Roadian. Wordian. Capitanian. Wuchia- pingian. Changh- singian. Jilh Formation. Sudair Formation. Khuff Red Bed in South Oman. Upper Gharif Red Bed.
Gharif and Khuff formations, subsurface Interior Oman
Depositional sequences of the Gharif and Khuff formations, subsurface Interior Oman Peter Osterloff, Aida Al-Harthy, Randall Penney, Pieter Spaak, Glen Williams, Farida Al-Zadjali, Neil Jones, Robert Knox, Michael Stephenson, Guy Oliver and Moujahed Al-Husseini ABSTRACT The Gharif Formation of the Haushi Group in subsurface Interior Oman is underlain by the glaciogenic sediments of the Al Khlata Formation, and is progressively transgressed by the marginal to fully-marine Khuff Formation. The Gharif Formation consists of three members that are interpreted as depositional sequences (DS). The Early Permian (Artinskian) Lower Gharif Member (about 65 m, 213 ft thick) corresponds to DS Permian 10 (DS P10) and consists of Gharif cycles 1 and 2. Gharif Cycle 1 contains Arabian Plate Maximum Flooding Surface MFS P10 in the ‘Marine Flooding Shale’ unit; Gharif Cycle 2 is characterised by the ‘Haushi Limestone’ in central and north Oman. The Middle Gharif Member (about 85 m, 279 ft; DS P13) corresponds to Gharif cycles 3 and 4; Cycle 3 is also Artinskian in age. The Upper Gharif Member (about 90 m, 295 ft; DS P15) consists of Gharif cycles 5-8 and by stratigraphic position it ranges in age from late Artinskian to early Wordian. Whereas Gharif cycles 5-7 are interpreted as Depositional Sequence 15, uppermost ‘estuarine’ Gharif Cycle 8 is only tentatively assigned to DS P15. The base of Gharif Cycle 8 is an apparent sequence boundary and this cycle may be related to the Khuff transgression. The Gharif Formation was deposited in environments ranging from continental to marginal marine, and its lithofacies can be organised in a hierarchical reservoir scheme. The boundaries between the three members and some of the intra-member cycles are sequence boundaries; characterised by incised channels (20-50 m deep) encountered in wells and sometimes imaged in seismic sections. The Khuff Formation of the Akhdar Group reaches a maximum thickness of 1,110 m (3,641 ft) in northern Oman, where it is dominated by carbonates with very little shale. Towards southeast Oman, the Khuff becomes progressively truncated (geographically and stratigraphically) and interspersed with, and finally dominated by, mainly ‘red bed’ clastic facies. In south Oman, the Khuff and Gharif red beds are palynologically barren and lithologically indistinguishable. The Khuff Formation is divided into three members and eleven transgressive-regressive cycles or possible depositional sequences. The Lower Khuff Member (up to 325 m, 1,066 ft thick) is interpreted as three depositional sequences (DS P17-19) that are Wordian and younger in age. DS P19 contains the Khuff Marker Limestone (KML) that is interpreted as Maximum Flooding Interval MFI P19; it provides a semi-regional datum in the red bed-dominated Lower Khuff Member in south Oman. The Middle Khuff Member (up to 640 m, 2,099 ft) is nearly equally bisected by the regional Middle Khuff Anhydrite (Khuff-D Anhydrite of Saudi Arabia). The Middle Khuff Member appears to consist of six depositional cycles: three below (DS P20, P23 and P27) and three above the Middle Anhydrite (DS P30, P35 and P40). The boundary between the Middle and Upper Khuff members is generally abrupt with claystones overlying carbonates. The first down-hole Permian microfauna occur within the Middle and Lower Khuff members; in contrast, microfauna is absent in the Upper Khuff Member. The Upper Khuff Member (up to 160 m, 525 ft) is Triassic in age and appears to consist of two depositional cycles (DS Tr10 and DS Tr20). The top Khuff carbonate boundary is conformable with the base of the Sudair Formation shale in north Oman; and unconformably overlain by progressively younger formations in south Oman.
83
Osterloff et al., 2004b Gharif Formation location map, Oman 53 E
56
55
54
57
58
59
60
IRAN
Arabian Gulf
25 N
25 N
Gulf of Oman
24
24
O
M
AN
UNITED ARAB EMIRATES
MO
UN
Safah 23
Lekhwair-70 (Figure 25)
Lekhwair
Dhulaima
TAI N
S
Al Barakah
23
Daleel FAHUD SALT BASIN Haban Fahud West Natih
Mezoon
Muwaythil-1 (Figure 4) Yibal-45 (Figure 43) Al-Bashair-1 (Figure 9) Al Huwaisah
Yibal
Hafar Baqlah
Abeer
Qalah
Fahud
Rasafah Kauther Afar Fushaigah 0 re 2 2 Figu Makarem Burhaan 3 Malih Saih Qarat Musallim Nihayda Al Milh Barakat Saih Farha-1 Rawl-2V Tawf Dahm (Figure 8) Saih Farha S Qarn Alam Rawl Mabrouk Habur Mafraq Barik Ramlat Al Ghubar GHABA SALT BASIN Sahmah Hassun OMAN Hazar Anzauz Hasirah-1 Zauliyah (Figures 21 and 22) (Figure 24) Hasirah C OM ENTR Mazeed Asfoor AN AL HIG Hawqa Wafra Maqtaa H Suwaihat-3 Wafra-6 (Figure 11) Bahja Haima Wadi Bahja-1 (Figure 10) Sayyala-29 (Figure 26) Al Khlata Rajaa Sayyala Nafoorah Safwan K Bahaa Zareef Mukhaizna AN Ghufos Safiq South-1 FL Nawal Sadad (Figure 3) N Naeem ER SOUTH OMAN Aseel ST SALT BASIN A E Rima Al Noor Nimr
22
SAUDI ARABIA 21
LEGEND Oil field Gas field 20
19
1 22
Al-Bashair
Hajal-1 Salwa Anuq Amin Hajal Omraan Ghamr Durra Marmul Amal-9 (Figure 13) Jisr Saqr Ihsan-2 Qaharir-4 (Figures 14a and 14b) Rahab Ghanuq Rahab-2 (Figure 12) Dhiab
21
Masirah Island
20
19
Birba North
Sakhiya Zalzala Sarmad
18
Dhahaban South
Arabian Sea 18
Jazal
Juzor Al Halaniyat
N
17 0
YEMEN
17 100
km
Scale: 1:5,000,000
16
53
54
55
56
57
58
59
Figure 1: Location map of representative wells and traverses of the Gharif Formation in Interior Oman. Also see Figure 23 for location map of Khuff traverses and wells. South Interior Oman includes the South Oman Salt Basin (SOSB), Eastern Flank and the region south of about 20°N. Central Interior Oman includes the southern part of the Ghaba Basin to the southwest of the Oman Mountains, in the region between about 20°-21°N. North Interior Oman includes the northern part of the Ghaba Basin, the Fahud Basin, and is limited to the area west of the Oman Mountains. In the Oman Mountains, the Al Khlata and Gharif formations are absent. 84
Gharif and Khuff formations, subsurface Interior Oman
Stratigraphy of the Gharif and Khuff formations, subsurface Oman BIOCHRONOSTRATIGRAPHY ZONES
LITHOSTRATIGRAPHY
SEQUENCE STRATIGRAPHY
1905
Jilh Formation Sudair Formation Upper
DS Tr20
MFS Tr10
DS Tr10
MFS P40
DS P40
Middle Member
DS P35 MFS P30
DS P30
Middle Khuff Anhydrite
DS P27 DS P23 DS P20 Khuff Marker Limestone (KML)
DS P19
MFI P19
DS P18 DS P17
Khuff Red Bed in South Oman
Upper Member
Upper Gharif Red Bed
Sakmarian
280.3 ± 2.6 Ma
LST/TST?
6
Middle
unit 3
'Playa Shale'
unit 2
Alluvial Clastics
unit 1
Basal Sandstone
submember-2
DS P15
5 4
DS P13 intra-Gharif unconformity
Fringe Clastics submember-3
Cycle 8 7
incised channels
Upper Gharif Sandstone
incised channels
Lower Member
Artinskian
Haushi Group
PDO 2190
272.2 ± 3.2
OSPZ3
OSPZ4
Kungurian
Gharif Formation
OSPZ5
Wordian
264.1 ± 2.2
Roadian
EARLY (Cisuralian)
MFS Tr20
Lower
Capitanian
OMAN-SAUDI ARABIA OSPZ6
PDO 2103 Biozone
Wuchiapingian
Khuff Formation
2351
253.0 ± 0.3
Akhdar Group
Changhsingian
Ma 251.1 ± 3.6
PDO 2252
MIDDLE (Guadalupian)
PERMIAN
LATE (Lopingian)
TRIASSIC
Basal Sandstone in South
3
Haushi LST HST
Cycle 2
Max Flood Shale
subseq 3
MFI
MFS P10
Bioturbated Sandstone
subseq 2
TST
Cycle 1
subseq 1
LST
DS P10
subunit 2 submember-1 subunit 1
2141 PZ2
Al Khlata Formation, Arabian Plate Glaciation 3 (AP G3)
Figure 2: Stratigraphy of the Gharif and Khuff formations in subsurface north and central Interior Oman. Towards south Oman, the Akhdar Group is progressively eroded by Triassic and younger unconformities. The Permian geological scale is after Jin et al. (1997). The two columns showing biostratigraphic zones are inhouse PDO and the Oman-Saudi Arabia Palynozones (OSPZ of Stephenson et al., 2003). Lithostratigraphic units are from Hughes Clark (1988), Guit et al. (1995), and Mohammed et al. (1997 PDO Report).
INTRODUCTION The Permian Gharif and the overlying Permian-Lower Triassic Khuff formations are widely distributed throughout the subsurface of Interior Oman (Figures 1 and 2). The Gharif clastic reservoirs constitute an important hydrocarbon production unit; however as yet, the Khuff Formation offers limited hydrocarbon potential–except in the oolitic grainstones of the Upper Khuff Member. In this paper these two formations are discussed in terms of historical and current lithostratigraphic definitions, as well as their palaeoenvironments, sequence stratigraphy, fossils and age. The Gharif Formation overlies older Palaeozoic strata, invariably the Carboniferous-Lower Permian Al Khlata Formation of the Haushi Group (Hughes Clark, 1988; Osterloff et al., 2004a), and is generally
85
Osterloff et al., 2004b
conformably overlain by the Khuff Formation in north Oman. The exception occurs in areas in south Oman where salt movement occurred, or in outcrop on the Haushi-Huqf Uplift (Angiolini et al., 2004). In these regions, both the Gharif and Khuff formations are directly overlain by the mid-Cretaceous Nahr Umr Formation and/or the Jurassic Sahtan Group, respectively. The Gharif and Khuff formations show lateral facies variations, both locally and regionally, and these are depicted in many representative wells and regional traverses (Gharif: Figures 3, 6-14; Khuff: Figures 24-39). To help clarify these variations, this paper will frequently distinguish between south (south of 20°N), central (about 20°-21°N) and north (north of 21°N) Oman. Three Gharif members are recognised regionally and these may be depositional sequences (DS P10, P13 and P15), with the oldest Lower Gharif Member containing Arabian Plate Maximum Flooding Surface, Permian 10 (MFS P10). The age of MFS P10, however, is here considered as Artinskian, rather than Sakmarian (Angiolini et al., 1997, 2004; Sharland et al., 2001; see discussion in Stephenson et al., 2003). In the Khuff Formation, three members and eleven transgressive-regressive cycles or depositional sequences are interpreted (DS P17-Tr20; Figure 2). Two of these Khuff sequences contain the corresponding Arabian Plate MFS P20 and MFS P40 as positioned by Sharland et al. (2001) in Hasirah-1. However, flooding surfaces MFS P30 and MFS Tr10 are repositioned lower down in this study, within their corresponding depositional sequences. An exact correlation between the Gharif and Khuff formations in subsurface and their counterparts in outcrop is not generally possible. This is particularly the case with the Gharif Formation, where the ‘red beds’ of the Middle Gharif Member do not offer any significant topographic features at outcrop. This renders it difficult to place red bed units and age-comparable palynomorph-barren sands within their correct stratigraphic positions. However, where appropriate, both this paper and that describing the outcrop (Angiolini et al., 2004) attempt to compare and correlate surface to subsurface units, recognising the absence of key horizons along the uplifted eastern margin of the Arabian Plate.
GHARIF FORMATION, HAUSHI GROUP Lithostratigraphy of Gharif Formation The subsurface reference section of the Gharif Formation is defined in Safiq South-1, south Oman (Figure 1; 55°31’00”E, 19°17’45”N), where it is 157 m (518.1 ft) thick (Figure 3; Hughes Clark, 1988; Mohammed et al., 1997 PDO Report). Reference sections in central Oman that represent the most-complete sequences include Bahja-1 (Figure 10; 56°07’18”E, 19°55’20”N; Mohammed et al., 1997 PDO Report), and the almost fully-cored Wafra-6 (Figure 11; 56°09’18.3748”E, 20°05’26.6441"N; Mohammed at al., 1997). South Oman well Rahab-2 (Figure 12; 55°06’31.9042”E, 18°01’08.5317"N; Hughes Clark, 1988; Mohammed et al., 1997 PDO Report; Stephenson and Osterloff, 2002; Osterloff et al., 2004a) offers one of the better-cored reference sections of the Al Khlata to Gharif transition as discussed in Osterloff et al. (2004a). Petroleum Development Oman (PDO) geologists have informally divided the subsurface Gharif Formation into Lower, Middle, and Upper members (Figure 2). In some areas the three Gharif members, and the top and base of the formation, are difficult to pick because of differential incision within the formation and rapid lateral facies changes. In south Oman, correlation is further complicated due to incomplete units associated with salt movement and erosion at the base Nahr Umr (base Cretaceous) unconformity. On a regional scale however, the three members can be correlated as shown in earlier studies (Figure 4, Blendinger et al., 1990; Figure 5, Guit et al., 1995) and here (Figures 6 and 7). In Figures 8-14, the Gharif members and submembers are shown in representative wells across Oman (Mohammed et al., 1997 PDO Report; Stephenson and Osterloff, 2002). These wells demonstrate important regional lithofacies variations that are highlighted in the following description of the Gharif Formation. Thickness maps for the Lower, Middle and Upper Gharif members are shown in Figures 15-17, respectively.
Lower Gharif Member In north and central Oman, the Lower Gharif Member is about 65 m (214.5 ft) thick, and consists of three submembers (Blendinger et al., 1990; Guit et al., 1995; Figures 2, 8-11).
86
Gharif and Khuff formations, subsurface Interior Oman Al Khlata and Gharif formations, Safiq South-1, south Oman Gamma Ray (API) 0
LOCAL STRATIGRAPHY
Sonic (µsec/ft)
Porosity (%)
LITHOLOGY
-3
45
Density (gm/cc)
The upper Gharif boundary is conformable and clearly marked by a change from limestones of the Khuff to the red shales of the Upper Gharif and by an increase in gamma, sonic and separation of the density/porosity logs. The Gharif Formation is subdivided into Upper, Middle and Lower. The Upper and Middle members are characterised by an interbedded sequence of continental red shales, siltstones and sandstones while the Lower comprises grey, marine limestones, shales, siltstones and sandstones. There are no visible differences between the lithologies of the Upper and Middle Gharif. It is impossible to distinguish between them whilst drilling.
Fringe Clastics
1,300
Bss
1,200
1,250
1,350
LITHOLOGICAL DESCRIPTION and COMMENTS
2.75
1.95
Lower
Khuff Fm
Upper Member Gharif Formation
100
Middle Member
2103 2252
Roadian-Wordian Artinskian-Kungurian
PDO Biozone 2190
LOWER
Lower Member
PERMIAN
MIDDLE
500
?
150
The Lower Gharif can be subdivided into two units - the Fringe Clastics and other Basal Sands (cf. Wafra-6 in Figure 11). The top is picked at the top of a thin, dark grey, lagoonal shale with shell fragments overlying the youngest lower Gharif limestone.
ARABIAN PLATE SEQUENCE STRATIGRAPHY
DS P17
DS P15
DS P13
DS P10 MFS P10
The upper boundary of the Al Khlata Formation is conformable and is taken at the base of the last Gharif sand here marked by the change from sandstones to shales of the Al Khlata. These uppermost shales of the Al Khlata in this well are not classified as the Rahab because (1) the well is not on the Eastern Flank; and (2) they appear to have more of an affinity to the true varved Al Khlata shales.
1,400
UPPER
CARBONIFEROUS
The Al Khlata Formation is an interbedded sequence of shales, sandstones and diamictites.
Al Khlata Formation
The sandstones of the Al Khlata are lithic to sublithic and more rounded and often coarser than the finer, angular subfeldspathic sandstones of the Gharif and the underlying Ghudun Formation.
1,500
DEPOSITIONAL SEQUENCE G3
1,550
The lower boundary of the Al Khlata Formation is an unconformity marked by a change from typical Al Khlata to Ghudun sandstones and green/red shales. The boundary is distinctive on logs as the Ghudun shows a very spikey gamma trace.
1,600 m
LOWER ORDOVICIAN Ghudun Formation
AP5 AP2
Figure 3: Lithostratigraphy, composite electrical logs, lithology, interpreted sequence stratigraphy, and age of the Gharif Formation in reference well Safiq South-1 (Hughes Clarke, 1988; Mohammed et al., 1997 PDO Report). Note PDO Biozones and stages shown in this and other figures are for reference only, and do not imply fossil recovery from the entire section. 87
Safah
Saih Rawl
Musallim
Mabrouk
Tawf Dahm
Malih
MO
Saih Nihayda
TAIN S
Qarn Alam
UN
Makarem
Natih Fahud
AN
57
0
59
40 km
km
N 100
Gulf of Oman
LEGEND Oil field Gas field
58
Density (gm/cc)
22
23
24
Gamma Ray (API) Density (gm/cc)
Sandstone
Argillaceous sandstone
Shale
Argillaceous limestone
Limestone
Dhulaima
100 m
pre-Gharif unconformity
Middle Gharif Member
Upper Gharif Member
Gamma Ray (API)
Muwaythil-1 Density (gm/cc)
LOCAL STRATIGRAPHY Khuff Formation
Haima Supergroup
Lower Palaeozoic
Gharif Formation
27 km
Khuff Formation
Figure 4: Traverse of gamma ray and density logs of the Gharif Formation in north Interior Oman (after Blendinger et al., 1990). The Gharif Formation is absent on the Lekhwair High.
SAUDI ARABIA
Yibal
Fahud West
56
Al Huwaisah
Muwaythil-1
Dhulaima
Lekhwair
UAE
55
infra-Cambrian Ara Group
pre-Khuff unconformity
Lekhwair
DS P17 DS P15 DS P13
Lower Member Upper Member Middle Member
South
MIDDLE LOWER
Gamma Ray (API)
OM
Lower Mbr
88
DS P10
Roadian-Wordian Artinskian-Kungurian
Gharif and lower Khuff formations, north Oman
PERMIAN
North
Osterloff et al., 2004b
89
Roadian-Wordian
Artinskian-Kungurian
MIDDLE
LOWER
PERMIAN
Lower Mbr
Upper Member
Middle Member
Khuff Fm
Gharif Formation
Akhdar Gp
Haushi Group
Al Khlata Formation
Lower Member
0
150
Q
68 km
0
API 150
Maximum Flooding Shale
Q
Q
27 km
0
Zauliyah API 150
Gamma Ray
Mix
Q
Q
Q
28 km
0
Hazar API 150
Gamma Ray
Mix
Mix
Mix
27 km
0
API 150
Gamma Ray
Tawf Dahm-1
Q
Q
Huqf
100 m
DS G3
MFS P10
DS P10
DS P13
DS P15
DS P17
SEQUENCE STRATIGRAPHY
Figure 5: Traverse of gamma ray logs of the Gharif Formation, west-central Interior Oman (Guit et al., 1995). The green and red dots correspond to different oil and gas types (Q, Huqf, mix) from different source rocks.
API
Hawqa
Gamma Ray
Sayyala
Gamma Ray
0 Km
Mediterranean Sea
500
Red Sea
ABIA I AR
Hazar
Sayyala
Waste zone/seal
Reservoir siltstone
Intra-formational seals
IRAN
OMAN
Arabian Sea
UAE
QATAR
Tawf Dahm-1
SAUDI ARABIA
Zauliyah Hawqa
Arabian Shield
BAHRAIN
KUWAIT
IRAQ
Ultimate top seal
OMAN
SAU D
Gharif and lower Khuff formations, west-central Oman
Gharif and Khuff formations, subsurface Interior Oman
Osterloff et al., 2004b
Lower Gharif submember-1 has a thickness of about 25 m (82.5 ft), and generally consists of crossbedded quartz sandstone that locally contains crinoid debris and brachiopods. The distribution of this unit is still not clear; across central Oman its thickness varies but does not exceed 30 m (98.5 ft). The relative increase in thickness of submember-1, when compared with units cited in Guit et al. (1995), is related to improved biostratigraphic calibration that lowers the top Al Khlata pick (Stephenson and Osterloff, 2002; Penney and Osterloff, in preparation). Lower Gharif submember-1 can be subdivided into two subunits that may be fourth-order sequences and represent a non-marine Lowstand System Tract (LST). It is overlain by marine-bioturbated shoreface sandstones that may represent a Transgressive System Tract (TST). Taken together, this pattern reflects a marine transgressive system unconformably overlying the Al Khlata Formation. Lower Gharif submember-2 (‘Maximum Flooding Shale’ of Guit et al., 1995, containing MFS P10 of Sharland et al., 2001) is less than 10 m (< 33 ft) thick. Core evidence suggests that it contains bioturbated lime-mudstones. Submember-2 corresponds to a lower gamma ray excursion as seen in Bahja-1 and Wafra-6 in central Oman (Figures 10 and 11), and Rahab-2 in south Oman (Figure 12). Guit et al. (1995) interpreted the flooding shale slightly higher up in Figure 5; microfossil evidence does not support this interpretation. A revised pick of the MFS can be correlated across Oman, even in south Oman where there are no Haushi carbonates. A characteristic upward-increasing series of gamma ray excursions is noted in many wells in south Oman (Figure 12). Lower Gharif submember-3 (‘Haushi Limestone’, about 35 m, 115.5 ft thick) is a cross-bedded oolitic packstone and grainstone, and stromatolitic mudstone. The Haushi Limestone occurs throughout most of central and north Oman, thickening towards the north. In central Oman two calcarenitic units, at most, are developed within the Haushi Limestone sensu lato. Mapping of these two units suggests that there was a dislocation of the barrier shoals of the ‘Haushi Sea’ in a northwesterly direction. This reflects the onset of progradation of the coastline within the ‘Haushi Sea’, a trend that continued during the deposition of the Middle Gharif Member. In south Oman, the Haushi Limestone passes laterally into the informally termed ‘Fringe Clastics’ (sometimes referred to as ‘Fringing Carbonates’) and ‘Basal Sandstone’ submembers (Figures 3 and 10; Mohammed et al., 1997 PDO Report). The Fringe Clastics generally include thin limestone beds which are difficult to differentiate from the Haushi Limestone sensu stricto. In southeast Oman, the Fringe Clastics pass laterally, in places, to paralic rocks reflecting a continued lateral facies change towards a more proximal coastline. Also in southeast Oman, the informally termed Lower Gharif Amal submember consists of a sandstone unit immediately above the Al Khlata Formation (Mohammed et al., 1997 PDO Report). This submember is not genetically related to the Haushi Limestone and Fringe Clastics, but rather to the lacustrine shoreface deposits associated with the Rahab Member (Stephenson and Osterloff, 2002). This unit is reassigned to the Al Khlata Formation as a coeval unit of the Rahab Member (Osterloff et al., 2004a). The shales of the Lower Gharif Member are grey and similar to the Rahab Member (Mohammed et al., 1997 PDO Report; Osterloff et al., 2004a), which has routinely caused problems in formation assignment in south Oman, resolvable only through palynostratigraphic application.
Boundary between Lower and Middle Gharif members In north and central Oman, the boundary between the Lower and Middle Gharif members is taken at the top of the Haushi Limestone (Guit et al., 1995). In these regions, the Haushi Limestone is capped by a thin, dark grey shale that separates it from the sub-feldspathic sandstones of the overlying Middle Gharif Member (Figures 8, 9 and 11; Mohammed et al., 1997 PDO Report). This shale is attributed to a lagoonal facies, reflecting post-Haushi Limestone onset of Middle Gharif progradation. The shale is overlain in west-central Oman by tidal barrier and shoreface systems, developed in association with renewed deposition triggered by the demise of the ‘Haushi Sea’. As with the two phases of calcarenitic shoals associated with the Haushi Limestone, a NE-trending barrier-shoreface system is developed in the lower Middle Gharif Member (Figure 6).
90
Gharif and Khuff formations, subsurface Interior Oman
In south Oman, where the Haushi Limestone is absent, the top of the Lower Gharif Member is often picked below the first appearance of continental red beds. However, as there are external controls on these red beds (i.e. a pronounced warming and drying of the climate), such a boundary could be diachronous, and controlled by laterally-shifting coastal and alluvial facies zones. Because of this difficulty, a carbonate-cemented ‘hardground’ surface has been taken as a proxy for the boundary in some south Oman wells, representing a potential sequence break (e.g. Rahab, Thuleilat and Qaharir fields). Indirect marine conditions may have periodically prevailed upon wettened lower coastal plains during the initial deposition of the Middle Gharif Member. This is evident in the area surrounding the Nimr field, based on the abundant palynomorph recovery from selected stratigraphic levels between palaeosols and pedified claystones. In the Haushi-Huqf Uplift, the contact between the Lower Gharif Member (Saiwan Formation, Angiolini et al., 2004) and Middle Gharif Member (intra-Gharif or supra-Saiwan unconformity, Figure 20) is an angular unconformity (Dubreuilh et al., 1992). Near the Oman Mountains, this boundary appears to cut into the Lower Gharif Member and Al Khlata Formation, and eventually brings the Khuff Formation into contact with the lower Palaeozoic Haima Supergroup (Figure 20; Blendinger et al., 1990). This unconformity is the source of conflicting lithostratigraphic interpretations and nomenclature involving the definitions of the Haushi and Akhdar groups, and the Gharif and Saiwan formations (see Angiolini et al., 2004). In the outcrops of the Oman Mountains, the Saiq and Mahil formations (Glennie et al., 1974) constitute the Akhdar Group, and the Gharif and Al Khlata are absent (Figure 20; Blendinger et al., 1990).
Middle Gharif Member In north and central Oman, the Middle Gharif Member is about 85 m (280.5 ft) thick and consists, from base up, of three informal units (Guit et al., 1995). Middle Gharif unit 1 (less than 10 m, 2 m thick) of variably argillaceous, very fine-grained sheetflood sandstones associated with the more ephemeral channel systems. Both floodplain and overbank deposits contain locally well-developed palaeosols that are indicative of the ephemeral and semi-arid system. Major stacked fluvial channel sandbodies, with similar reservoir potential to those occurring in Type Environment B1 form the best reservoirs, e.g. Upper Gharif DS P15 (cycles 5-7). Individual channel widths are calculated to be less than 10 m, but the channel sandbodies are vertically stacked, producing composite sandbodies characterised by poor to good channel-to-gross ratios (generally 0.3-0.6). The channel sandstones are fringed by heterolithic overbank deposits that locally consist of thick sheetflood sandstones that may flank the channel by several hundreds of metres. The sheetflood sandstones have poorer reservoir quality than the stacked channel sandstones, with the more proximal deposits having an average porosity of about 19% and horizontal permeability of about 7 mD (geomean). These deposits laterally and vertically connect some of the channel sandbodies, and as such, the channel sandstones are variably well interconnected; the resulting stacked channel belt width is generally interpreted to reach up to one kilometre across. However, the thick, proximal sheetflood deposits are typically interbedded and laterally gradational with, poorer reservoir, low-energy sheetflood sandstones, locally forming a wide sandflat environment. The entire sandflat may reach an approximate width of up to 2.5 km (though it is generally about one kilometre wide), but vertical connectivity is likely to be relatively poor due to the heterolithic character of these deposits. Type Environment B3 generally occurs in the most basinal part of the alluvial plain depositional system and may join with the marginal marine depositional system, typical of the Middle Gharif (DS P13, Cycle 3), especially in west-central and northwestern Oman. It consists of a laterally extensive sandflat complex that is made up of very fine-grained, clean to argillaceous sheetflood sandstones and minor, very fine-grained, generally clean, low sinuosity ephemeral channelised deposits. No major stacked
103
Osterloff et al., 2004b
Gharif Formation, Qaharir-4, south Oman Gamma Ray (API) 0
LOCAL STRATIGRAPHY
Sonic (µsec/ft)
LITHOLOGY
40
-3
45
Density (gm/cc)
LITHOLOGICAL DESCRIPTION and COMMENTS
ARABIAN PLATE SEQUENCE STRATIGRAPHY
2.75
1.95
Middle
140
Khuff Fm
Porosity (%)
150
1,150
The Upper Gharif shale comprises a monotonous sequence of micaceous red shales. Its upper boundary is taken at the base of the last down-hole Khuff limestone marked by an increase in gamma ray, sonic and separation of the density/porosity logs.
Upper Gharif shale
Roadian-Wordian-Capitanian
MIDDLE
1,200
?
previously "Khuff red beds"
? The Upper Gharif shale was fomerly referred to as the Lower Khuff red beds. 1,250 Limestones are of an incipient development within the Upper Gharif shale and the strong unambiguous development of such would lower the basal Khuff and create a red shale sequence which would be attributed to the Lower Khuff (see Amal-9, Figure 13).
Upper Gharif SST Middle Gharif Member
Artinskian-Kungurian
PDO Biozone 2190
1,400
1,450
Al Khlata Rahab SST
Sak
The upper boundary of the underlying Upper Gharif sandstone is marked by a change from red shales to sub-feldspathic sandstones marked by a decrease in gamma ray and sonic and a sharp break on the density/porosity logs.
DS P15
The top of the Middle Gharif is taken at the top of a thick red 'Playa Shale' unit underlying sands of the Upper Gharif.
DS P13
1,500 m
2141B
LOWER
?
1,350
Lower Gharif Member
2252
PERMIAN
Gharif Formation
1,300
104
The top Lower Gharif is marked by the first appearance of dark grey micaceous shales and siltstones. These shales are very similar to the Rahab shales of the Al Khlata Formation. The Middle and Upper Gharif shales are by contrast red in colour. Base of the Gharif is marked in this well by the first occurrence of diamictites representing the top of the Al Khlata Formation.
DS P10
DS G3
Gharif and Khuff formations, subsurface Interior Oman
Gharif and Khuff formations, Qaharir-4, south Oman Gamma Ray (API) 0
LOCAL STRATIGRAPHY
Porosity (%)
150
Sonic (µsec/ft) 140
40
LITHOLOGY
-3
45
Density (gm/cc)
clastics 1,050
Mesozoic clastics are siltstone, variably silty, mottled purple, red, light green. Sandstone, quartzite, (lithic), fine-coarse, friable, moderately sorted, very light grey.
?
Upper?
The upper boundary of the Khuff Formation is an unconformity marked by a change from the clean sandstones of the Mesozoic clastics to the calcareous grey shales and siltstones of the Middle/Upper(?) Khuff, reflected by a sharp increase in gamma.
Middle
The Middle Khuff is composed of lime-wackestone with thin interbeds of shale. The sequence is fossiliferous characterised by Permian fauna.
MIDDLE
1,150 m
?
The lower boundary of the Khuff is taken at the base of the last limestone reflected by an increase in gamma, sonic and separation of the density/porosity logs.
(submember)
Upper Gharif shale
PERMIAN
Gharif Formation
ARABIAN PLATE SEQUENCE STRATIGRAPHY
2.75
1.95
MESOZOIC
Khuff Formation
LITHOLOGICAL DESCRIPTION and COMMENTS
Figure 14a (facing page) and 14b: Lithostratigraphy, composite electrical logs, lithology, interpreted sequence stratigraphy, and age of the Gharif and Khuff formations in Qaharir-4, south Oman (Mohammed et al., 1997 PDO Report). See Figure 1 for location.
channels occur within this environment. The sandflat facies are considered to represent sheetflood deposition on a very low-relief alluvial plain with locally confined flow in small scale rivers and streams that constantly switched positions. This type of fluvial system is likely to produce channel belts with a relatively high width-to-depth ratio. Both floodplain and overbank deposits contain locally welldeveloped palaeosols that are indicative of the ephemeral and semi-arid system, especially well developed to the north of a hinge line running across central Oman through Asfoor to Bahja (Figure 1). The main reservoir facies in Type Environment B3 are small-scale, straight ephemeral stream deposits and enclosing high-energy sheetflood sandstones, which together make up part of the sandflat environment. These are typical of the lower net-to-gross systems developed in the upper part of Cycle 3 and lower part of Cycle 4 within DS P13 (Middle Gharif Member). The deposits pass laterally into more argillaceous and heterolithic sheetflood deposits. The cleaner parts of the sandflat effectively generate a composite sandbody, with the ephemeral stream deposits no more than a few metres wide; but the stream plus high-energy sheetflood deposits can be up to several hundred metres across. It is likely that the laterally-extensive sandflat complexes are laterally and vertically separated by mud-dominated overbank/floodplain sediments. Poor vertical connectivity is highlighted by the presence of relatively low channel and proximal overbank-to-gross channel belt ratios; these are predominantly less than 0.4 and commonly less than 0.2. The entire sandflat facies typically forms a laterally extensive, but heterolithic sandbody that ranges from less than 500 m to 2.5 km in width. Type Environment B4 encompasses sediments that represent the transition from marginal to offshore marine facies and has been subdivided into two sub-environments based on the presence or absence of carbonate deposits. Marginal marine deposits that include limestone beds have been assigned to Type Environment B4c (Figure 18), while Type Environment B4s (Figure 18) represents an entirely clastic
105
Osterloff et al., 2004b Lower Gharif Member thickness map
SAUDI ARABIA
OMAN
N 0-18 m 18-31 31-40 40-47 47-53 53-57 57-62
Arabian Sea
62-69 69-79 79-92
Figure 15: The Lower Gharif Member (DS P10) varies in thickness from zero to about 90 m in the central Rub’ Al-Khali Basin. Middle Gharif Member thickness map
SAUDI ARABIA
OMAN
N 0-28 m 28-46 46-59 59-68 68-74 74-78 78-84 84-92
Arabian Sea
92-110 110-120
Figure 16: The Middle Gharif Member (Depositional Sequence P13) varies in thickness from zero to about 120 m in the central Rub’ Al-Khali Basin. 106
Gharif and Khuff formations, subsurface Interior Oman
Upper Gharif Member thickness map
SAUDI ARABIA
OMAN
N 0-31 31-54 54-72 72-86 86-96 96-100 100-110
Arabian Sea
110-120 120-130 130-140
Figure 17: The Upper Gharif Member (DS P15) varies in thickness from zero to about 140 m in the central Rub’ Al-Khali Basin.
succession. This subdivision appears to be stratigraphically controlled, with the wholly clastic sequence (B4s) forming the base of the Lower Gharif Member (DS P10 representing the Cycle 1 LST and TST deposits in west-central Oman). The mixed (carbonate-clastic) sequence (B4c) is restricted to the top of the Lower Gharif Member (‘Haushi Limestone’, equivalent to the upper part of Cycle 2 or upper part DS P10). In Type Environment B4 (B4c and B4s), the tidal flat sediments are predominantly sandy and are interpreted as a discontinuous, but fairly extensive linear system. This tidal flat system is gradational with the sandflat of Type Environment B3, and extends from the supratidal zone through the intertidal zone. The tidal flat sandstones may be separated from a shoreface barrier system by a series of terrigenous and carbonate lagoons. In Type Environment B4c, the barrier system is composed of cross-bedded oolitic grainstones; whilst in Type Environment B4s, the shoreface is composed of sandstone bars. These environments are seen across west-central Oman, in the Asfoor, Hawqa and Hasirah areas (Figure 1) within the base of Cycle 3 (lowermost part of the Middle Gharif, i.e. DS P13). In both systems, the shoreface sediments fine offshore into lower shoreface sandstones and into the highly argillaceous and poor or non-reservoir offshore transition zone facies. The carbonate-rich environment (Type Environment B4c) contains no true reservoir facies; all limestones and sandstones have been heavily cemented by calcite. In contrast, the lowermost clastic environment (Type Environment B4s) contains shoreface sandbodies that have good reservoir characteristics, with porosity values up to 27%, and horizontal permeability values up to 2,000 mD. The lagoon shoreline and mouthbar complexes and beachface-upper shoreface deposits display lobate to sheet-like sandbodies that should have excellent lateral connectivity. However, vertical connectivity between individual sandbodies is variable and dependent on the thickness and lithology of intervening lagoonal sediments. In a typical vertical sequence, these facies have a sandbody-to-gross ratio of between 0.35-0.65. The sandstone bars are stacked, typically into units approximately 4 m thick, which migrate with transgressions and regressions to form a sandsheet approximately 250 m in width.
107
Osterloff et al., 2004b
Marginal Marine Offshore Transition Zone
Shoreface
Alluvial Plain Type Environment B1
Backshore/ Foreshore
Type Environment B2
AC1
Type Environment B4c AC1
AOs
AC3 AOp
AOp
AFp
AOd AFd
MSc
MBt/ MBg MBm
MSd
AFd
AC3 AOp
MOc
AOd AFp
MBt
AFd
MBg/MBy b MC
MBy MOs MSu MSs MOm/MOd
B3
Type Environment B4s Code: Description AC1: Major/stacked fluvial channels MBm: Inter-tidal mudflats AC3: Ephemeral fluvial channels MBt: Inter-tidal sandflats AFd: Floodplain - Lake with distal sheetflood MBy: Marine - Bay AFp: Floodplain - Ephemeral Pond/Lake; MOc: Marine - Sheet to wedge-shaped carbonate storm beds low preservation potential MOs: Marine - Heterolithic, shallow marine silts and sands AOd: Low energy sheetflood MSc: Marine - Carbonate shoals AOp: High energy sheetflood MSd: Marine - Carbonate (storm) wash-over lobes AOs: Crevasse splay/laterally restricted sheetflow MSs: Marine - Lower shore-face sandbodies MBg: Marine - Lagoonal MSu: Marine - Foreshore/upper shore-face
Figure 18: Block diagram summarising the distribution and geometry of the principal facies bodies of the Gharif Formation, in the basinal alluvial plain and marginal marine environments (B1-B4).
Proximal Setting P Area (Figure 19) Type Environment P1 is considered to occur in the most proximal part of the Proximal Setting alluvial plain. It consists of relatively straight, ribbon-like, major fluvial channels containing inter-channel bar deposits. These channels may link together to form a braided sandstone sheet, or broad ribbon-shaped channel belt. Channel-fill and channel bar sediments are dominated by poorly-sorted, medium- to very coarse-grained or granule-grade sediment, and minor siltstone and mudstone (occurring as channel abandonment facies). An example of this setting is present in the Mukhaizna field, within both Middle and Upper Gharif members (DS P13 and P15, cycles 3-7) (Figure 1). The channel system of Type Environment P1 may have resulted from a large influx of coarse sediment. However, the flow of sediment may have been highly variable, and indicative of ephemeral, and possibly seasonal flow. This type of system is likely to generate channel belts with relatively high width-todepth ratios and poorly to unconfined overbank deposits (sheetfloods) resulting from floods and channel breaching during erratic and possibly seasonally-controlled channel discharge; point-sourced overbank (crevasse) deposits will not occur. Variable concentrations of detrital clay cutans occur, and are a result of the ephemeral system. Areas to the south of Asfoor-Bahja in central Oman (Figure 1) represent this type environment. Fluvial channel sandstones form the dominant reservoir facies in Type Environment P1 with an average measured porosity of about 25% and horizontal permeability of about 270 mD (geomean). The individual channels are considered to be very small, approximately less than 10-20 m wide, which is consistent with outcrop data. However, the individual channel deposits are vertically stacked, producing composite sandbodies up to approximately 20 m thick and characterised by very high channel-to-gross ratios (e.g. >0.8). Overbank deposits are relatively rare in this environment, and are likely to have had a low preservation potential due to the high energy and channel-dominated system. The resulting sandbody
108
Gharif and Khuff formations, subsurface Interior Oman
Marginal Marine Shoreface
Sediment Source Area
Alluvial Plain P1
Backshore/ Foreshore P3a AFd
AC3b
AOp
MBg/ MBy AOp AOd
AOs
MBt/MBm
AC1b AFd MCb
AFp MSu
Type Environment P2
Type Environment P4
P3
Code: Description AC1b: Major/stacked fluvial channel bars MBg: Marine - Lagoonal AC3b: Ephemeral fluvial channel bars MBm: Inter-tidal mudflats AFd: Floodplain - Lake with distal sheetflood MBt: Inter-tidal sandflats AFp: Floodplain - Pond/Lake MBy: Marine - Bay AOd: Low energy sheetflood MCb: Marine - Small tidal channel bar AOp: High energy sheetflood MSu: Marine - Foreshore/upper shore-face AOs: Crevasse splay/laterally restricted sheetflow
Figure 19: Block diagram summarising the distribution and geometry of the principal facies bodies of the Gharif Formation, in the proximal alluvial plain and marginal marine environments (P1-P4).
is composed of thick channel bar sandstones and rare overbank deposits producing well-connected tabular or broad ribbon sandstone deposits. The sandbodies may be laterally connected, forming belts typically about two kilometres wide, and resulting in high reservoir amalgamation and intercommunication. In this type of environment, local vertical permeability baffles are caused by infiltrated clays within some of the channel bar and overbank sediments, the presence of locally preserved abandonment deposits and locally interbedded floodplain muds. Type Environment P2 contains variably thick stacks of relatively coarse-grained and poorly-sorted fluvial channel sandstones. The textural similarity of these channels to the thicker, stacked channel deposits occurring in Type Environment P1 suggests that the rivers were likely to have had similar geometries, but probably represent more temporary river activity during flood events. The overbank facies associated with major and ephemeral channel stacks, in this type environment, are similar to those identified in Type Environment P1. However, overbank and floodplain facies associations sometimes form a major constituent of Type Environment P2. High- and low-energy sheetflood sandstones represent unconfined to poorly-confined flow, with lateral extent often controlled by the erratic channel discharge. Fluvial channel sandbodies are the main reservoir facies, which include both stacked/major channel sandstones and ephemeral channel sandstones. Reservoir quality is good for both facies with an average measured porosity of about 25% and 24%, and horizontal permeability of 271 mD and 67 mD (geomean), respectively. The overall lower permeability for similar porosity of ephemeral channel sandstones, compared to that of stacked channels, is interpreted to be a response to more abundant clay cutans (infiltrated clays) in the former.
109
Osterloff et al., 2004b
As in Type Environment P1, the individual upward-fining units (indicative of minimum bankfull depth) are thin (mainly
