen’s algal-

rich Infracambrian carbonates are iden-

tical to those producing in Oman.

However, there is no information to

confirm that the Yemeni carbonates

have the same sealing salts as those in

the Huqf Field.

Fig.2.19: ARMCHAIR EXPLORATION: This computer-generated LANDSAT false-colour composite

image of Yemen was acquired using the Thematic Mapper sensor. The image covers an area of

185km by 178km with a ground cell resolution of 28.5m by 28.5m. The LANDSAT satellite orbits

the earth at an altitude of 700km and produces images using

two visible and one reflected infra-red spectral bands. This image has been used to map structural

features such as anticlines, synclines, faults and fractures as well as general photogeologic

stratigraphic units.

†C. Andre, 1989: Photogram Eng. Remote

Sensing, v. 55, no. 8, p 1129-1136.

27Number 12, 1992.

In addition, special processing of the LANDSAT data (see insert) has revealed areas which may indicate the presence of hydrocarbons near or

at the surface. Several promising areas were detected from the images and the prospects were subsequently upgraded after hydrocarbons were

detected. The satellite information also provides a ‘road map’ for seismic surveying, site development, environmental benchmarking and

environmental monitoring. In general, LANDSAT data provides vast amounts of geological and environmental information in a short time and at

low cost. (Image kindly provided by Texaco E & P Research Division, Houston, Texas, USA).

Sunah Field

Hemiar Field

Camaal Field

Heijahl Field



Are there more Maribs?

Canadian Occidental’s four oil discover-

ies (Sunah, Camaal, Heijah and Hemiar

fields) in anticlinal structures to the east

of the Marib basin are in Cretaceous

Qishn Formation sandstones. Canadi-

anOxy’s reserves are estimated to be

about 750 million barrels, based on the

discovery of Hemiar Field in 1992. This

field is located 18km east of Sunah

Field. The production is expected to

increase to 100,000B/D by the end of

1993.

In this area there are numerous pos-

sible structural traps, in the form of

faulted anticlines, along Yemen’s south

coast. Here, the continental shelf is very

narrow, averaging 20km-30km wide and

lies in about 1,000m of water. Wider sec-

tions of shelf occur in three main areas:

• West of Aden, opposite the mouths

of the Wadi Turban and Wadi Bana,

• In the Sayhut/Ras Sharwayn area,

• At Wadi Jeza.

The most promising horizons for

hydrocarbon exploration are found in

the Paleocene and Eocene-age lime-

stones. Evaporitic shales may be a

potential source rock and the persistent

Eocene Rus evaporite is probably a

good seal.

AGIP has carried out most of the

exploration in this region and has

drilled eight wells, all of which have oil

and/or gas shows. The most promising

is the Sharmah well which has been

tested in three different zones. Heavy

oil flowed from one zone but the

Oligocene Ghadyah Formation pro-

duced 300B/D of light oil (40.5°API).

The Eocene Habshiya Formation pro-

duced 1,800B/D but this flow rate was

almost doubled after acid treatment.

Even so, the field was considered non-

commercial.

Geochemical examination of oil from

the Habshiya and Cretaceous Harshiyat

Formations, undertaken by AGIP, sug-

gests that the Eocene oil was generated

by the Rus or Jeza formations or the

Paleozoic Radhuma Formation. Oil

recovered from Cretaceous formations

seems to have its source in rocks of the

same age.

An excellent oil-prone source rock

has been found in the Upper Creta-

ceous in thick deposits which were laid

down before the Gulf of Aden was creat-

ed. The high geothermal gradient in the

region indicates that any oil found in

the Gulf of Aden and environs will prob-

ably exist at shallow depths between

1,750m and 3,500m.

Studies of organic materials extract-

ed from offshore wells have shown that

oil buried below these depths would

eventually be cracked into methane.

This is why the Tima Basin in the Red

Sea is expected to contain gas reserves.

However, the decrease in the thermal

gradient throughout much of the Sayut

Basin in southeast Yemen means that

oil could still be present in these rocks.

The World Bank-financed Red Sea

and Gulf of Aden project recently sum-

marized Yemen’s offshore hydrocarbon

potential. It concluded that prospective

traps formed in the Gulf of Aden during

Early Oligocene rifting. Since then, sub-

sidence associated with sea-floor

spreading under the Gulf of Aden has

led to these traps becoming buried to a

depth as great as 2,000m - the same

depth as the estimated oil window.

Yemen’s Red Sea coastal shelf is

proving to be an attractive exploration

target. Not only does it have a relatively

wide graben shelf but it also has thick

salt deposits which could provide excel-

lent seals for underlying reservoir

rocks. In addition, organic matter asso-

ciated with the major marine evaporites

could provide an excellent hydrocar-

bon source.

Black, organic-rich shale deposits are

often found below the thick salt layers

in this region. These are usually

radioactive because uranium has accu-

mulated in the minerals associated with

organic matter. These pre-evaporitic

deposits vary in content and this dic-

tates the type of hydrocarbon they

would produce if brought to maturation.

Those with sapropelic (marine origin)

organic material yield oil while those of

humic origin will be gas prone.

Dark shales within these pre-evapor-

ites have been found in a number of

Tihama basin wells. Their total organic

carbon ranges in value between 1% and

2.9% and they are often interbedded

0 50km

x x x xx x x x

x x

CoastJ-1

MN-1C-1SF-1

Coastal basin Salt basin Salt wall

Thio-1Amber-1

Dhunishub B-1

+

+

++

+

+

+

++

+++

++

+++

++

++

++

+

+

++

++

+

+

+

++

++

+

+++

+

++

xx

SW

Miocene SaltOcean crust

Redbeds-Lavas-Evaporites

Rift Valley Erosion

Continental crust

Oceanic crust

Fig. 2.21 (Below): Cross section through

the southern end of the Red Sea. As the

crustal plates pull apart the pre-evaporite

reservoir carbonates and sands are

subjected to complex faulting and

folding. Thick Miocene salts provide an

excellent seal but they also hinder

seismic exploration.

Fig. 2.20: DON'T PANIC!: The continental crust in the Red Sea will

continue to pull apart and, in many millions of years, a large ocean will

exist between Arabia and Africa.

Modified from JCDoornenbal et al., 1991;Geology and HydrocarbonPotential of the TihamaBasin, Yemen: SPE MiddleEast Oil Show & E Savoyatet al., 1989; PetroleumExploration in the EthiopianRed Sea: Jour. Pet. Geol.,v. 12, no. 2.

29Number 12, 1992.

with the overlying Miocene evaporite

sequence. Shows of medium gravity oil

have also been found.

The extremely low radioactivity of

the overlying salt does not indicate a

change in environmental conditions. It

probably reflects the increased sedi-

mentation rate which occurs in hyper-

saline brines and dilutes the effect of

the organic accumulation which per-

sists at a high rate.

The thick salt layers of Miocene

evaporite which overlie the known

organic source rock units are attractive

features for explorationists. However,

the salt layers act as a low-velocity fil-

ter, hampering investigation of these

rocks using surface seismic studies.

Crustal rifting in the central part of

the Red Sea, combined with the cre-

ation of new sea floor, has split the orig-

inal evaporites and pre-evaporites.

There are now two evaporite

sequences running parallel to each

other on each side of today’s Red Sea

rift basin.

Western fringe basin

Axial trough

Central part of the salt basin

Salt basin axis Coast

Eastern fringe basin Mountain

escarpment

++

+ ++

+++

++

++

+ ++

++ +

++

++

+

++

+++ +

+++

++

+

++

++

+

NE

+

Miocene SaltContinental crust

The burial of the thick salt deposits

is accompanied by flowage and doming

of the salt which adopts a low-density

and plastic nature under pressure. This

salt flow is often the most important

hydrocarbon trapping sequence in post-

evaporite deposits but it does make

investigation of deeper structures more

difficult. Unfortunately, the Miocene salt

sequence has resulted in irregular

depositional geometry and erratic distri-

bution of post-evaporite deposits. In

onshore wells, clastics dominate this

sequence but, further offshore, carbon-

ate zones become increasingly impor-

tant.

km

0

1

2

3

4

5

Hathout 1 Taur 2 Al Fatk 1X Sharmah 1

Gulf of AdenJesa Basin

N. Hadramaut high

Rub Al Khali

��Upper Tertiary

Upper Cretaceous

Jurassic Limestone

��

��

��

��

��

Sunah 1

Sunah 2

Kohlan ss reservoir

Qishn ss

Sandstone (L. Cretaceous Qishn & Jurassic Kohlan)

Lower Tertiary

Lower Cretaceous

S Mills, 1992: Oil Discoveries in theHadramaut: How CanadianOxy Scored inYemen, Oil & Gas Jour., v. 90, no. 10,March 9, p. 49-52.

Fig.2.22: CanadianOxy’s

Sunah Field was the first

commercial discovery outside

the Marib area. This has

reservoir sands in the

Cretaceous and the thin basal

Jurassic Kohlan Formation.

Modified from SJ Mills, 1992 & SK Paul, 1990: ClassicPetroleum Provinces: Geol. Soc. Special Pub. no. 50.

looking for yemen's hidden treasure - schlumberger

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