STRATIGRAPHY AND PETROLEUM SYSTEMS OF THE …taylors/es486_petro/readings/Al-Saad_et_al-2016.pdfH....

STRATIGRAPHY AND PETROLEUM SYSTEMS OF THE PALAEOZOIC (PRE-KHUFF) SUCCESSION, QATAR

H. Al-Saad* and F. N. Sadooni**+

A thickness of more than 1500m of Palaeozoic (pre-Khuff) siliciclastic strata were encountered in three deep wells (Matbakh A, and Dukhan A and B) drilled in the State of Qatar. These sedimentary rocks have no formal entries in the Qatari Geologic Lexicon and were not included in sequence stratigraphic schemes of the Arabian Plate region. The rocks are investigated here using cores, well logs and other materials. The pre-Khuff succession has been divided from the base up into the Qasim, Qusaiba, Sharawra, Tawil and Unayzah Formations based on age, stratigraphic status, lithology, sedimentary structures and correlation with the Saudi stratigraphic nomenclature. The succession is divided into a series of third-order sequences based on the identification of the principal maximum flooding surfaces. Deposition is interpreted to have occurred in a wide range of environments ranging from fluvial to shallow-marine but with the absence of the glacial facies which are prevalent in the equivalent Saudi section. Core and log analyses indicate that the Qusaiba Formation represents an important source rock which became mature for hydrocarbon generation during the Late Permian; it generated oil until the Late Jurassic and then began to expel gas and condensates, continuing until the present day. Both the Qasim and Sharawra Formations may include potential reservoir rocks which merit further study. The new nomenclature reported here will assist with the understanding of the Palaeozoic palaeogeography across the Arabian Peninsula and will help to delineate reservoir rocks in the Qatari Palaeozoic succession.

* Center for Sustainable Development, College of Arts and Sciences, Qatar University, PO Box 2713, Doha, Qatar.** Research Office, Qatar University, PO Box 2713, Doha, Qatar.+ Author for correspondence, [email protected]

Key words: Qatar, Palaeozoic, pre-Khuff, stratigraphy, petroleum systems, source rocks, reservoir rocks, third-order sequence, Qusaiba Formation.

INTRODUCTION

Palaeozoic strata of the Arabian Plate are increasingly important exploration targets for gas, oil and condensates. A considerable amount of data is available on the Permian Khuff Formation but much less data on the pre-Khuff succession has been published; this is

about to change following the recent discoveries of gas and oil in Saudi Arabia and to a lesser extent in Oman and Iraq. With the exception of only one semi-internal report (Focke et al., 1986), the pre-Khuff succession in Qatar has not been investigated before in any detail.

Palaeozoic rocks of the Arabian Plate are exposed in two broadly north-south oriented zones (Fig. 1). A western zone extends from southern Jordan to Asir in Yemen. A second zone, located further east, extends

357Journal of Petroleum Geology, Vol. 39(4), October 2016, pp 357-374

© 2016 The Authors. Journal of Petroleum Geology © 2016 Scientific Press Ltd

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from the Amanus Mountains in southern Turkey through the Taurus and Zagros Mountains in Iraq and Iran, to the Oman Mountains and the Huqf region in Oman (Beydoun, 1985). Between these two zones, Palaeozoic strata are overlain by a younger sedimentary succession which may reach up to 16 km in thickness, and which has been penetrated by relatively few wells.

Compared to the Mesozoic, the Palaeozoic succession has only been investigated relatively recently following the discovery of hydrocarbons and other mineral deposits in Saudi Arabia and Oman (Laboun, 1982, 1988, 2010; McGillivray and Husseini, 1992). Most of the work published has focused on the Permian Khuff Formation on account of its major hydrocarbon potential. Laboun (1986) reviewed the stratigraphic status and hydrocarbon occurrence in the Widyan and Tabuk Basins in Arabia. Beydoun (1986) provided the fi rst comprehensive review of strata older than the Khuff, including their economic importance. Alsharhan and Kendall (1986) reviewed the stratigraphic succession from the Precambrian to the Jurassic in the Arabian Gulf region. They noted that the fi rst commercial volumes of oil found in the region were located in the fl uvial Ordovician to Permian sandstones of the Haima and Haushi Groups at the Marmul fi eld in southern Oman, and that hydrocarbons

were probably sourced from organic-rich intervals in the underlying Huqf Group. Laboun (1993) compiled the fi rst Lexicon of the Palaeozoic and Lower Mesozoic strata of Saudi Arabia.

Ali and Silwadi (1989) documented hydrocarbon potential within the pre-Khuff clastics of Abu Dhabi, United Arab Emirates, from wells Hair Dalmah-3, Shah-16 and Satah-19. The thicknesses of this section were 241, 439 and 452m respectively. They divided the succession into two parts separated by an unconformity associated with the Hercynian orogeny based on lithologic characterization, palynological data and log signatures. The supra-unconformity section was assigned to the Permo-Carboniferous, and the section below was considered as Devonian and older. The succession is in general composed of continental clastics grading into fl uvial/ deltaic sandstones, shales, claystones and dolomites. Gas was reported from three reservoir units (PK-1, PK-2 and PK-3) in well Hair Dalmah-3.

Upper Carboniferous to Lower Permian strata of the southern Arabian Peninsula were investigated by Alsharhan et al. (1993), who reported that these sediments in Yemen, Saudi Arabia and Oman are composed of diamictites, varvites with dropstones, sandstones and shales, which they interpreted as

Fig. 1. Main map shows the outcrop distribution of Palaeozoic rocks in the Arabian Peninsula; inset shows the locations of the three studied wells in Qatar. (Main map courtesy of A. Laboun).

N0 100

Km

Arabian Shield

Wajid Basin

Hail Arch

Central Arabian ArchArabian Shelf

Red Sea

Glacial Outcrops

Tabuk Basin

Khu�

Unayzah

Shajra

Aba ar-RuwathJawbahJawfTawil

Sharawra

HawbanSarahSaqiyahQassim

Saq

Quwayrah

Siq

Basement

QusaybaBaq’a

Perm

ian

Carb

onif.

Dev

Silu

rian

Ord

ovic

ian

Cam

bria

nPC

Subsurf.

BAHRAIN

QATAR

YEMEN

Qat

ar A

rch

Dukhan Field

Doha

DukhanArch

NorthField

Al-Khalij

MaydanMahzam

Bul Hanine

Idd Al-SharqiNorth

South

Al-Karkara

00 KM 50

51 E E E52 53

2726

25N

NN

Matbakh A

Dukhan ADukhan B

IRAQ

KUWAIT

SAUDI ARABIA

The Gulf

EGYPT

JORDAN

N0 100

Km

Arabian Shield

Wajid Basin

Hail Arch

Central Arabian ArchCentral Arabian ArchCentral Arabian ArchCentral Arabian ArchCentral Arabian ArchArabian Shelf

Red Sea

Glacial Outcrops

Tabuk BasinTabuk BasinTabuk Basin

Khu�

Unayzah

Shajra

Aba ar-RuwathJawbahJawfTawil

Sharawra

HawbanSarahSaqiyahQassim

Saq

Quwayrah

Siq

Basement

QusaybaBaq’a

Perm

ian

Carb

onif.

Dev

Silu

rian

Ord

ovic

ian

Cam

bria

nPC

Subsurf.

BAHRAIN

YEMEN

QATAR

Qat

ar A

rch

Dukhan Field

Doha

DukhanArch

NorthField

Al-Khalij

MaydanMahzamMaydanMahzamMaydan

Bul HanineBul HanineBul Hanine

Idd Al-SharqiNorth

South

Al-Karkara

00 KM 50

51 E E E52 53

2726

25N

NN

Matbakh AMatbakh AMatbakh A

Dukhan ADukhan B

IRAQ

KUWAIT

SAUDI ARABIA

The Gulf

EGYPT

JORDAN

358 The Palaeozoic (pre-Khuff) succession, Qatar

Sadooni.indd 358 01/09/2016 10:44:49

grounded tillites and glacio-lacustrine to marine deposits. They noted that oil accumulations in Oman occur in the sandstones of the Haushi Group whose petroleum was probably sourced from Late Precambrian marine algal limestones. de la Grandville (1982) reported that oil is produced from a number of oilfields in south Oman from Ordovician siliciclastics and from Permo-Carboniferous periglacial clastics and Permian sediments. Abu Ali et al. (1999) developed a geochemical model to explain the history of migration and entrapment of Palaeozoic hydrocarbons in the Permian Unayzah Formation in central Saudi Arabia, concluding that the Silurian Qusaiba “hot shale” is the main source rock for these accumulations.

Senalp and Al-Duaiji (2001) published a comprehensive analysis of the Ordovician Qasim Formation in central Saudi Arabia, and Senalp and Al-Laboun (2000) detailed Upper Ordovician glacial sediments in this area from surface and subsurface data. They reported that Upper Ordovician strata show glacially formed unconformities as well as a combination of glacial, marine and fluvial sediments which fill deep, incised valleys. They suggested that these sediments were deposited in intermediate or even high latitudes in southern Gondwana.

Bugti et al. (2010) attempted to interpret seismic data and investigated six deep wells from the Awali field, Bahrain. They suggested that deep structures and reservoirs may occur in the Jubah, Jauf, Tawil, Ra’an, Hanadir and Saq Formations which may be sourced from the Qusaiba Shales. They suggested that the Devonian Jauf Formation is gas-bearing and is associated with a fault-trap play.

Mamam and Nasrulla (1989) published the first evaluation of drilling operations and reservoir characteristics of the pre-Khuff succession in well Matbakh-2, offshore Qatar. They documented the presence of gas in the tight pre-Khuff strata, while the more porous and permeable section was found to be water-bearing.

More recently, van Buchem et al. (2014) reviewed the depositional systems and hydrocarbon habitat of Qatar. Based on tectonic history, depositional regimes and climatic changes, they divided the Phanerozoic succession of the country into six tectono-sedimentary phases. The second phase encompasses Palaeozoic shallow-marine clastic sediments, and was characterized by the presence of conspicuous surfaces of erosion or non-deposition which represent the local effect of the Hercynian orogeny.

MATERIALS AND METHODS

The study is based on data from three wells which penetrated the pre-Khuff siliciclastics in the State of Qatar, namely wells Matbakh A, Dukhan A and Dukhan

B (Fig. 1). Data include well logs, cores, thin sections and some porosity and permeability measurements. Additional information came from a semi-internal report published by Qatar Petroleum (Focke et al., 1986) which hitherto has not been widely available.

Detailed descriptions were carried out on the available cores, recording lithology, sedimentary structures, diagenetic features, and visible porosity such as mouldic and fracture porosity. These data were plotted on the well logs. The studied wells were correlated and the succession was divided into stratigraphic intervals by analogy with the equivalent Saudi section, as modified recently by the Saudi Stratigraphic Committee (2013). Well logs were also processed to calculate the petrophysical properties of potential reservoir intervals in the studied section. Analysis of both cores and logs enabled the main stratigraphic sequences to be delineated, and these are compared to those identified by Sharland et al. (2001) for the Arabian Plate.

STRATIGRAPHIC FRAMEWORK

The geologic lexicon of Qatar (Sugden and Standring, 1975) has no entries for pre-Khuff strata which consequently have not been formally named, defined or described. No rocks older than Tertiary are exposed at outcrop in the country except for the diapiric Infracambrian material which occurs in some salt-cored islands (Sadooni et al., 2004). The only available synthesis on the pre-Khuff succession is that presented by Focke et al. (1986).

Palaeozoic strata in Saudi Arabia, however, have been studied for many decades. The first stratigraphic scheme was introduced by Powers (1986), who suggested the term “Tabuk Formation” to encompass the entire Ordovician – Silurian succession. He divided the formation into six members, two of them un-named. McClure (1978) was the first to describe Gondwanan continental glaciation (Hirnantian, Ordovician) from Saudi Arabia (Beuf et al., 1971; Miller and Al-Ruwailli, 2007). This discovery led to an extensive revision of the Powers (1968) stratigraphic scheme by Vaslet et al. (1987) and Vaslet (1989, 1990), and the succession was divided accordingly into glacial and non-glacial intervals. The Ordovician glaciation event in Arabia was characterized by two phases of ice-sheet advance which are represented by the Zarqa and Sarah Formations. Clark-Lowes (2005) described the evolution of palaeo valleys within the Sarah Formation and suggested that these glacial sediments may represent potential reservoir rocks.

The stratigraphic scheme for the pre-Khuff succession in Saudi Arabia introduced by the Saudi Stratigraphic Committee (2013) recognised four Groups, namely Tayma, Tabuk, Qalibah and Huj. The

359H. Al-Saad and F. N. Sadooni

Sadooni.indd 359 01/09/2016 10:44:50

Tayma Group includes all the sediments which were deposited before the Late Ordovician glacial period. The Tabuk Group is composed of Late Ordovician glacial and periglacial deposits. The Qalibah Group includes the regressive Lower Silurian siliciclastics which form the main source rocks in the Palaeozoic succession. The Huj Group comprises the Upper Silurian – Lower Carboniferous succession.

The first data on the pre-Khuff succession in Qatar became available in 1983 after the drilling of well Matbakh A on the Qatar Arch, where around 1500m of pre-Khuff strata were penetrated. Another two deep wells (Dukhan A and B) were subsequently drilled, and these wells were used by Focke et al. (1986) to describe the section. As this work is a semi-internal publication of Qatar Petroleum, it is little known outside the company and the Qatari section has therefore not been included in regional studies of the pre-Khuff succession including for example Sharland et al. (2001). Following the Powers (1986) scheme but with some modifications, Focke et al.(1986) divided the succession in Qatar into the Tabuk, Sharawra, Tawil and Haushi Formations.

Regional correlation of the pre-Khuff Qatari succession with its Saudi equivalent has led the present authors to change some of the terms used to make them more compatible with the Saudi nomenclature, in order

to improve regional correlations and understanding of these strata across the Arabian Peninsula. There is insufficient data to confirm the existence of the Ordovician Tabuk Group in Qatar, and the pre-Khuff section is therefore divided into three groups, namely: the Early Cambrian – Ordovician Tayma Group, represented by the Qasim Formation; the Silurian Qalibah Group, represented by the Qusaiba and Sharawra Formations; and the Late Silurian – Early Carboniferous Huj Group represented by the Tawil Formation. This is unconformably overlain by the Permian and younger Buraydah Group comprising the Unayzah and Khuff Formations. This nomenclature is open for the insertion of the Tabuk Group if future work proves its existence in the subsurface of Qatar.

Fig. 2 shows the proposed new stratigraphic scheme compared to that of Focke et al. (1986). The Groups are summarised in the following paragraphs which are followed by a review of the potential Palaeozoic petroleum system in Qatar.

Tayma Group (late Early Cambrian –early Late Ordovician)

The Tayma Group (Vaslet et al., 1987) includes all the Lower Palaeozoic sediments which were deposited before the Late Ordovician glaciation in Saudi Arabia.

Fig. 2. Chart showing the Focke et al. (1986) nomenclature for the pre-Khuff succession in Qatar, and also the revised nomenclature proposed in this paper.

Age Formation Member

Lithology Focke et al., 1986 This Study

Formation

Khu�

Period

LateMid

Artinskian

Sakmarian

Haushi

K-5

Frasnian

GivetianEifelian

Pre-EmsianReworked

Silurian

Silu

rian

Ord

ovic

an

Ludlovian

Wenlockian

Llandoverian

Late

Mid Tabuk

Tawil

Lower

Khu�

Qasim

Qal

ibah

Tawil

SharawraSharawra

Qusaiba

UpperSandy

LowerShaly

Upper

D e

v o

n i

a n

P e

r m

i a

n

Taym

aH

uj

Group

Bura

ydah

Unayzah

Lower Porous Zone

Member

Upper

Lower

Lower Porous Zone


Sadooni.indd 360 01/09/2016 10:44:50

The Group includes the Siq, Quweira, Saq and Qasim Formations. Only the Qasim Formation has been encountered in Qatar.

Qasim FormationThe Qasim Formation was described by Williams et al. (1986) from Habashi Mountain in the Qasim region in northern Saudi Arabia, and replaces the lower parts of the Tabuk Formation of Powers (1968). The formation is divided at its type locality into four members from bottom to top: Hanadir, Kahfah, Ra’an, and Quwarah (Fig. 3). It consists of up to 1600m of shallow-marine clastics ranging from claystones to sandstones. In some areas, the upper beds are incised by glacial palaeovalleys associated with the fall in sea level during the Late Ordovician glaciation (Ghazwani, 2012).

In Qatar, Focke et al. (1986) used the term “Tabuk” for these strata, but here the term Qasim will be used to replace the previous, now obsolete term. The Qasim

Formation in Qatar is of Middle to Late Ordovician age based on the presence of the trace fossils Cruziana and Skolithos and on the palynological assemblages recovered from well Matbakh A. However, the formation was assigned to the Early Silurian in well Dukhan B, and this age controversy cannot be resolved given the present quality of the data (Fig. 4).

The Qasim Formation is composed of micaceous and arkosic sandstone with siltstones and shales. The sandstones contain plagioclase (up to 34%) and quartz, and are thinly bedded (beds are less than 15 cm in thickness) showing fining upward sequences. Locally, the sandstones pass into shales (Focke et al., 1986).

In well Dukhan B, the formation consists of larger units up to 15m thick which coarsen upwards. Focke et al. (1986) suggested that these units may have been deposited in storm-agitated waters or by traction currents. The fine-grained parts of the formation are intensely bioturbated, and common ichnofacies are vertical or oblique burrows of Skolithos and Cruziana.

Fig. 3. Lithostratigraphy, sedimentary structures and interpreted depositional settings of the Ordovician – Silurian succession exposed at the surface in Central Saudi Arabia (modified after Clark-Lowes, 1980, 2005).

Shallow Marine/Deltaic

HanadirMember

Tidal /SubtidalSand

BivalvesGraptolitesTrilobites

Skolithos

Skolithos

Llanvimian

KahfahMember Llandeilian

Zoophycus

Bifungites

ShallowMarine/Deltaic

Fluvial/ Glacial

Q

A

S I

M

F

O

R

M

A T

I

O

N

Ra’anMember

QuwarahMember Caradocian

Ashgilian

Llandoverian

Wenlockian

S T R A T I G R A P H Y

AgeInterpretation

Faunaand

Trace FossilsPaleo-current

Data

Grain SizeSedimentary Structures

and Lithology

Sub-LittoralSandsOrthocone-

Nautiloids

TrilobitesGraptolitesBivalvesNot Exposed

Sara

h M

embe

r com

pris

ing

glac

ial,

�uvi

al a

nd m

ud�o

w d

epos

itscu

ts d

own

into

und

erly

ing

Saq

Form

atio

n

100M

Shal

eSi

ltsto

ne

F IMC

Sand

ston

e

Gro

up Formation Member

T a

y

m

aQ

a

l

i b

a

h

Sharawra

Qusaiba

Sarah


Sadooni.indd 361 01/09/2016 10:44:51

The main lithologic units of the formation include:1. Silty shales with load structures and de-

watering features.2. Fine-grained sandstones with well-developed

cross-lamination, scouring and contortion. 3. Claystones alternating with thin, contorted

sandstone laminae. Root-like features protrude from the sandstone into the claystones, and are probably cast-load features of sandy material penetrating into the relatively soft claystone.

4. Sandstones with unidirectional ripple marks draped and mixed with bioturbated shales and some sandy nodular features, which may indicate local reworking.

At the Matbakh A well, the formation is dominated by fi ner-grained materials with relatively high gamma-ray readings (Fig. 4). This section may be correlated with the Ra’an Shale Member of the Qasim Formation in Saudi Arabia.

The presence of the trace fossil Cruziana has been reported from many parts of the Arabian Plate, including in the Khabour Quartzite Formation in Iraq,

the Qasim Formation in Saudi Arabia and the Andam Formation in Oman (van Bellen et al., 1959; Seilacher, 2007).

In general, the Qasim Formation is interpreted to have been deposited in shallow marine, locally deltaic settings. The fl uvial/glacial sediments of the Sarah Formation, which have been reported from many parts of Saudi Arabia, have not been recognized in Qatar, and there is no evidence to support the extension of the glacial conditions into Qatar at that time.

Qalibah Group (Silurian)

The Qalibah Group was proposed by Janjou et al. (1996) to include the Silurian Uqlah, Qusaiba and Sharawra Formations. Only the last two of these formations have been encountered in the subsurface of Qatar.

Qusaiba FormationThe Qusaiba has been upgraded to formation status; previously it was considered as a member of the so-

Fig. 4. Lithology and gamma-log response of the Qasim Formation in wells Dukhan B and Matbakh A. The shale-rich section at Matbakh A may indicate the presence of the Ra’an Member of the Qasim Formation at this location (modifi ed from Focke et al., 1986).

FormationGamma Ray

(API)00 100 200

LithologyDepth

4710

4740

4770

4800

4830

4668.5m

Gamma Ray(API)100 20000

Lithology

4710

4740

4770

4800

4830

Q

A

S I

M

)(M BDF

MATBAKH A100 KMQ

USA

IBA

DUKHAN B


Sadooni.indd 362 01/09/2016 10:44:52

called Qalibah Formation. A new type section has been described from the Ad Dahkiyah area in Saudi Arabia (Janjou et al., 1996). In central and NW Saudi Arabia, the formation is dated as Llandoverian based on the presence of trilobites, graptolites and chitinozoans (Clark-Lowes, 1980) (Fig. 3).

At its type section the formation is divided into fi ve units; in general, it is composed of alternating siltstones and micaceous fi ne-grained sandstones with ripple marks, cross bedding and bioturbation. Some parts of the formation are correlatable with the “hot shale beds” of central Saudi Arabia which are considered to be the main source rocks in the Palaeozoic succession (Jones and Stump, 1999).

In Qatar, Focke et al. (1986) named this interval as

the Lower Shaly Member of the Sharawra Formation and noted that it can be correlated with the Qusaiba and Ra’an shale members in Saudi Arabia, although these members appeared to be of different ages. The member is composed of a uniform section of grey to blackish mica-bearing siltstones and claystones in the lower part and these lithologies appear to be similar to those in the Saudi section (Fig. 5).

The formation in Qatar is composed of marine transgressive shales that contain sapropelic organic matter and may represent the source rock for the gas pockets which are reported in the pre-Khuff succession, as well as the major gas accumulation in the Khuff Formation in the North fi eld (Focke et al., 1986) (see below).

Fig. 5. Lithology and gamma-log response of the Qusaiba and Sharawra Formations in well Matbakh A (modifi ed from Hamam and Nasrulla, 1989).

Marine

transgressive

shale

Subtidal sandstone,

siltstones and shales

(API)

00 50 100Gro

up

FM

Lith

olog

y

FDC LogDepositional Environment(gm/cc)

2.45 2.70 2.952.20

Strat.Seq.

GR Log

Q U

S A

I B

AS

H A

R A

W R

A

Q

A

L I

B

A

H

AP3


Sadooni.indd 363 01/09/2016 10:44:53

Sharawra FormationThe average thickness of the Sharawra Formation at the new type locality in the Al Qalibah quadrangle is 433m (Saudi Stratigraphic Committee, 2013). The formation was divided into four members by Janjou et al. (1996). In general, the formation is composed of upward-coarsening sequences of sandstone and siltstone with red micro-conglomeratic clasts and dark-red phosphatic grains with ball-and-pillow structures. Al-Ruwaili (2000) assigned a Ludlovian (Late Silurian) age to the formation.

In Qatar, the Sharawra Formation is composed of thin-bedded sandstones alternating with siltstones with shales in its upper part. The sandstone units are relatively thick (up to 6m), and are characterized by

the presence of cross-bedding, and fl aser and ripple structures. The formation represents a transgressive marine system that generated relatively coarse sediments which were deposited in a shallow-water subtidal setting (Figs 3 and 5).

Huj Group (Late Silurian to Early Carboniferous)

The Huj Group was introduced by Janjou et al. (1996) to describe the Upper Silurian – Lower Carboniferous rocks in NW Saudi Arabia, where the group includes the Tawil, Jauf and Jubah Formations. In Qatar, only the Tawil Formation has been encountered in the subsurface (Fig. 6).

Fig. 6. Lithology and gamma-log response of the Tawil Formation in well Matbakh A (modifi ed from Fockeet al., 1986).

30 0Porosity

%LithologyGamma Ray (API)

0 100 200

Upp

er

Taw

ilLo

wer

Por

ous

Zone

T A

W I

L

F O

R M

A T

I O

N

Shal

low

Mar

ine

Cont

inen

tal (

Allu

vial

)

Formation Member StratigraphicSequence

AP3


Sadooni.indd 364 01/09/2016 10:44:55

Tawil FormationThe Tawil Formation was described for the fi rst time by Steineke et al. (1958). The type locality of the formation is in the Al Jawf area where it reaches a thickness of about 200 m thick. The Tawil Formation consists of large-scale cross-bedded, medium- to coarse-grained sandstones interbedded with red shales, particularly in the lower part. At the base of the formation, there are beds of coarse-grained to micro-conglomeratic sandstone with a lenticular band of quartz pebbles in the lower part.

The formation has no major fossils and hence its age is still controversial. Al-Laboun (1982) assigned an Early Devonian (Lochkovian) age for the formation based on microfl ora from well samples, and on the

Middle Devonian age of the overlying Jauf Formation. Janjou et al. (1996) suggested a Late Silurian to Early Devonian age because of the Lochkovian – Pragian age of the basal member of the Jauf Formation.

In Qatar, the Tawil Formation is assigned to the Devonian (and the upper part probably to the Carboniferous) and is divided into three sections: a lowermost un-named portion; a middle “Lower Porous Zone”; and the overlying Upper Tawil. The formation in general consists of a monotonous succession of thin sandstone beds with shale horizons which are around 1 m thick. Sandstones in the Upper Tawil section contain low-angle cross-bedding with plant remains and mica fl akes, and are cemented with quartz and siderite and to a lesser extent with gypsum.

Fig. 7. Lithology and gamma-log response of the Unayzah Formation in well Matbakh A (modifi ed from Focke et al., 1986).

Khu�

L O

W

E R

UPP

ER

Porosity

30 15 00

(%) Lithology

Gamma Ray

(API)0 120

Formation

T

E

R

R

E

S

T

R

I

A

LM

ARI

NE

U

N

A Y

Z

A

H

Strat. SequenceEnvi.

D10402 Ma)(

AP5


Sadooni.indd 365 01/09/2016 10:44:56

The Lower Porous Zone is around 100m thick and represents the most important gas reservoir in the entire pre-Khuff succession in Qatar. The sandstones are cross-bedded and contain wavy and fl aser bedding (Fig. 5) (Focke et al., 1986).

The formation represents a shallowing sequence since the lower part is composed of marine sediments that gradually pass into fl uvial/continental deposits with a considerable increase of shale content. These changes are probably the result of a combination of tectonic, depositional and climatic changes (van Buchem et al., 2014).

Buraydah Group (Mississippian – Pennsylvanian)

The Buraydah Group was introduced into the Saudi stratigraphic system by Vaslet et al. (1988) to include strata across the Palaeozoic – Mesozoic boundary. The Palaeozoic part of the group encompasses the Berwath, Unayzah and Khuff Formations.

Unayzah FormationIn Saudi Arabia, the Unayzah Formation was described and formally defi ned by Laboun (1987) as a siliciclastic succession underlying the Khuff carbonates in the Al Qasim area (Saudi Stratigraphic Committee, 2013).

Fig. 8. Tectonostratigraphic megasequences and maximum fl ooding surfaces of the Cambrian – Devonian succession in Central Saudi Arabia. The pre-Khuff interval of Qatar is shaded in green colour in the Stratigraphy column. (Modifi ed from Sharland et al., 2001).

0 -5

KM-100 Meters

Striations ’ groove marks

Varves

Slumping

BurrowsFormations found in Qatar

Basement

Tabu

k G

roup

Formation

SaqFormation

Taym

a G

roup

D20 (393 Ma)

)

)

D10 (402

S10 (440 Ma

)

Qasim

Tawil

SharawraFormation

QusaibaFormation

FormationSarah

Formation

Formation

Huj

Gro

up

ZarqaFormation

Central Saudi Arabia

StratigraphyLithology

SequenceStratigraphySharland et al

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Ashgill

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Llandoilo

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AP3

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AP2

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TawilFormation

Formation

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QusaibaFormation

SharawraFormation

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asim

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Sarah Formation

Saq Formation

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ZarqaFormation

ZarqaFormation


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Originally these strata were named the “pre-Khuff clastics” (Laboun, 1982) or the fifth lower member of the Khuff Formation (Delfour et al., 1982). At its type locality, the Unayzah Formation consists of alternating thinly-bedded sandstones, siltstones, shales and marls, with minor lithologies including limestones and anhydrite, together with varicoloured silty, gypsiferous claystones, alternating at the bottom with yellowish to grey, silty dolomite. In some locations the formation contains fine-grained, cross-bedded sandstones in the middle part and dolomite beds which are stained red by iron oxides and manganese at the top. In the subsurface, the Unayzah Formation is divided informally into the A, B, and C members from top to bottom (Saudi Stratigraphic Committee, 2013).

In Qatar, these strata were named the Haushi Formation by Focke et al., (1986). This was unfortunate because the Haushi was already used as a Group name for Carboniferous – Permian sediments in Oman. We suggest that the term is replaced by “Unayzah” in the Qatari stratigraphic nomenclature. The Unayzah Formation in Qatar is Lower Permian, and it rests unconformably on the Tawil Formation and is in turn overlain by the Khuff Formation (Fig. 7).

The Unayzah Formation consists of fine-grained to pebbly sandstones, conglomerates and breccias, siltstone and clays, shales with some anhydrite, and includes lignite and silty coal beds. Sandstone is the dominant lithology with cross- and graded bedding with load-cast structures and fracturing.

The lower part of the formation represents a continuation of the continental conditions which prevailed during the deposition of the underlying Tawil Formation, although the sandstone beds are thicker and better developed. Marine conditions were re-established in the upper part of the formation, and evolved into a wide-scale transgressive system which deposited the carbonates of the Khuff Formation. The upper part of the formation contains some sandy limestones and shales which suggest shallow-marine conditions under the influence of clastic input from the nearby land.

SEQUENCE STRATIGRAPHY AND DEPOSITIONAL SETTINGS

In order to classify the Palaeozoic succession of Qatar within the sequence stratigraphic framework established for the Arabian Plate, a comparison was made with the sequence stratigraphy of age-equivalent units in central Saudi Arabia. Fig. 8 shows the main tectonostratigraphic megasequences (TMSs) and Maximum Flooding Surfaces (MFSs) of the equivalent units in Qatar.

According to the stratigraphic scheme suggested by Sharland et al. (2001), the sediments of the Tayma

Group (i.e. Saq and Qasim Formations) represent the AP2 megasequence which extends from the Early Cambrian to the Late Ordovician (520-445 Ma). Only the Qasim Formation have been encountered in the subsurface of Qatar. Two maximum flooding surfaces (O30 and O40) were recognized within this formation: (i) the Middle Ordovician O30 surface (middle Llanvirn) represents the base of the Qasim Formation, and was placed at the basal shale unit (the Hanadir Shale member of Saudi Arabia); and (ii) the Late Ordovician O40 surface (Late Caradoc), which was placed at a middle shale unit (the Ra’an Shale member of Saudi Arabia).

During the deposition of the AP2 megasequence, the Arabian Plate including Qatar was represented by a broad and shallow-water clastic shelf adjacent to the Palaeo-Tethys passive margin, which received a supply of continental clastic material in the south and which passed into more marine conditions to the north.

The AP2 megasequence consists of two cycles: the first is within the Saq Formation; and the second is delineated by a TST under the O30 Maximum Flooding Surface at the base of the Qasim Formation. This cycle represents the resumption of normal marine conditions after the deposition of the mixed, unsorted clastics of the underlying Saq Formation. The Qasim Formation in Qatar is composed of alternating sandstones and shales at well Dukhan B and predominantly of shales at well Matbakh A; the presence of trace fossils indicates the prevalence of shallow-marine conditions.

In Saudi Arabia, the AP2 megasequence ends with an erosional unconformity at the Zarqa / Sarah Formation which represent a glacial phase. Corresponding sediments are not reported from Qatar although glacial deposits are recognized in the Zagros to the east and as far as north as southern Turkey; however they were not recognized in the materials studied. Hence the top boundary of this megasequence is placed at the bottom of the Silurian Qusaiba Formation (the Lower Shaly Member of the Sharawra Formation of Focke et al., 1986).

In general, the lithology of the formation indicates a variable depositional environment with thin beds of sandstones and shale alternating at a small scale and containing nodules, flasers and load cast structures. Such a setting and the presence of trace fossils suggest a shallow-water, coastal marine environment. Similar conditions at that time extended over a large area from northern Iraq to Oman.

Megasequence AP3 encompasses the succession between the Upper Ordovician and the Upper Devonian (445-364 Ma) (Fig. 8). The lower part of this megasequence is placed above the Upper Ashgill unconformity which marks the top of the Qasim Formation in Qatar and other parts of Arabia. In Qatar, this megasequence contains neither glacial


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sediments (as in central and west Arabia) nor volcanic materials as in the northern part of the Arabian Plate. It consists of the Silurian Qalibah Group (Qusaiba and Sharawra Formations) as well as the Tawil Formation of the Devonian Huji Group. There is a disconformity between the Sharawra and Tawil Formations (Focke et al., 1986).

Two maximum flooding surfaces are recognized in the AP3 megasequence in Qatar: the Early Silurian S10 (mid-Aeronian) MFS, which is located at the base of the Qusaiba Formation; and the Late Silurian S20 (late Pridoli) MFS at the base of the Tawil Formation. The S10 represents a “second order” depositional sequence which followed the melting of the Late Ordovician glaciation. Volcanic activity in the northern part of the Arabian Plate indicates a shift from the passive margin conditions that prevailed during AP2 to the onset of back-arc rifting, with the start of SW-directed subduction (Sharland et al., 2001).

There is not enough evidence to support the extension of the glaciation to present-day Qatar. The sediments indicate a continuation of low-energy marine conditions, as indicated by the deposition of the shales of the Qusaiba Formation. However, the melting of the ice cap may have resulted in the establishment of a long-lasting TST which was accompanied by deposition of the “hot shale” within the Qusaiba Formation.

The sandstones of the Sharwra Formation represent a TST which was probably associated With regression after the development of the S10 MFS. The Presence of the Cruziana ichnofacies indicates a subtidal marine environment with little variation in lithology or facies. Most of the sediments are characterized by bioturbation which is recorded in both Saudi Arabia and Qatar.

The MFS D10 of Sharland et al. (2001) passes through the top shale member of the Tawil Formation. Janjou et al. (1996) proposed a Late Silurian to Early Devonian age for this formation on the basis of the Lochkovian – Pragian age of the basal member of the overlying Jauf Formation in Saudi Arabia, and on the minimum age of the top of Sharawra Formation of the Qalibah Group (Saudi Stratigraphic Committee, 2013).

It is difficult to infer the detailed depositional environment of the Tawil Formation in Qatar from the present data, but the presence of land plant spores in the upper part led Focke et al. (1986) to suggest that it may have been deposited partly in a fluvial braided river plain. The lower part, however, is thought to have been deposited in a shallow-marine environment due to the presence of wavy, ripple and flaser bedding as well as of marine acritarchs. The lower part of the Saudi section represents a major transgressive event that developed into an alluvial braided river system. The sedimentary structures of the top strata may indicate a renewed transgression characterized by tidal sand bars

Fig. 9. Burial history curve of the Qusaiba Formation in well Matbakh A (modified from Focke et al., 1986).

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900

1800

2700

3600

4500

P A L A E Z O I C M E S O Z O I C CENOZOIC

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formed in a deltaic to estuarine complex, marking the Early Devonian transgression (Janjou et al., 1996).

AP4 sediments have not yet been encountered in Qatar. AP4 (Late Devonian to Late Carboniferous) strata are missing or were not deposited over large parts of the Arabian Plate including Qatar, possibly due to regional uplift.

The AP5 megasequence extends from the Late Carboniferous to the mid-Permian (295-255 Ma), and includes the relatively thin Permo-Carboniferous clastics which are interpreted to have been deposited in a partly syn-rift regime. The Arabian Plate remained in the same extensional back-arc setting but migrated into subtropical latitudes which led to the deposition of marine carbonates and evaporites. The base of this megasequence is placed at the Hercynian Unconformity and its top at the pre-Khuff unconformity (Sharland et al., 2001). In Qatar, the Unayzah Formation belongs to this megasequence.

THE PALAEOZOIC PETROLEUMSYSTEM IN QATAR

The hydrocarbon potential of the pre-Khuff strata in Qatar has not yet been investigated in any detail, although important oil accumulations have been reported from equivalent units in Saudi Arabia. In Saudi Arabia, the pre-Khuff Palaeozoic petroleum system comprises the Qusaiba Formation as the main source rock, together with reservoir units in the Devonian Jauf Formation and the Carboniferous-Permian Unayzah Formation. The basal-Khuff clastics and evaporites represent a regional seal, with structural traps related to the Hercynian Orogeny (Cole et al., 1994).

Source Rocks The Qusaiba Formation (the Lower Shaly Member of the Sharawra Formation of Focke et al., 1986) consists

Fig. 10. Litho-saturation cross-plot of the Qasim Formation in well Dukhan B.

0.2 0.4

4672.7

4702.7

0.6 0.8 1.0

4732.7

4762.7

4792.7

4822.8

4852.7

Depth (M)Volume

MatrixShale Water SaturationResidual Movable

HydrocarbonsHydrocarbons


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of grey-black siltstones and claystones, and source rock intervals contain sapropelic organic matter. The total organic carbon content ranges up to 7.3% wt.

Fig. 9 shows the burial history of the shales in the Qusaiba Formation in well Matbakh A. The figure assumes an average geothermal gradient of 3.1 C° per 100m based on the general geothermal gradient prevalent in the Arabian Plate and a surface temperature that ranges from 10 to 25 C° after the Permian glaciation. It seems that the Qusaiba shale reached maturity during the Late Permian (240 million years ago) and may have generated oil until the Late Jurassic, and then condensate and wet gas after that. Dry gas began to be generated during the Cretaceous possibly continuing locally to the present-day in areas where the source rocks have not been consumed.

Reservoir rocks

Qasim FormationThe litho-saturation cross-plot of the Qasim Formation in well Dukhan B (Fig. 10) shows a shale volume ranging between 2% and 85% (average, 23%) in the upper part (4668-4710m), intercalated with matrix varying between 1% and 88% (average, 67%), and a water saturation of 1-6% (average, 3%). Movable hydrocarbons have values ranging between 6% and 13% (average 9%) in this interval. In the middle part of the formation (4710-4800 m), the shale volume varies between 4% and 34% with an average matrix value of around 84%. The water saturation is about 3%, while movable hydrocarbons have an average of 8%.Within the lower interval of the Qasim Formation, the matrix

Fig. 11. Litho-saturation cross-plot of the Sharawra Formation in well Dukhan B.

3363.6

3515.2

3666.7

3818.2

3969.7

4121.2

4272.7

4424.2

4575.8

0.0 0.2 0.4 0.6 0.8Volume

Dep

th(M

)

MatrixShale Water SaturationResidual Movable

HydrocarbonsHydrocarbons


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is the main component with an average of 91%, and shales increase gradually with an average of 10%. Water saturation ranges between 1% and 6%, while movable hydrocarbons reach 10%.

Porosity values range between 4% and 14% (average 9%) and permeability is around 0.5 mD. The formation is made up of fine-grained sandstones which were cemented with feldspar and calcite cement. Some of the porosity originated from the decay of the feldspar or the partial dissolution of the carbonate cement creating mouldic porosity. Pores however are small in size and not connected. Also, the Qasim Formation has poor reservoir characteristics because of the thin laminations of the sandstone beds, which rarely exceed 5 cm.

Sharawra FormationThe Sharawra Formation shows shale volumes ranging between 2% and 40 % in the upper part (3330-3630m) with an average of 12%, intercalated with matrix with volumes of between 52% and 92%, with almost zero water saturation. Movable hydrocarbons range up to 94%, while residual hydrocarbons vary from 0% to 2% in this interval (Fig. 11). In the middle part (3630-4080 m) the shale volume ranges between 1% and 95% (21%), while the matrix varies from 0% to 69%. Movable hydrocarbons, the main hydrocarbon constituent, are about 15%, while residual hydrocarbons are about 2%, and the water saturation is almost zero. The lower interval of this formation (4080-4668m) shows shale values ranging between 4% and 99% (average 32%), intercalated with matrix of between 0% and 88%. Movable hydrocarbons reach 15% with residual hydrocarbon saturation of about 1%, and zero water saturation.

Although the Sharawra Formation includes thick shale intervals, it has also some well-developed sandstone intervals with good porosity and permeability. Porosity ranges between 3% and 21% (average 10%), and a maximum value was recorded at 3870m (20%). Sandstones are mainly quartz arenites with minor amounts of feldspar. The main porosity is mouldic resulted from the dissolution of calcite cement with some fine intergranular porosity.

CONCLUSIONS

The Palaeozoic succession of the Arabian Plate is becoming important as an exploration target for gas, oil and condensates. Although a considerable amount of data is available for the Permian Khuff Formation, there is little data on the pre-Khuff succession. The first data on the pre-Khuff in Qatar became available after the drilling of well Matbakh A on the Qatar Arch, which penetrated about 1500m of pre-Khuff strata. Another two deep wells, Dukhan A and B,

were drilled subsequently. Palaeozoic strata in Qatar were not included in previous publications such as the Qatari Geologic Lexicon or the sequence stratigraphic framework of the Arabian Plate.

A new stratigraphic nomenclature is suggested here and is comparable with that suggested for the equivalent succession in Saudi Arabia. The Tabuk Formation is now named the Qasim Formation; both the Qusaiba and Sharawra are upgraded to formation status as a part of the Qalibah Group, and the Haushi Formation is replaced by the Unayzah Formation.

The newly introduced nomenclature is updated with the sequence stratigraphic scheme of Sharland et al.(2001).

The available data suggest that the Qusaiba Shale is an important source rocks that generated oil until the Late Jurassic and then started to generate gas since then. Both the Qasim and Sharawra Formations represent potential hydrocarbon reservoir rocks.

ACKNOWLEDGMENTS

The authors are indebted to Qatar Petroleum for providing data and for allowing access to cores. We are grateful to M. Lofty for his help with the well log analyses, and to Andrew Horbury for reviewing an early version of the manuscript. The manuscript benefitted from extensive reviews by A. A. Laboun (King Saud University), who kindly supplied Fig. 1 and many additional resources; and Owen Sutcliffe (Neftex) who also provided many insights and materials.

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