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A New Look at the Middle to Lower Cretaceous Stratigraphy,Offshore Kuwait

Abdul Aziz Al-FaresKuwait Oil Company (KSC)

Mark Bouman and Pete Jeans Shell International Exploration and Production B.V.

ABSTRACT

Offshore exploration in Kuwait commenced in 1961 with the award of a 5,600 squarekilometre offshore concession to Shell. Some 6,300 kilometres of 3-fold analogue seismicwere acquired in 1961, and 3 wells were drilled during 1962 and 1963. In the sameperiod, Kuwait Oil Company (KOC) also drilled their first 3 offshore exploration wells.In 1981, KOC embarked upon a second offshore exploration campaign, acquiring some6,000 kilometres of seismic data and, during 1983 and 1984, drilling two wells. None ofthese wells was a commercial discovery.

Between 1995 and 1997, an integrated team of KOC and Shell explorers undertook areview of the hydrocarbon potential of Offshore Kuwait. In order to establish anintegrated sequence-stratigraphical framework for the prospective Lower to MiddleCretaceous interval, a quantitative biostratigraphical study was made. Some 790biostratigraphical analyses (10% core samples; 90% cuttings) from eleven wells werecarried out: the nanno-fossil data was particularly important in providing accuratechronostratigraphical calibration, and this data has been used to constrain a “Time-Rock Synopsis”.

KOC’s lithostratigraphical nomenclature proved to be basically sound and has beenmaintained as the basis for the present stratigraphical framework. However, the studyrevealed the existence of two substantial and hitherto unsuspected hiati: one betweenthe Ratawi and Zubair formations of Early Valanginian to Mid-Hauterivian age; andthe other, representing the whole of the Early Albian, within the Burgan Formation.This latter result, if it can be further substantiated by more exhaustive study in theonshore area, would neccessitate a re-definition of the Burgan Formation and the erectionof a new formation to describe the clastic sequence of Late Aptian age which lies betweenthe Early Albian hiatus and the top of the Shu’aiba Formation, and which has hithertobeen included within the Lower Burgan Formation.

INTRODUCTION

On 27 September, 1995, a Joint-Study Agreement was signed between Shell International Explorationand Production B.V. and Kuwait Oil Company (KOC) whereby an integrated team of Shell and KOCgeologists and geophysicists was established in The Hague (The Netherlands) to undertake a review ofthe hydrocarbon potential of Offshore Kuwait, including Kuwait Bay and Bubiyan Island (Figure 1).

The results of the Study regarding plays and prospectivity of the Offshore area are still deemed to beconfidential by both parties and so will not be discussed here. This paper, however, presents the resultsof one aspect of the work, on the Cretaceous Stratigraphy of Offshore Kuwait, which has noconfidentiality implications and which, it is hoped, may be of interest to the wider community.

Prior to the discussion of the details of the work, overviews of the history of exploration of OffshoreKuwait, and of the tectono-stratigraphic evolution of Kuwait through the Cretaceous, will be presented.

The standard international sub-division of the Cretaceous comprises of an Upper and Lower Cretaceous,with the boundary being at the top of the Albian. However, in the Middle East a three-fold Cretaceous

GeoArabia, Vol. 3, No.4, 1998Gulf PetroLink, Bahrain

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sub-division has traditionally been used (Christian, 1997), with boundaries at the top of the Aptianand at the top of the Turonian, respectively. This practice reflects, far more usefully, the tectono-stratigraphic evolution of the area, and is maintained in this account.

HISTORY OF PREVIOUS EXPLORATION

A brief history of onshore exploration and an overview of the geology and oil fields of onshore Kuwaitis most recently provided by Carman (1996), whilst detailed accounts of the Burgan and Raudhatainfields are presented by Brennan (1990a, b). The first, and arguably still the best, discussion of theTertiary and Cretaceous stratigraphy of Kuwait was provided by Owen and Nasr (1958). Yousif andNouman (1997) provide the latest overview of the Jurassic stratigraphy of Kuwait, whilst Khan (1989)reviews the Permo-Triassic stratigraphy. An excellent list of additional references is provided by Carman(1996).

Offshore exploration in the Northern Gulf commenced in the mid-1950s in the Partitioned NeutralZone of Kuwait and Saudi Arabia, and the first offshore oil field was discovered in 1958 (Khafji: thenorthern continuation of Saudi Aramco’s Safaniya field). Possibly prompted by this and by the IPACconsortium’s exploratory activity in Iranian waters, offshore exploration in Kuwait commenced in1961 with the award of a 5,600 square kilometre (sq km) offshore concession to Shell, of which theeastern part of the area was effectively ‘off-limits’ due to boundary disputes between Kuwait, Iran,and Saudi Arabia.

In 1961 Shell shot 6,300 km of 3-fold analogue seismic, from which it became apparent that the ‘open’part of the concession comprised a gentle, north-easterly-dipping monocline, and the only three

Figure 1: Location map of Kuwait Offshore Study Area and location of wells from which sampleswere taken for biostratigraphic analyses. The locations of the cross-section in Figure 2, and theseismic line in Figure 9, are indicated.

Dorra

Lawhah

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Medina

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Figure 2: Geological cross-section, Kuwait Offshore and coastal area. For location see Figure 1.

Figure 3: Kuwait Offshore: Depth Map, Top Ahmadi Formation (in metres subsea).

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structures which could be identified were of very low relief, with mapped closures of less than 35metres (m). These were drilled between 1962 and 1963. The least discouraging results were obtainedfrom the first well drilled, which tested at an initial rate of 720 barrels of oil per day (bopd) of 38°-40°API oil from Lower Cretaceous Ratawi limestones. Unfortunately, production declined to only 103bopd after 5 days. Only minor oil and gas shows were noted in the other two wells. All three wellswere abandoned. In the same 1962-63 period, KOC also drilled their first offshore exploration wells, inthe Bubiyan Island-Kuwait Bay area.

In 1981, the then-nationalised KOC embarked upon a second offshore and shallow-water explorationcampaign, acquiring some 6,000 km of seismic data. The new seismic data was interpreted in the lightof the results of the three Shell wells, and a detailed review was made of the stratigraphy andhydrocarbon potential of Offshore Kuwait (Al-Kandari, 1981). Between 1983 and 1984, KOC drilledtwo wells: the first, a follow-up of the Shell non-commercial discovery, was a dry hole with onlyshows of oil; the second recovered a maximum of 420 bopd on test from the Minagish Formation.

Figure 4: Plate Wander Path (in red) for Kuwait, Late Ordovician to Recent. Numbers (in red)indicate age in million years before present. Green ‘G’ indicates timing of Paleozoic glaciationsand the red hash marks ‘/ / / /’ indicate timing of periods of major rifting or deformation. Shellinternal compilation.

Middle-Lower Cretaceous Stratigraphy, Kuwait

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STRUCTURAL OVERVIEW OF THE KUWAITI OFFSHORE AREA

As shown by the geological cross-section and the seismic depth map (Figures 2 and 3), the structure inOffshore Kuwait is dominated by a gentle regional dip to the north-east with no significant structurationintervening between the Burgan Arch to the west, and the Khafji-Nowruz Arch to the east.Consequently, all the offshore wells to date have been drilled on very low relief structures.

The most prominent offshore feature is the north-northeast-trending Khafji-Nowruz Arch, located inthe extreme east of the Kuwaiti Offshore. The structure plunges gently to the north-northeast; isasymmetric with a steeper western flank; and, in common with the Burgan Arch, the upper part of theMiddle Cretaceous sequence appears to be truncated below the Base Upper Cretaceous Unconformityover the structure. Structural growth is also apparent during Neogene times.

KUWAIT DURING THE CRETACEOUS: AN OVERVIEW

The geological history and stratigraphic succession of Kuwait has been determined by its location onthe north-eastern margin of the Afro-Arabian Plate, and by the motions and stresses (near and far-

Figure 5: Paleogeographic reconstruction, Late Cretaceous times (~85 Ma, approximating BaseUpper Cretaceous Unconformity time. Shell internal compilation).

E Q U AT O R

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field) to which that plate has been subject throughout its evolution. Hence this overview will attemptto place the key elements of Kuwait’s Cretaceous history in a global plate-tectonic context, as illustratedin Figures 4 and 5.

At the beginning of Cretaceous times, Kuwait was located just north of the Equator, and the large scalebasin configuration had just changed from one of a differentiated passive-margin of shallow shelvesand deeper, intra-shelf basins which characterized the Jurassic (Murris, 1980) to that of a very lowrelief passive-margin ramp setting, with the stable Arabian shelf passing northeastwards into the deeperwater realm of the Mesopotamian-Northern Gulf Basin. Although the location of the shelf-to-basintransition oscillated through time with successive eustatic sea-level changes, the overall ramparchitecture remained little changed in Offshore Kuwait throughout the Cretaceous.

Consistent with its equatorial location throughout the Cretaceous, and the low-relief nature of theArabian margin, the Cretaceous succession in Offshore Kuwait is dominated by carbonates, generallymud-supported (Figure 6). However, around 130 million years before present (Ma), the opening of theSouth Atlantic, and then, some 25 Ma later, the opening of the Central Atlantic Ocean, induced a majorchange in plate motion direction (Figure 4) with consequent impact, through far-field stresses, on theAfro-Arabian Plate. Uplift of the cratonic hinterland of Arabia resulted in the flooding of the passivemargin with extensive tongues of deltaic, shallow-marine sands, forming the Zubair and Burganreservoirs, of Barremian-Early Aptian and Mid-Late Albian age, respectively. Reduced clastic influx,

Figure 6: Traditional Lower - Middle Cretaceous lithostratigraphy of Kuwait, and the revisedstratigraphy described in this paper, with the eustatic sea-level curve in red (after Haq et al.,1988). Timescale is after Harland et al., 1990.

? ?

Rumaila

Wara

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"Unnamed Clastics"

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Hith

THIS PAPER

Mishrif

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TRADITIONAL

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coupled with rising sea-level, resulted in the re-establishment in Offshore Kuwait of cyclic carbonate-shale deposition through the balance of the Middle Cretaceous.

At approximately 90 Ma, there occurred a pause in the steady northwards progress of Kuwait, whichreflects the onset of ophiolite obduction and nappe emplacement at the leading edge of the ArabianPlate, the site of the present-day Zagros Suture. In Kuwait, this deformation reactivated older structuralfeatures, and generated the Base Upper Cretaceous Unconformity, which downcuts progressivelytowards the west and south, causing significant thinning and loss of section of the Middle Cretaceousover the major anticlines of coastal Kuwait. Growth, though more subdued, of these major structurescontinued throughout the Upper Cretaceous, which in Kuwait developed in a dominantly carbonateshelf setting, whilst to the east, in the Mesopotamian-Northern Gulf Basin, a marl and shale-dominatedforedeep developed.

The effectiveness of far-field stresses as a mechanism for generating tectono-stratigraphic responsesover large areas of craton is illustrated by a comparison between Arabia and Central Africa. Guiraudand Maurin (1992) describe two phases of intra-cratonic rifting from Central Africa which appear tobe co-eval with the Zubair and Burgan sand influxes of Arabia, and with the two-phase opening of theAtlantic. In the reverse direction, the Santonian obduction event on the Arabian margin is co-eval withthe inversion of many of the Cretaceous rifts in Central Africa (Guiraud and Maurin, 1992).

Figure 7: Kuwait Offshore: Depth Map, Top Rus Formation (in metres subsea.)

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Figure 8: Middle to Lower Cretaceous nanno-fossil zones and major nanno-fossil events, OffshoreKuwait (after Varol Research, 1996).

Formation

Ratawi Limestone

Mishrif

Rumaila

Ahmadi

Wara

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Burgan

'UnnamedClastics'

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Minagish

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L. pseudoquadratus

B. hanckokii, G. nanum

G. chiasta, G. praeobliquumRothia sp. (persistent), R. irregularis

N. fragilis

T. phacelosus P. cretacea

B. stenorhetha, B. bigelowi

G. theta, H. paleocaudiculus, P. constansO. partitum, R. achlyostaurion

R. undosus, O.perspicuum, A. albianus, B. enormis

N. circularis, L. houghtonii

M. obtusus, D. lehmanii

F. biforaminis

N. circularisW. manivitea

C. margerelii , C. margerelii grandis

N. steinmannii minor, N. kamptneri minor,D. rectus, N. quadratus

N. neocomianus

P. parvistellatus, P. maltica

P. senaria

C. mexicana minor

N. steinmannii minor, N. kamptneri minor

Frequent

Common

Abundant

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Late

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Early

Late

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Middle

Early

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Hauterivian

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Berriasian

Tithonian

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Tectonic quiesence and shallow-water carbonate deposition characterized the Paleogene of Kuwait.The onset, in Lower Miocene times, of the Zagros Orogeny, representing the final collision of Arabiaand Eurasia caused uplift and erosion in Kuwait, represented by the Top Dammam Unconformitywhich shows truncation and gentle arching over most of the major oil fields, whilst the succeedingdevelopment and infill of the Zagros foredeep generated the regional north-easterly dip which is sucha feature of the Offshore area today (Figure 7).

STRATIGRAPHY OF THE LOWER-MIDDLE CRETACEOUS SEQUENCE

Introduction

Given the low relief, monoclinal nature of much of Offshore Kuwait, it was felt that stratigraphictrapping would provide the only possibility of significant hydrocarbon entrapment within the Lower-Middle Cretaceous reservoirs (prolific producers in the Onshore fields). In order to explore forstratigraphic traps, a high-resolution sequence-stratigraphic framework is essential, building onquantitative biostratigraphical data in order to provide as detailed a chronostratigraphical and paleo-environmental calibration of this sequence as possible.

Over 500 samples were collected from eleven wells (highlighted in orange in Figure 1): most of thesesamples were split, and a total of some 790 biostratigraphic analyses were made, by Robertson ResearchInternational Ltd. (foraminifera), and Varol Research (nannoplankton). Approximately 90% of thesamples analyzed were cuttings. Previous analyses commissioned by KOC on ostracod distributionwere re-evaluated by Lacustrine Basin Research (LBR).

This study is the first to utilize nannoplankton analysis on Kuwait samples, and the data so obtainedwere crucial in providing accurate chronostratigraphical calibration, far more than would have beenobtained from the microfaunal data alone. A zonation scheme specific to Offshore Kuwait has beendeveloped (Figure 8). It utilizes a combination of global markers and local nanno-fossil events whichare consistently recognizable in the wells studied (Varol, 1996).

Results

The quantitative biostratigraphic data was plotted on a well-by-well basis and a well log and seismiccorrelation framework established (Figure 9). From this it was apparent that the parallel reflectiongeometry seen on the seismic data concealed no significant diachronicity in the investigated sequence,and that KOC’s lithostratigraphical nomenclature was sound. Hence it was retained as the basis forthe Offshore stratigraphical framework. A new Time-Rock Synopsis (Figure 10) was constructed. Theparallel nature of the formation boundaries reflects the lack of diachronicity and the parallel nature ofthe seismic reflections across the area.

Plotting of the biostratigraphic and well-data revealed the presence of two major disconformities/hiati, with no data representing the Late Valanginian-Early Hauterivian, or Latest Aptian-Early Albianbeing recorded. It was also apparent that the age and stratigraphic position of these gaps may havebeen incorrectly identified in the past, namely:

(1) that the regionally-identified Aptian-Albian disconformity does not occur at the Burgan-Shu’aibacontact, but within the lower part of the Burgan Formation, and that the basal shales of the BurganFormation lie below this hiatus, and are of Late Aptian age. The age control on the Lower Burgansands is poor, and the hiatus is assumed to lie just below the incoming of the massive BurganFourth sands. The presence of a major hiatus at this time is attributed to the significant change inplate motion that affected Arabia at ~110 Ma (Figure 4).

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WELL C SYNTHETIC SEISMOGRAM

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Figures 9a: Synthetic Seismogram for Well C, typical of Offshore Kuwait. The fine pink linesrepresent formation boundaries; the heavier blue lines correspond to seismic reflectors or othermarkers.

Middle-Lower Cretaceous Stratigraphy, Kuwait

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(2) that the disconformity separating the Zubair and Ratawi formations is of Early Valanginian-EarlyHauterivian age, and not co-incident with the Hauterivian-Barremian boundary as previouslysuggested. A hiatus of similar age is reported from southwest Iran by Shakib (in Simmons, 1994),and corresponds to a major global eustatic sea-level lowstand.

Summary of the Lower Cretaceous Stratigraphy

The Lower Cretaceous sequence of Kuwait ranges in thickness from 3,800 ft to 4,400 ft (Figure 11),becoming thicker to the north-northeast as the Mesopotamian trough is approached. The sequencecan be sub-divided into two cycles:

Makhul, Minagish and Ratawi FormationsThe sequence comprises a relatively featureless succession of mud-supported limestones and inter-bedded shales. The Makhul Formation is composed of dense grey-dark grey limestones and interbed-ded dark grey shales and averages 450 ft in thickness in the Offshore area. The succeeding MinagishFormation ranges in thickness from 800 ft in the south of the area to 1,200 ft in the north and comprisesa sequence of grey argillaceous lime mud-wackestones with interbedded green-grey calcareous shales.The oolitic grainstones that characterize this formation in Onshore Kuwait have not been encounteredin the Offshore. The Ratawi Formation is divided into two members, the Lower Ratawi Limestone andthe Upper Ratawi Shale, reflecting the proportion of lime mud-wackestone and calcareous shales andmarls in each unit, which average about 500 feet in thickness.

The environment of deposition based on ostracod analysis is overall inner shelf (i.e. low tide down to~40 m) with a conjectural 20-50 m water-depth towards the north (Lacustrine Basin Research, 1996).

The age of the sequence is ?latest Tithonian to Early Valanginian (Varol Research, 1996).

Zubair, Shu’aiba and “Unnamed Clastics” FormationsHigh global sea-level prevailed during Late Hauterivian to Late Aptian but despite this, uplift of theArabo-Nubian craton resulted in the influx of the Zubair delta from the west, inundating the entire

Figure 9b : Seismic line, Offshore Kuwait, illustrating themonoclinical nature of the offshore area between the Burgan andthe Khafji-Nowruz Arches (Figure 1). The key interpreted reflectionscorrespond to the Middle Cretaceous Ahmadi, Upper CretaceousTayarat and the Tertiary Rus formations. Blue vertical lines indicatebends in the seismic line (see inset map). The synthetic seismogramfor Well C is shown on Figure 9a.

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n

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er Middle LowerLowerUpperMiddle

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RA

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et a

l., 1

990)

Middle-Lower Cretaceous Stratigraphy, Kuwait

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Offshore area with an interbedded sequence of sands (fine-very fine grained) and shales. The sequencethins from ~1,350 ft to less than 1,000 ft, and the sand percentage decreases, in a northeasterly direction,from 50% near the southern coast to ~20% in the vicinity of Bubiyan Island (Figure 11).

Ostracod faunas indicate that the Zubair Formation was deposited on a broad, relatively shallowcontinental shelf with average water depths between 20 and 40 metres (Lacustrine Basin Research,1996). Nannoplankton ages for the Zubair range from Late Hauterivian to Early Aptian.

Figure 11: KuwaitOffshore, LowerCretaceous isopach mapin feet (black); andNet-to-Gross (i.e sandpercentage) contours forthe Zubair Formation(orange).

Figure 12: KuwaitOffshore, “UnnamedClastics” isopach mapin feet.

Bahrah

Sabiriyah

Kuwait City

4,40

0

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4,000

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Bahrah

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N

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100

Sabiriyah

0

Km

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Kuwait City

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Figure 13: RepresentativeLog for the OffshoreKuwait Lower-MiddleCretaceous sequence, andsuggested Type Log forthe ‘Unnamed Clastics’Formation (Well F).

NA

NN

O-

PL

AN

KTO

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BU

ND

AN

CECHRON

STRAT

DEPTH(ft)

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RIE

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SP-200 0 200

GAMMARAY0 150

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190 40

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?Ju Hith Hith

T.D. 14,622

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Shu'aiba

"UnnamedClastics"

Burgan

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Mauddud

Rumaila

Ahmadi

Mishrif8,000

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Middle-Lower Cretaceous Stratigraphy, Kuwait

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The Shu’aiba Formation (Early-Late Aptian in age) marked a temporary return to shallow marinecarbonate deposition, and it comprises a light grey-buff, lime wackestone, which is frequently fractured,locally vuggy (and hence a frequent lost circulation zone) and averages 300 ft in thickness.

At the close of Shu’aiba times, clastic sedimentation resumed through to the end of the Aptian. Thisshale sequence is considered to conformably overlie the Shuaiba (as observed by Owen and Nasr,1958), and has hitherto been referred to the lowermost part of the Burgan Formation. As a result of thisstudy, however, a major hiatus or disconformity has been identified seperating these shales from therest of the Burgan Formation, and hence it is felt that a new formation name is required (the informalterm ‘Unnamed Clastics Formation’ has been used in Study Team reports). This interval ranges inthickness from ~80 ft in Burgan field, to some 300 ft in the east of the Offshore (Figure 12), and wasdeposited in an open marine inner neritic environment (Robertson Research, 1996). The easternmostoffshore well has been selected as the proposed type log for the ‘Unnamed Clastics’, which wereencountered between 10,245 ft and 10,560 ft (depths below derrick floor) (Figure 13).

Summary of the Middle Cretaceous Stratigraphy

During the Middle Cretaceous, global sea level rose to its highest level in Mesozoic-Cenozoic history,reaching a maximum during the Cenomanian/Turonian. However, across the entire Arabian passivemargin, a renewed clastic influx occurred, spreading clastics across large parts of the basin. Twosedimentation cycles can generally be recognised during this period, both ranging from clastics at thebase to carbonates at the top. In Offshore Kuwait the lower cycle (Burgan and Mauddud formations)is readily distinguished. However the upper cycle (Wara, Ahmadi, Rumaila and Mishrif formations)in the offshore consists essentially of carbonates and shales only. The isopach of the Middle Cretaceoussequence (Figure 14) increases from 1,600 ft to 2,800 ft, though the ‘thin’ over the Burgan Arch is theresult of erosion below the base Upper Cretaceous unconformity.

Burgan and Mauddud FormationsAfter the deposition of the Zubair and Shu’aiba formations and the (as yet unnamed) basal shale ofthe Burgan Formation, the Middle Cretaceous commences with a newly-recognised hiatus ofapproximately six (to possibly 11) million years duration. (Due to age dating problems within theBurgan sandstones (only ditch cuttings were used) no firm conclusions can yet be drawn regardingthe precise duration of this hiatus).

Figure 14: Kuwait Offshore, MiddleCretaceous isopach map in feet,(black); and Net-to-Gross (sandpercentage) contours for the BurganFormation (orange).

Bahrah

Burgan

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Uplift of the cratonic source areas to the west caused a renewed influx of sands over large parts of thebasin in the early Middle Albian. The Burgan Formation is characterised by thick deltaic sands in thewest, thinning somewhat to an average thickness of 1,100 ft in the Offshore (including the basal shales).Towards the east the sand percentage diminishes but not so dramatically as in the Zubair, such that anet-to-gross of ~45% is still apparent in the easternmost well of Offshore Kuwait (Figure 14). Carbonatesonlapped in a westerly direction and progressively displaced the clastics westward into interior Kuwait,resulting in the deposition of the Upper Albian Mauddud carbonates. The Mauddud Formation showsa gradual overall thickening towards the east and northeast, ranging from the depositional feather-edge in the Minagish area to some 425 ft in the Bubiyan area.

Wara, Ahmadi, Rumaila and Mishrif FormationsFollowing the deposition of the thick Albian Burgan-Mauddud sequence a number of relatively short-duration carbonate-shale cycles comprise the balance of the Middle Cretaceous.

The original depositional thickness of these intervals gradually increases towards the northeast, butthis pattern is highly modified by the syn-depositional thinning, and end-Middle Cretaceous erosion,over the Burgan and Khafji-Nowruz Arches, from which the Rumaila and the Mishrif formations havebeen completely removed.

The lithologies in the offshore area are relatively uniform and are characterised by alternations of(“highstand”) carbonates (i.e. mainly wackestones) and (“transgressive”) shales. Water depths basedon ostracods consistently range from ~20 m (shallow inner shelf) to a possible maximum of ~70 m(middle shelf) during deposition of the Ahmadi shale (Lacustrine Basin Research, 1996).

CONCLUSIONS

The principal conclusions from this study are as follows:

• There is no significant diachronicity apparent in the Lower and Middle Cretaceous sequencesacross Offshore Kuwait. The layer-cake appearance of the Time-Rock Synopsis mirrors the parallelnature of the seismic reflectors across the area.

• Nanno-fossils have the potential to provide a far more detailed sub-division of the Cretaceousof Kuwait than has hitherto been available.

• A significant hiatus or disconformity is present across the Aptian-Albian boundary, but it isthought to lie, not at the Shu’aiba-Burgan contact, but within the Burgan at the contact of thebasal shales (now thought to be of Late Aptian age) with the base of the Burgan Main or Fourthsand (of ?Mid-Albian age).

• A second major hiatus or disconformity is thought to lie at the contact of the Ratawi Shale (nowthought to be no younger than Early Valanginian) with the base of the Zubair Formation (ofLate Hauterivian age).

The majority of the samples studied were cuttings, and hence the above results should be regarded asprovisional or indicative until they can be substantiated by more detailed examinations based oncores and sidewall samples. If the assignment of the basal Burgan shales to the Late Aptian as indicatedhere can be substantiated by more detailed work, then it is recommended that this sequence be elevatedto formation status and a formal name, preferably local, be assigned.

Middle-Lower Cretaceous Stratigraphy, Kuwait

559

ACKNOWLEDGEMENTS

The authors wish to thank Kuwait Oil Company and Shell International for permission to publish theresults of this study. The authors also wish to thank the anonymous reviewers and Gulf PetroLinkstaff for redrafting some of the figures.

REFERENCES

Al-Kandari, A. 1981. Kuwait Offshore Geology. Kuwait Oil Company, Internal Report.

Brennan, P. 1990a. Raudhatain Field, Kuwait, Arabian Basin. American Association of PetroleumGeologists Atlas of Oil and Gas Fields, Structural Traps, 1, p. 187-210.

Brennan, P. 1990b. Greater Burgan Field, Kuwait, Arabian Basin. American Association of PetroleumGeologists Atlas of Oil and Gas Fields, Structural Traps, 1, p. 103-128.

Carman, G.J. 1996. Structural Elements of Onshore Kuwait. GeoArabia, v. 1, n. 2, p. 239-266.

Christian, L. 1997. Cretaceous Subsurface Geology of the Middle East Region. GeoArabia, v. 2, n. 3, p. 239-256.

Guiraud, R. and J.C Maurin 1992. Early Cretaceous Rifts in West and Central Africa. Tectonophysics, 213,p. 153-168.

Haq, B.U., J. Hardenbol and P.R. Vail 1988. Mesozoic and Cainozoic Chronostratigraphy and Cycles of SeaLevel Change. In C.K. Wilgus et al. (Eds.), Sea Level Changes, An Integrated Approach. Society ofEconomic Paleontologists and Mineralogists Special Publication no. 42, p. 71-108.

Harland, W.B., R.L. Armstrong, A.V. Cox, L.E. Craig, A.G. Smith and D.G. Smith 1990. A GeologicalTimescale. Cambridge University Press, Cambridge.

Khan, A. 1989. Stratigraphy and Hydrocarbon Potential of the Permo-Triassic Sequence of Rocks in the Stateof Kuwait. Organization of Arab Petroleum Exporting Countries Seminar on Hydrocarbon Potentialof Deep Formations in Arab Countries, Abu Dhabi, 1989, p. 3-29.

Lacustrine Basin Research 1996. Environmental Interpretation of Ostracod Faunas from Wells Onshore/Offshore Kuwait. Kuwait Joint Study Team Internal Report.

Murris, R.J. 1980. Middle East Stratigraphic Evolution and Oil Habitat. American Association PetroleumGeologists Bulletin, v. 65, no. 5, p. 597-618.

Owen, R.M.S. and S.N. Nasr 1958. Stratigraphy of the Kuwait-Basra Area. In ‘Habitat of Oil’ AmericanAssociation Petroleum Geologist Memoir 1, p. 1252-1278.

Robertson Research International 1996. Mid and Lower Cretaceous, Coastal/Offshore Kuwait;Lithostratigraphy, Biostratigraphy and Palaeoenvironments of 7 wells. Kuwait Joint Study Team InternalReport.

Simmons, M.D. 1994 (Ed.) Micropalaeontology and Hydrocarbon Exploration in the Middle East. Chapmanand Hall, London.

Varol Research 1996. Report on the Stratigraphical Implications of Nannoplankton Analyses on Eleven Wells.KJST Internal Report.

Yousif, S. and G. Nouman 1997. Jurassic Geology of Kuwait. GeoArabia, v. 2, n. 1, p. 91-110.

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Paper presented at 3rd Middle East Geosciences Conference and Exhibition,GEO’98, Bahrain, 20-22 April, 1998

Manuscript received 23 June 1998

Revised 27October 1998

Accepted 5 November 1998

ABOUT THE AUTHORS

Mark Bouman gained a MSc in Stratigraphy from theUniversity of Utrecht, and joined Shell in 1982. Followingassignments in Peru, London, and Cairo, he returned to TheHague in 1995 and was Senior Stratigrapher in the Kuwait JointStudy Team. Mark is currently Course Director for Geosciencesat the Shell Learning and Development Centre, Noordwijkerhoud,in The Netherlands. His areas of expertise include sequencestratigraphy, basin analysis, and stratigraphical computing.

Abdul Aziz Al-Fares graduated from Kuwait University witha BSc degree in Geology in 1992, and in the same year he joinedKuwait Oil Company as a Wellsite Geologist. In 1994 AbdulAziz was assigned to work as a Geophysicist at KOC, and fromNovember 1995 to December 1996 he was a member of the KOC/Shell Joint Study Team. He is currently working as a 3-D SeismicInterpreter.

Pete Jeans is currently a Regional Business Advisor with ShellEP International Ventures’ New Business Development Group.Prior to this, he was Senior Geologist and then Project Leader ofthe Kuwait Joint Study Team. Pete graduated with a PhD inGeology from Birmingham University in 1973, and worked inOman, Jakarta, Houston, and Brunei before returning to TheHague in 1989. His particular interest is prospect and playgeneration.