Henk Droste (formerly Shell) and Ian Sharp (Statoil) over many years were helpful for developing some of the ideas presented herein. REFERENCES. Al-Aslani ...
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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
Help to minimise risk in frontier areas with our suite of global exploration tools From plate to pore-scale, Exploration Insights™ product suite provides the tools with which to build a robust framework for frontier exploration. Model the unknown and generate meaningful geological interpretations in areas with limited data-control to better understand exploration risk.
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
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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.
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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
Neftex Exploration Insights | 21
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?
Local humid climate proxies?
Eustatic fall
Latest Aptian-Middle Albian (“Burgan”)
Strong
Late Aptian rifting
Local humid climate proxies?
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
East Mediterranean rifting
Local humid climate proxies?
Immediately postdates eustatic fall
Late Norian (“Minjur”)
Strong
East Mediterranean rifting
Humid episode
Major eustatic fall
Olenkian (“Sudair”)
Strong
Palmyride rifting
Humid episode
Major eustatic fall
Local humid climate proxies?
Table 5: Episodes of Major Siliciclastic Input onto the Arabian Plate and Possible Connecting Causes.
22 | Neftex Exploration Insights
Neftex Exploration Insights | 23
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.
24 | Neftex Exploration Insights
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.
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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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