Dive into the Frontier Dive into the Frontier

Dive into the Frontier Dive into the Frontier

Regional Controls on Siliciclastic Input into Mesozoic Depositional Systems of the Arabian Plate and their Petroleum Significance by Roger Davies, Mike Simmons, Thomas Jewell and Joanne Wyton elements of petroleum plays. The world’s second largest oilfield, Burgan in Kuwait, and the world’s largest offshore siliciclastic oilfield, SafaniyahKhafji, shared by Saudi Arabia and the Partitioned Neutral Zone, produce from Middle Cretaceous sandstone reservoirs, as do the supergiant Rumaila and Zubair fields of southern Iraq. Key siliciclastic seals include the regional shales of the Triassic Sudair Formation and the Cretaceous Gadvan, Nahr Umr and Ahmadi formations; whereas, major siliciclastic-dominated source rocks include

MIDDLE EAST STRATIGRAPHY – LOOKING BEYOND THE DOMINANCE OF CARBONATES

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For many petroleum geologists, the Mesozoic succession of the Arabian Plate is instinctively associated with carbonate sediments that form the most significant hydrocarbon reservoirs in the world, not least the Jurassic Arab Formation of Saudi Arabia, Qatar and Abu Dhabi. However, during the Mesozoic Era, episodes of siliciclastic input onto the dominantly carbonate Arabian shelf form important SAUDI ARABIA Maastrichtian

LATE

»

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Santonian Coniacian Turonian

CRETACEOUS

»

Aruma

Continental sediments Campanian

Cenomanian

Albian

Ilam Ilam

Tanuma

Fine-grained deep-water siliciclastics (marine)

Mishrif Ahmadi Wara

Shallow-water carbonates

Sarvak Kazhdumi

Kazhdumi

Salina/saltern evaporites Shuaiba

Dariyan Gadvan

Barremian

Biyadh

Hauterivian

Garau

Zubair

K150

Lowering

K110

Ratawi

K90

JURASSIC

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Regional uplift

K70 K60 K50

Levantine extension

Hanifa

Dhruma

Aalenian

Lowering J80-100 J70 J60

Gondwana seperation

J50

Surmeh

Sargelu Alan

K30 K20 J110

Najmah

Tuwaiq Mtn Lst

J40

Upper Marrat

Cooling Humid

J30 J20

Lowering Toarcian

EARLY

Marrat

Marrat

Ocean Anoxic Event Humid Cooling

J10

Pliensbachian

Marrat

Neyriz Lowering

Sinemurian

East Mediterranean rifting

Hettangian Rhaetian

Humid

Minjur Baluti

TRIASSIC

Humid Cooling

K10

Makhul Hith Gotnia

Hith Arab Jubaila

D7 D6

Fahliyan

Cooling

K80

K40

Yamama Minagish

Sulaiy

Oxfordian

MIDDLE

Humid

K100

Lowering

Kimmeridgian

Callovian Bathonian Bajocian

Regional uplift

K120

Valanginian Berriasian

Climate

Syrian Arc event

K160

Lowering

Shuaiba

Tectonics North Arabia rifting and inversion

K180

K140

Nahr Umr

Burgan

Major eustatic events

K130

Mauddud

Wasia

Deep-water carbonates

C O N T R O L L I N G FA C T O R S MFS

K170

Sadi

Fine-grained shallow-water siliciclastics (marine)

Aptian

LATE

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Gurpi Hartha

EARLY

Predict stratigraphic fill and generate models for the organisation of systems tracts using our high-resolution, temporally calibrated, sequence stratigraphic model. Map sediment pathways, analyse compositional risk and characterise potential reservoir and source rocks using our unique paleoclimate model.

ZAGROS

Tayarat

Coarse-grained shallow-water siliciclastics

Tithonian

»

KUWAIT/ARABIAN GULF

LATE

Humid

Lowering East Mediterranean/ Palmyride rifting

D Sefidar Dolomite C B

Jilh

Ladinian

EARLY

Tr80

Norian

Carnian

MIDDLE

Butmah

A

Anisian Olenekian

Sudair

Induan

Khuff

Khuff

Aghar Shale Kangan

Tr70

Dashtak

Tr60

Lowering

Humid

Lowering

Humid

Tr50 Tr40 Tr30 Tr10-20

Humid Humid

Lowering Palmyride rifting

Figure 1: A simplified chronostratigraphic chart for the Mesozoic of the central part of the Arabian Plate, highlighting episodes of significant siliciclastic influx. Tectonic, climatic and eustatic events are also highlighted.

Super-Greenhouse

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Geologic Age

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the Middle Cretaceous Kazhdumi Formation. It is, therefore, apparent that large volumes of siliciclastic sediments were periodically input onto the dominantly carbonate Mesozoic shelf of the Arabian Plate and have petroleum significance as reservoirs, seals and source rocks.

Siliciclastic Influence

Mid Turonian–Maastrichtian

minor

Late Cenomanian–early Turonian

weak

Late Albian–middle Cenomanian

moderate

Latest Aptian–middle Albian

strong

Early Aptian

weak

Rhaetian–Aalenian

weak

Late Norian

strong

Anisian–middle Norian

weak

This review examines the temporal and spatial extent of these siliciclastic episodes. It then compares them against known tectonic, climatic and eustatic events affecting the Arabian Plate that may have been acting independently or coincidently to control siliciclastic input by means of hinterland uplift, influence on denudation and run off, incision and creation of sediment pathways and accommodation space. Understanding the controls on siliciclastic occurrence can be insightful to predict their distribution and significance as key elements of petroleum systems. Moreover, understanding the regional controls on siliciclastic input can lead to insights for new exploration plays, for example, sandstone-rich lowstand shelves and turbidites.

Olenekian

strong

STRATIGRAPHIC ORGANISATION

Induan

weak

Late Valanginian–Barremian

strong

Berriasian–early Valanginian

moderate

Late Kimmeridgia –Tithonian

weak

Mid Kimmeridgian

minor

Late Oxfordian–early Kimmeridgian

weak

Early Oxfordian Callovian Late Bathonian Late Bajocian–early Bathonian Early Bajocian

moderate weak moderate weak moderate

Regionally extensive clay-rich marls and shales can be traced across much of the Arabian Plate

Table 1: Episodes of Siliciclastic Influence on Mesozoic Arabian Plate Stratigraphy.

CLIMATE  RAINFALL INTENSITY/SEASONALITY

WEATHERING SLOPE PROFILE RUNOFF

LITHOLOGY VARIATIONS TECTONICS  UPLIFT V.S. SUBSIDENCE

TIDES/LONGSHORE DRIFT

cla

ici

Sil

s

c sti

s ate

n rbo

Ca

EUSTASY

©E

x

r plo

at i

o

ns nI

igh

ts

(Sharland et al., 2001; van Buchem et al., 2011). In the eastern part of the plate, they form low permeability barriers that partition the carbonate reservoirs (Strohmenger et al., 2006); but moving westward toward the sediment source, sandstone content increases and stratigraphic trapping reservoir potential exists (e.g., recent discoveries in the Zubair Formation of Kuwait (Tanoli et al., 2011) and Dhruma Formation of Saudi Arabia (Stewart et al., 2016)). The sedimentary record of the interior of the shelf is recorded by the succession preserved in the outcrop belt, extending from Saudi Arabia through Jordan into Sinai. In these locations, the Mesozoic succession is rich in siliciclastic sediments, suggesting that erosion of the Arabian Shield and its dominantly siliciclastic Paleozoic sedimentary cover occurred throughout much of the Mesozoic. What is interesting to document are the times when siliciclastic-dominated sedimentary belts extended onto the greater part of the shelf, including into any intra-shelf basins. Figure 1 provides a regional chronostratigraphic chart (significantly modified from Sharland et al., 2001, but using their sequence nomenclature), extending from the interior of the shelf in Saudi Arabia to the Neotethys margin in the Zagros, to help explain the timing of major pulses of siliciclastic sedimentation and forms the basis for the discussions that follow. Additionally, Table 1 summarises the nature of the episodes of siliciclastic influence. FUNDAMENTAL CONTROLS OF CARBONATE VERSUS SILICICLASTIC DEPOSITION The near-equatorial position of the Mesozoic Arabian Plate favoured the deposition of carbonates. However, although the topography of the hinterland to this shelf is poorly known, there were most likely highlands forming part of a “Variscan Range,” running from modernday southern Europe to Equatorial Africa and composed of Pre-Cambrian granitic basement (the Arabian Shield) and quartzose Paleozoic sedimentary cover that could be a ready source of siliciclastic sediment. Given the proximity of this highland hinterland, the controls on carbonate versus siliciclastic deposition are, thus, a complex interplay between tectonics, climate and eustasy, as schematically shown in Figure 2. Tectonics

Figure 2: Cartoon illustrating the interplay of factors influencing siliciclastic versus carbonate deposition of a sedimentary shelf.

Although tectonically-driven subsidence is a factor in providing accommodation space for sediments to accumulate, the net uplift of the siliciclastic

sediment source area is probably more important. This is because net uplift exposes basement rock types and/or older sedimentary rocks to weathering and erosion; it is influential in creating the slope profile, which in turn will influence sediment transport rate and quantity to the shelf. It is important to remember the thickness of the Mesozoic section dominated by carbonates that overlie the potential sources of siliciclastic sediment. Late Permian to Jurassic well sections in the Hawtah area close to the Arabian Shield are approximately 2,000 m thick and dominated by carbonates with subordinate siliciclastics and evaporites (Al-Aslani, 1994). The volume of Cretaceous sandstones in the Zubair, Burgan and equivalent formations seems much greater than that present in the Triassic and Jurassic succession in proximal locations in Saudi Arabia (Le Nindre et al., 1990) and unlikely to be supplied by reworking of Triassic and/or Jurassic sandstones. Allied to the chemical maturity of the Cretaceous sandstones (Al-Eidan et al., 2001), together these observations support the proposal that reworked Paleozoic sandstones were the most likely provenance for the Cretaceous sandstones. Table 2 highlights some of the main tectonic events to have influenced the Arabia Plate during the Mesozoic. Geologic Age

Tectonic Event

Early Triassic

Palmyride rifting

Norian

East Mediterranean/Palmyride rift related uplift

Early Jurassic

East Mediterranean rifting and associated uplift

Oxfordian

Separation of India from Arabia with development of Yemen rift and Arabian intrashelf basins; potential for riftrelated uplift

Early Cretaceous

Extension on Levantine margin, mantle plume or slab pull-related; substantial uplift in northern and western Arabia

Late Aptian

Uplift and fracturing in the southern and eastern part of the plate of uncertain origin (obduction related?)

Turonian

Uplift and fracturing in the southern and eastern part of the plate in relation to the initiation of obduction

Late Santonian

Syrian arc event; inversion of structures across Arabia and ophiolite obduction

Campanian– Maastrichtian

Post-inversion north Arabia rifting allied with ongoing uplift along compressional northeastern and northern plate margins and slab-pull effects from subducting margin

Table 2: Main Mesozoic Tectonic Events Affecting the Arabian Plate.


Cretaceous

Timing of siliciclastic input Campanian-Maastrichtian (“Amiran-Tanjero”) Mid Turonian-Coniacian (“Tuwayil-Surgeh”) Late Albian-Middle Cenomanian (“Ahmadi-Wara”) Latest Aptian-Middle Albian (“Burgan”) Late Valanginian-Barremian (“Zubair”)

Jurassic

Berriasian-Early Valanginian (“Ratawi”) Mid Kimmeridgian


Arid climate indicators on the Arabian Plate

Coals and laterities in Yemen and Oman Evaporites in Kifl Formation of southern Iraq Detrital kaolinite in Khasib, Tanuma and Sadi formations of Iraq Evaporites in Jawan Formation of northern Coals in Wasia Formation of Saudi Arabia and Iraq Wara Formation of southern Kuwait. Amber in upper Wara sandstones of Burgan Az Zabirah bauxite in Saudi Arabia. Amber in Burgan and Nahr Umr formations as well as ageequivalent sediments along Levant margin Detrital kaolinite and amber in Zubair Formation of southern Iraq and Kuwait. Coals, lignite and kaolinitic clays from Israel and Lebanon.

Evaporites of the Arab, Hith, Gotnia and Basarin formations

Early Oxfordian (“Kidod”) Late Bathonian (“Upper Dhruma”) Early Bajocian (“Lower Dhruma”)

Coals in the Dhruma Formation. Uncertain relationship of coals in Early Bathonian of Sinai Coals in the Dhruma Formation. Uncertain relationship of coals in Early Bathonian of Sinai Possible thin coals in Minjur Formation. Major fluvial drainiage reporesented by Mulassa ‘F’ Formation in Euphrates Graben

Late Norian (“Minjur”) Triassic

Humid climate indicators on the Arabian Plate

Olenkian (“Sudair”)

Anhydrite beds in Sudair Formation and basal sections of Geli Khana and Dashtak formations

Table 3: Empirical Evidence of Climatic Variation Affecting the Arabian Plate During Periods of Significant Siliciclastic Input.

Climate Climate is intimately linked to tectonics at both global and local scales. Plate movements and geodynamic reorganisation can profoundly affect the areal extent and placement of climatic belts; whereas, local tectonics can lead to orographic effects. Aridity versus humidity is a particularly important climatic factor because it can control rainfall intensity and duration; thereby, influencing weathering, run-off and vegetation. Given the frequency of carbonate and evaporite sediments in the Mesozoic succession of the Arabian Plate, it is tempting to regard the climate as being persistently hot and arid. Nonetheless, with a near-equatorial position during the Mesozoic, the Arabian Plate was in the area affected by the Inter-Tropical Convergence Zone and would have experienced a monsoonal climate with potentially strong seasonal rainfall. Climates may have often been humid, rather than arid, but undoubtedly always hot (mean annual temperatures typically >35°C). The climate models of Sellwood and Valdes (2007) demonstrate this effectively. Tectonic uplift would have generated some local climate effects, especially the creation of a dominantly onshore wind direction. These are, however, difficult to disentangle from other factors, especially in the absence of high-resolution climate models. With this in mind, changes in global climate

are important because they may drive changes in humidity versus aridity. Although the Mesozoic is often regarded as simply a time of a greenhouse Event

Authors Cited

Late Olenekian

Recognized by Embry (1997), Ogg (2012). Regression R1 of Gianolla and Jacquin (1998). Recognized by Embry (1997), Ogg (2012), Simmons et al. (2007). Regression R2 of Gianolla and Jacquin (1998). Recognized by Simmons et al. (2007), Ogg (2012). Regression R3a of Gianolla and Jacquin (1998). Recognized by Embry (1997), Ogg (2012). Regression R3b of Gianolla and Jacquin (1998). Recognized by Ogg and Hinnov (2012), Simmons et al. (2007). Regression R4 of Jacquin et al. (1998). Recognized by Ogg and Hinnov (2012), Simmons et al. (2007). Regression R6 of Jacquin et al. (1998) Recognized by Ogg and Hinnov (2012), Simmons et al. (2007). Regression R9 of Jacquin et al. (1998) Recognized by Ogg and Hinnov (2012), Simmons et al. (2007), Haq (2014). Regression R11 of Jacquin et al. (1998). Recognized by Zorina (2014), Haq (2014), Ogg and Hinnov (2012), Simmons et al. (2007). Regression R13 of Jacquin et al. (1998). Recognized by Haq (2014), Ogg and Hinnov (2012), Simmons et al. (2007).

End Ladinian Mid-Carnian End Norian Late Sinemurian Late Toarcian– Aalenian Late Kimmeridgian– early Tithonian Early Valanginian Late Aptian

Mid-Turonian

Table 4: Major Events of Mesozoic Eustatic Sea Level Lowering and Agreement between Authors.

climate state, increasingly workers acknowledge that conditions were not uniform, even during the Cretaceous “greenhouse” period (e.g., Sames et al., 2016), and growing evidence (see review in Simmons, 2012) indicates that phases occurred in which significant polar ice caps developed and/or global climate cooled significantly. Conversely, there are periods best described as “hothouse” episodes, with extreme global warming during times of super-elevated atmospheric pCO2. Empirical evidence for paleoclimate, such as climate sensitive lithologies (e.g., coals and evaporite minerals), fossil plants, palynology and fossil soils (e.g., bauxitic laterites), provides insight into Mesozoic climate variation across Arabia. Unfortunately, the use of reported paleoclimate proxy data is often limited by poor age control. Nonetheless, there are reports of bauxites, laterites and coals from Arabia (Table 3). Eustasy Eustasy is the final factor to be considered, as it significantly contributes to relative sea-level change, especially on time scales of a few million years or less, and because this process occurs more rapidly than many tectonic events. Sea-level lowering can lead to river systems incising and potentially maintaining or increasing sediment supply. It will lead to progradation of marine sedimentary systems (of deltas, for example) and ultimately, in the case of forced regressions, to exposure of the shelf and Timing of siliciclastic input

deposition basinward of the former shelf break. Conversely, sea-level rise will cause sedimentary systems to retrograde, change sedimentary distribution points into estuaries and overall reduce external sediment supply into the basin. Table 4 lists major events of Mesozoic eustatic sealevel lowering for which there is some agreement between authors. EXPLORING CAUSALITY The combination of key events is important because they can amplify the external sediment supply. The coincidence of tectonic uplift with humid, at least seasonally wet, climates and eustatic sea-level fall is particularly interesting. The times of this coincidence would, on first principals, be likely to create potentially significant siliciclastic input. It is instructive to compare the timing of the major Mesozoic episodes of siliciclastic input onto the Arabian Plate with the major tectonic, climatic or eustatic events, as documented in Table 5. The most extensive episodes of siliciclastic input correspond to the synchronous combination of tectonic uplift of the hinterland, particularly humid/seasonally wet climates and major eustatic falls. These factors will have acted in concert to drive clastic sediment onto the Arabian shelf. More moderate episodes of siliciclastic input are related to perhaps more localized factors, such as tectonics, or remain enigmatic in their origins.

Degree of siliciclastic influence

Tectonics

Campanian-Maastrichtian (“Amiran-Tanjero”)

Minor

Compressional uplift along northern and north-eastern plate margins

Mid Turonian-Coniacian (“Tuwayil-Surgeh”)

Minor

Obduction

Late Albian-Middle Cenomanian (“Ahmadi-Wara”)

Moderate

Climate

Eustasy

Local humid climate proxies?

Major eustatic fall?


Eustatic fall

Latest Aptian-Middle Albian (“Burgan”)

Strong

Late Aptian rifting


Major eustatic fall

Late Valanginian-Barremian (“Zubair”)

Strong

Levantine Margin extension; uplift in Local humid northern and western Arabia climate proxies?

Major eustatic fall

Berriasian-Early Valanginian (“Ratawi”)

Moderate

Levantine Margin extension; uplift in northern and western Arabia

Eustatic fall

Mid Kimmeridgian

Minor

Possible localised differential uplift (e.g. eastern UAE/Oman and Fars province)

Early Oxfordian (“Kidod”)

Moderate

India - Arabia separation

Warming/humidity

Late Bathonian (“Upper Dhruma”)

Moderate

Early Bajocian (“Lower Dhruma”)

Moderate



Immediately postdates eustatic fall

Late Norian (“Minjur”)

Strong


Humid episode

Major eustatic fall

Olenkian (“Sudair”)

Strong

Palmyride rifting

Humid episode

Major eustatic fall


Table 5: Episodes of Major Siliciclastic Input onto the Arabian Plate and Possible Connecting Causes.



2650

600

Hith

Meters

0

Legend v v v

Shallow-marine carbonates with evaporites

Organic-rich sediments

Shallow-marine shales

Evaporites

Deep-marine shales

The advance of the most significant siliciclastic events appears to have been rapid. High net-to-gross ratio sandstone packages, such as the Zubair and Burgan formations, contain thick, sharply based fluvialdominated sandstones, even when they overlie shaledominated packages and form sandstone-dominated units up to 400 m thick (Figure 3). Although evidence of cold climates and associated high amplitude sea level fluctuations exist that are coincident with the rapid onset of these sandstones (Bodin et al., 2015), eustasy alone seems unlikely to account for such large volumes of sandstone because the periods of highest sandstone supply coincide with periods of active tectonism. In our view, this implies that hinterland uplift, probably related to extension along the Levantine Margin and associated uplift in northern and western Arabia (Table 5), was very important in generating provenance areas.

Siliciclastic rocks form important seals on the Arabian Plate. They can be classified into three categories:

150

Shuaiba

KN35

K100 SB

200

Shuaiba

K90

3150

3200

Meters

0

K9

250

MFS

0S

B

100

200

(b) Transgressive shales. These sediments represent the retreat of siliciclastic systems under the influence of sea-level rise and hinterland denudation. They form the ultimate top seals to the main sandstone reservoirs of the Middle East. In proximal locations, where the Zubair or Burgan/Nahr Umr formations have very high net-to-gross ratio, the transgressive shales at the top of the formation are the only effective seal. These uppermost shales are overlain by regional carbonates (e.g., Shu’aiba, Mauddud) that represent continued transgression. Transgressive shale seals are also important above some regional

350

450

Legend

200

Fluviatile

(a) Progradational shales and marls. These sediments characterise highstand deposition, where the advancing siliciclastic system “poisoned” carbonate deposition in an existing platform. Marls include the Upper Minagish Member in Kuwait. The Ratawi Shale is a prominent seal for the Yamama Formation reservoirs in southern Iraq, and locally contains its own sandstone reservoirs.

300

400

100

Figure 3: Regional Valanginian to early Aptian sequence stratigraphic correlation depicting the contrast between the fluvio-deltaic siliciclastic deposition in onshore to offshore Kuwait and the carbonate-dominated deposition over the Qatar Arch and in offshore Abu Dhabi. In Kuwait, a marked change occurs from proximal sandstone-dominated to more distal shale-dominated settings. Maximum flooding surfaces (MFS) are picked in regional shales. These shales provide important intraformational seals for stacked reservoirs in the largest Zubair Formation fields. The ultimate top seal for the Zubair Formation reservoirs is provided by the transgressive shale at the top of the formation. Pronounced thinning over the Qatar Arch identifies this area as an active high. In Qatar and the UAE, the siliciclastic component derived from western parts of the Arabian Plate is represented by thin transgressive shales and marls, respectively. This cyclicity is important in partitioning the Thamama Group reservoirs. Wireline log for Hamuur-1 is spontaneous potential. All other wireline logs are gamma rays (data modified after Tanoli et al., 2011; Al Fares et al., 1998; van Buchem et al., 2014; Ishibashi, 1997). Red lines = interpreted 3rd order sequence boundaries; blue lines = interpreted 3rd order maximum flooding surfaces.

Seals

2250

Deep-marine carbonates

Deep-marine conglomerates

PETROLEUM SIGNIFICANCE

100

2300 3100

Location Map

Shallow-marine carbonates

PAZANAN

2200

Dariyan

Fluviatile Undifferentiated fluvio - deltaic to shallow-marine sandstones Deep-marine sandstones

3050

2750

Location Map

Dair

Nahr Umr

2700

3000

Sarvak

2600

Kazhdumi

S

ONSHORE IRAN

2150

Late Cretaceous

550

MF

K100 MFS

Early Cretaceous

500

2950

NOWROOZ

Burgan

K40

2550

Burgan

450

OFFSHORE IRAN

Cretaceous

Aptian

Albian

Nahr Umr

Shuaiba

Albian

K50 MFS

Indeterminate identification

2900

2500 400

Thamama

Indeterminate identification

K60 MFS

Aptian

Ratawi

Valanginian

450 Ratawi

Ratawi

K40 SB 450

3700

2850

KN41 - KN40

3650 400

400

2800

2400

"Unnamed Clastics"

K50 SB

0

K4

3600

Mauddud

Early Cretaceous Albian

350 350

FS

M

HAMUUR_1_WELL_F

2350

2450 350

Early Cretaceous Hauterivian-Barremian

300

KN49 - KN47

3550

Hauterivian

300

3500

K70 MFS

300

Hawar

KN49 KN48

250

K50 MFS

250

KN51 KN50B

250

K6

Zubair

B 0S

Early Cretaceous

Zubair

Early Cretaceous

200

3450

Zubair

Early Cretaceous Barremian

150

200

Aptian

MFS

Kharaib

3400

K60

OFFSHORE KUWAIT

GHASHA_WMB_1

Ratawi

3350

100 100

150

ABU DHABI

Yamama

B K70 S

KN41 KN40 3300

Early Cretaceous Barremian Hauterivian - Berriasian

50

Aptian

50

KN44 - KN42

Shuaiba

Shuaiba

Shuaiba

0

QATAR CRETACEOUS_OFFSHORE_N_QATAR

HAMUUR_1_WELL_F

Shuaiba

OFFSHORE KUWAIT RA

Sulaiy

ONSHORE KUWAIT MU

500

Shallow-marine carbonates

Undifferentiated fluvio - deltaic to shallow-marine sandstones Deep-marine sandstones

v v v

Shallow-marine carbonates with evaporites Deep-marine carbonates

Deep-marine conglomerates

Organic-rich sediments

Shallow-marine shales

Evaporites

Deep-marine shales

Figure 4: Regional late Aptian to Albian sequence stratigraphic correlation depicting the contrast between the fluvio-deltaic sandstonedominated deposition in offshore Kuwait, mixed siliciclastic and carbonate deposition over the Safaniyah-Nowrooz-Hendijan Arch and organicrich deposition in the Kazhdumi intrashelf basin. Biostratigraphic data is from Al Fares et al. (1998). The Safaniyah-Nowrooz-Hendijan Arch appears to have been an active positive structure that influenced deposition. Red lines = interpreted 3rd order sequence boundaries; blue lines = interpreted 3rd order maximum flooding surfaces; and green lines = interpreted 3rd order maximum regression surfaces.

unconformities, notably the Laffan Formation seal to the Mishrif Formation reservoirs in the southern Arabian Gulf. (c) Shales associated with maximum flooding surfaces. These sediments form the most prominent intraformational seals in the Zubair and Burgan/Nahr Umr formations. They record maximum retreat of the siliciclastic system between two regressive pulses. The siliciclastic fields with the highest reserves are located where there are intraformational seals to create stacked reservoirs. They are best illustrated by the Zubair Formation of Kuwait and southern Iraq in fields including Raudhatain, Rumaila and Zubair. It seems obvious that shales thin and become less effective seals away from their provenance areas. Ultimately, this characteristic defines one margin to play fairways. For example, the pinchout of the Aghar Shale is an important control on the limit

to the Dalan-Kangan play fairway, but it probably permitted some leakage into overlying reservoirs in the Middle to Late Triassic Dashtak Formation. Fluvial - Paralic Reservoirs Siliciclastic input created some of the most prolific hydrocarbon reservoirs on Earth. The best reservoirs (Zubair and Burgan/Nahr Umr) are clearly associated with a favourable combination of all three contributory factors (tectonically driven hinterland uplift, humid climate and eustatic lowstands) that introduced very high net-to-gross ratio, fluvio-deltaic reservoirs with high permeabilities, which generate high recovery factors under natural aquifer drive. However, the stratigraphic organisation means that heterogeneity increases upward in many reservoirs; it is of escalating importance for late field development because remaining reserves are increasingly located in these complex reservoirs as the “easy oil” is progressively exploited.


Lowstand Plays The Middle Cretaceous Tuwayil Sandstone lowstand reservoir of Abu Dhabi is notable as a rare example of a working lowstand siliciclastic play on the Arabian Plate. The previous discussion highlights the stratigraphic intervals where tectonism, climate and major sea-level falls have led to marked advances of siliciclastic systems with high net-togross ratios, notably in the Valanginian and Albian. It is predicted that basin margin delta systems, plus slope and basin gravity flow deposits fed by these advances, are the most attractive targets for lowstand reservoirs on the Arabian Plate. Explorers should be looking for these plays. Source Rocks The Kazhdumi Formation is a major source rock deposited in an intrashelf basin that is down system tract from the Burgan/Nahr Umr fluvio-delta system (Figure 4). Structures such as the Kazerun Fault and the Safaniyah-Nowrooz-Hendijan Arch were demonstrably active in the late Aptian to early Albian (van Buchem et al., 2010). These and possibly other structures presumably played a role in creating an intrashelf basin prone to organic-rich deposition. The Kazhdumi Formation is a siliciclastic source rock, even though it has changed gradationally into a more carbonate-dominated equivalent seen at the outcrop in Kuh-e Bangestan (van Buchem et al., 2010). As previously noted, nutrients were brought in by the “Burgan Delta” and preserved as a result of water stratification linked to a freshwater overhang in front of the delta (Bordenave and Burwood, 1995). CONCLUSIONS Particularly important phases of siliciclastic input occur in: (i) the Early Triassic (Olenekian “Sudair Shale”) coincident with major eustatic lowering, an episode of humid climate and rifting on the northern part of the Arabian Plate; (ii) Late Triassic (late Norian “Initial Minjur Sandstone”) coincident with East Mediterranean rifting, a humid episode and a major eustatic sea-level fall; (iii) Middle Jurassic (early Bajocian, “Initial Dhruma Sandstone”) coincident with localised uplift and a humid climate and immediately postdating an eustatic sea-level fall in the Aalenian; (iv) Early Cretaceous (late Valanginian–Barremian “Zubair Sandstone”) postdating a Valanginian eustatic lowering and coincident with humid climate and uplift in northern and western Arabia; (v) Mid-Cretaceous (Latest Aptian–middle Albian “Burgan Sandstones”) coincident with Arabian Shield uplift, humid climate and a eustatic low. Other episodes of siliciclastic input also occur, although they tend to be more localised.


Important seals are formed during the progradation of siliciclastic systems “poisoning” carbonate shelves or during transgression when distal pro-delta siliciclastic systems retreat back across the shelf, capping upsystems tract fluvial or shelfal sandstones, or when they are located above major unconformities, capping carbonate reservoirs. Siliciclastic reservoirs include the well-known and prolific fluvial and paralic sandstones that contribute, for example, to the Burgan Field in Kuwait and to the Zubair and Nahr Umr reservoirs of the northern Gulf. Lowstand sands (both lowstand deltas and slope and basin gravity flow deposits) form viable, but underexplored, reservoir targets. Source rocks may be deposited in front of prograding delta systems linked to high nutrient supply and water stratification caused by freshwater overhang, leading to anoxia and preservation of organic matter. A wellknown example is the Kazhdumi Formation of the Zagros. A better understanding of the fundamental controls on siliciclastic input onto the Arabian Plate will enable better predictions of these key petroleum play elements and a better understanding of the subsurface risk associated with their occurrence. ACKNOWLEDGEMENTS Conversations with Frans van Buchem (Halliburton), Henk Droste (formerly Shell) and Ian Sharp (Statoil) over many years were helpful for developing some of the ideas presented herein.

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