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GROUPFORMATIONMEMBER
SahtanJubaila 
Hanifa 
Tuwaiq Mountain 
Dhruma 
MafraqUpper Mafraq
Lower Mafraq
GROUPFORMATIONMEMBER
SahtanJubaila 
Hanifa 
Tuwaiq Mountain 
Dhruma 
MafraqUpper Mafraq
Lower Mafraq

Authors: The Sahtan Group was defined by Glennie et al. (1974) in the Al Hajar Mountains outcrops to cover the shallow-marine carbonate Jurassic rocks lying between the Triassic dolomites of the Mahil Formation (Akhdar Group) and the overlying fine-grained limestones and marls forming the lower part of the Kahmah Group. See also Hughes Clarke (1988).

Introduction

The stable carbonate platform that was established on the Arabian Peninsula during Late Permian – Triassic times became notably influenced by regional scale tectonics during the Late Triassic. Even prior to this, doming of the Arabian Platform led to part emergence along the Al Huqf High. The greater part of the Arabian Platform became exposed at the end of the Triassic. Late Triassic structural movements took place throughout Oman along old north-south trending basement faults, especially in the north. These movements may have resulted from changes in the spreading of the Neo-Tethys that had developed during the separation of Gondwana from Permian times onwards. Spreading of the Neo-Tethys along its previous axis (the Hawasina Basin) ceased, and active spreading shifted eastwards (Glennie, 1995). Halokinesis of the Ara salts continued in the northern salt basins, with some growing diapirs piercing the surface (Peters et al., 2003), and the associated development of rim synclines. Substantial salt-withdrawal basins formed in the Ghaba Salt Basin, with up to 2,000 m thick Triassic sections preserved. The siliciclastics of the basal Sahtan Group (Mafraq Formation) were deposited over the shelf margin as well as over the foreland interior. From then on, shallow-marine carbonates were to dominate the shelf area and source the carbonate turbidites that characterise much of the Jurassic and Cretaceous platform-margin and basinal Hawasina deposition of Neo-Tethys (Glennie et al., 1974). This time interval, corresponding to the Dhruma to Jubaila carbonates, comprises successive shallowing-up, regressive cycles of mudstones grading to grainstones. The mudstones in adjacent UAE developed into source rocks under anoxic conditions and have been tied to one of the petroleum systems recognised in the northeast of Oman (the Tuwaiq Petroleum System; Terken et al., 2001). These overall condensed and strongly regressive sequences, deposited when globally sea level was rising, indicate continued continental-margin uplift(s). This may be responsible for the post-Tuwaiq Mountain and post-Sahtan unconformities, where particularly the Upper Jurassic uplift and erosion seems to be observed over the entire eastern edge of the Arabian Plate (Rousseau et al., 2006).

Figure 8.1:

Location map: Sahtan Group.

Figure 8.1:

Location map: Sahtan Group.

Rousseau et al. (2006) identified three major depositional episodes:

  • (1) a Pliensbachian – Toarcian coastal encroachment in a southward direction, represented by the dominantly clastic deposition of the Lower Mafraq Formation upon the Permian – Triassic carbonates, and as identified herein, clastics of the Late Triassic, Minjur Formation;

  • (2) a general Late Bajocian marine flooding diachronously overstepping the Lower Mafraq, comprising a hybrid facies of marginal-marine environments of the Upper Mafraq Formation, followed through the Bathonian – Callovian by the carbonate Dhruma-Tuwaiq Mountain System. This evolved through time from a low-angle, homoclinal ramp dipping in a (north) westwards direction, to a purely aggradational, flat-topped platform; and

  • (3) a Kimmeridgian – Tithonian onlap in an eastwards direction of fine-grained limestones (Hanifa-Jubaila) upon a post-Tuwaiq Mountain unconformity. They describe a significant post-Tuwaiq Mountain unconformity, with possibly up to 200 m of erosion in the east, which in time can be associated with a widespread Middle – Upper Jurassic unconformity, that has been recognised all over the Arabian Peninsula (Murris, 1981).

Type section: Wadi as Sahtan, north flank of Al Jabal Al Akhdar. For additional subsurface reference sections see individual formations.

Lithology: The Sahtan Group evolved from a lower clastic succession into a shallow-marine carbonate shelf succession with grainstones, aphanitic mudstones and wackestones with dolomites developed locally (Figures 8.3, 8.7 and 8.11). However, in some areas, for example, in the northwest of the Fahud Salt Basin, the dolomites predominate.

Subsurface recognition: The limestone sequence subdivision of Jubaila, Hanifa and Tuwaiq Mountain is reliant on the recognition of wackestone/mudstone (bottom) to grainstone (top) sequences, reflected by sharp breaks on the Density-Neutron logs, and on biostratigraphy. These are difficult to distinguish when incomplete with progressive erosion of the younger units southwards.

Boundaries: An erosional unconformity is visible at the base of the Sahtan Group in Al Jabal Al Akhdar outcrops. It lies unconformably on Minjur Formation clastics in the greater Lekhwair area and on progressively older Akhdar Group carbonates in a general southeasterly direction. The upper boundary with the Kahmah Group is generally unconformable but in North Oman the Jubaila Formation may pass conformably into the Rayda Formation (Rousseau et al., 2006).

Distribution: The Sahtan Group occurs throughout Oman, but passes northwards into the Musandam Group (see Glennie et al., 1974), equating to most of the Lower Musandam in Ruus Al Jibal. The Jubaila, Hanifa and the Tuwaiq Mountain formations are most complete in the westernmost areas of North and Central Oman (Butabul-1, Figure 8.2) and are truncated towards the east and south. The thinning southwards is partly a combination of erosional truncation (Tithonian unconformity as well as internal truncation) and onlapping of the Akhdar Group. It may also be reduced by truncation to the northeast in the Al Hajar Mountains outcrops (see Kahmah Group). A marked age gaps develops progressively towards the southeast between the Sahtan and underlying Akhdar Group, caused by low-amplitude structural movements (block-faulting and tilting), particularly thinning and truncation towards the Al Huqf axis. This was a time of extensive soil formation by deep weathering associated with the base-Jurassic unconformity, preserved in North Oman. Considerable peneplanation also took place, as the basal Sahtan only shows evidence of a weak palaeorelief.

Figure 8.2:

Composite electrical logs, lithology and lithological description of the Jubaila/Hanifa/Tuwaiq Mountain formations, Sahtan Group, in well Butabul-1, Central Oman (Mohammed et al., 1997). See Figure 8.1 for location.

Figure 8.2:

Composite electrical logs, lithology and lithological description of the Jubaila/Hanifa/Tuwaiq Mountain formations, Sahtan Group, in well Butabul-1, Central Oman (Mohammed et al., 1997). See Figure 8.1 for location.

Subdivision: In the subsurface sections in western Oman five formations are recognised in the Sahtan Group: Jubaila, Hanifa, Tuwaiq Mountain, Dhruma and Mafraq (clastic/carbonate), the first four being entirely carbonate units originally defined in Saudi Arabia. In one western well only an uppermost thin evaporitic unit is present (Hughes Clarke, 1988). This unit may represent the feather-edge of the evaporitic Arab and Hith formations of Saudi Arabia, but these rock units are not discussed here.

Sequence Stratigraphy: The Sahtan Group represents the upper AP6 (Lower Mafraq Formation only) and AP7 Megasequences of Sharland et al. (2001). Sharland et al. (2001) correlate their MFS J70 to J10 surfaces into the Group (see formational discussions).

Age: Early to Late Jurassic, Late Pliensbachian – Early Tithonian, ca. 187–147 Ma.

Biostratigraphy: The Jubaila/Hanifa formations and the major part of the Tuwaiq Mountain Formation belong to Biozone F47 (Kurnubia jurassica associated with Crassicollaria spp.). The lower part of the Tuwaiq Mountain Formation belongs to Biozone F45 (Pfenderina salernitana, P. trochoidea), as do the Dhruma Formation and Upper Mafraq Member. The Lower Mafraq Member comprises Biozone F30 (Pseudocyclammina liassica, Orbitopsella praecusor). However, the zonation remains relatively crude in places, e.g. the Hanifa is poorly characterised, with little additional insight since Sikkema (1992). Sikkema (1992) extends the Mafraq into the Rhaetian (unzoned and hypothetical).

Kharusi (1986) set up a fourfold palynological zonation, with two subzones, for the Sahtan Group and then updated the Mafraq (Kharusi, 1989) with two more zones and four subzones, extending the age of the Lower Mafraq down to the Norian – Carnian. These original schemes can be combined and summarised as follows:

ZoneSubzoneMarker speciesRelative ageFormation
4214 Systematophora cf. areolataLate Oxfordian – Early TithonianHanifa – Jubaila
4009 Ctenidodinium cf. ornatumLate Bathonian – Middle OxfordianDhruma – Tuwaiq
4009BCtenidodinium cf. ornatum(Middle Oxfordian)
4009A(downhole decrease S. cf. areolata, increase C. cf. ornatum)(Late Bathonian – Early Oxfordian)
4191 Dichadogonyualax spp.Late Bajocian – ‘middle’ BathonianMafraq – Dhruma
2262 Spheripollenites scabratusEarly Toarcian – Early Bajocian
2262B**Spheripollenites scabratus(Early Toarcian – Early Bajocian)
2262A(absence Callialasporites spp.)(Early Toarcian)
2242** Corollina spp.?Hettangian – PliensbachianMafraq
2255* Bartenia communisCarnian – ? RhaetianMafraq*
2247Vesicaspora schemili(Norian - ?Rhaetian)
2241Partitisporites verrucosus(Carnian – Norian)
ZoneSubzoneMarker speciesRelative ageFormation
4214 Systematophora cf. areolataLate Oxfordian – Early TithonianHanifa – Jubaila
4009 Ctenidodinium cf. ornatumLate Bathonian – Middle OxfordianDhruma – Tuwaiq
4009BCtenidodinium cf. ornatum(Middle Oxfordian)
4009A(downhole decrease S. cf. areolata, increase C. cf. ornatum)(Late Bathonian – Early Oxfordian)
4191 Dichadogonyualax spp.Late Bajocian – ‘middle’ BathonianMafraq – Dhruma
2262 Spheripollenites scabratusEarly Toarcian – Early Bajocian
2262B**Spheripollenites scabratus(Early Toarcian – Early Bajocian)
2262A(absence Callialasporites spp.)(Early Toarcian)
2242** Corollina spp.?Hettangian – PliensbachianMafraq
2255* Bartenia communisCarnian – ? RhaetianMafraq*
2247Vesicaspora schemili(Norian - ?Rhaetian)
2241Partitisporites verrucosus(Carnian – Norian)
*

Now included in the Minjur Formation.

**

Strong facies dependency, see Mafraq Formation, Biostratigraphy discussion.

As with the Cretaceous palynological zonations, this scheme has not been consistently applied since the 1980s and is now in need of comprehensive review and upgrade. Additional insight and improved interpretation have come from individual wells and play specific studies, which are discussed in detail within the individual Formation sections below. Virtually with every study done, zonal characteristics and age determinations have been adjusted and in places significantly changed, e.g. the intra-Mafraq age gaps (see formational discussions for updates).

The early age determinations of the Sahtan Group carbonates, as discussed in Hughes Clarke (1988), were based on comparison with better dated Saudi Arabian sections and rather limited microfaunal and palynological data. Improvements in understanding have been rather piecemeal and often difficult to reconcile with one another, i.e. they have given conflicting ages. Such age uncertainties may in part relate to diachronous formational shifts but more detailed further study is required.

Many uncertainties remain, and alternative age assignments are discussed in the appropriate sections, but a best fit current understanding of the Sahtan Group bio- and chronostratigraphy is proposed as follows:

FormationMicropalaeontological ZonePalynological ZoneRelative age
JubailaF47 (Sub-biozones F479 - F475)4214late Kimmeridgian – Early Tithonian
HanifaF47 (F475)4214Kimmeridgian
Tuwaiq MountainF47-45 (F475, F473, upper F457)4009 - 4191Late Bathonian – Callovian
DhrumaF45 (F457 - F455)(?4009) 4191 - 2262Late Bajocian – Bathonian
Upper MafraqF45 (F455 - ?F453)4191/2262B - 2262B*Toarcian – Late Bajocian
Lower MafraqF302262A - 2242*Late Pliensbachian – Early Toarcian
FormationMicropalaeontological ZonePalynological ZoneRelative age
JubailaF47 (Sub-biozones F479 - F475)4214late Kimmeridgian – Early Tithonian
HanifaF47 (F475)4214Kimmeridgian
Tuwaiq MountainF47-45 (F475, F473, upper F457)4009 - 4191Late Bathonian – Callovian
DhrumaF45 (F457 - F455)(?4009) 4191 - 2262Late Bajocian – Bathonian
Upper MafraqF45 (F455 - ?F453)4191/2262B - 2262B*Toarcian – Late Bajocian
Lower MafraqF302262A - 2242*Late Pliensbachian – Early Toarcian
*

Strong facies dependency, see Mafraq Formation, Biostratigraphy discussion.

The advances in understanding resulting from recent publications and in-house PDO palynological work significantly challenge the ages assigned to the historical Biozones of Sikkema (1992). This is further justification of the need for a comprehensive review and upgrade of the bio- and chronostratigraphy of this Group.

Jubaila Formation

Authors: Steineke (unpublished, 1937), see Powers (1968).

Introduction

This rather undistinguished unit is referred to the Saudi Arabian Jubaila Carbonate Formation, based mainly on its position above the Hanifa Formation. The Hanifa Formation in the interior parts of the Arabian epeiric carbonate platforms is characterised by a clear cyclicity of organic mudstones and carbonates (Droste, 1990). Oman’s marginal position is characterised by continuous carbonate sedimentation and even emergence. The mudstones of the Jubaila Formation correspond to a full drowning of the epeiric carbonate platform including the Oman margin (J70 MFS of Sharland et al., 2001, at the base of the Formation). This drowning seems to be immediately followed by uplift of the Oman margin, eroding deeply into the Jubaila Formation and underlying formations of the Sahtan Group to the south and east.

Type and reference section: Near al Jubaylah, Saudi Arabia. The Oman subsurface reference section is Butabul-1 (Figure 8.2).

Lithology: The sediments of the Jubaila Formation are predominantly slightly-argillaceous, fine-grained wacke-/mudstones with some fine dolomite layers, becoming ‘aphanitic’ towards the base (Figure 8.3b). The textures are, for the most part, mud-supported. This contrasts with the porous grainstones in the upper part of the underlying Hanifa.

Figure 8.3:

Ditch cuttings from the Sahtan Group: (a) Limestone-mudstone from Ramlat-1, Jubaila Formation; (b) Limestone-mudstone (aphanitic) from Lekhwair-85, Hanifa Formation; and (c) Limestone-packstone/grainstone from Ramlat-1, Hanifa Formation (scale grid is 1 x 1 mm) (Mohammed et al., 1997).

Figure 8.3:

Ditch cuttings from the Sahtan Group: (a) Limestone-mudstone from Ramlat-1, Jubaila Formation; (b) Limestone-mudstone (aphanitic) from Lekhwair-85, Hanifa Formation; and (c) Limestone-packstone/grainstone from Ramlat-1, Hanifa Formation (scale grid is 1 x 1 mm) (Mohammed et al., 1997).

Subsurface recognition: Whilst drilling, only hot-shot samples for biostratigraphy can signal the presence of Jurassic faunas. Post-drilling the Jubaila Formation can be picked on log pattern, in conjunction with biostratigraphic analysis.

Boundaries: The upper boundary with the Rayda Formation of the Kahmah Group is generally unconformable, but may be conformable in northern Oman, where both formations onlap the underlying older units of the Sahtan Group, which progressively get older to the east. Rousseau et al. (2006) consider the Jubaila to be part of the Late Jurassic flooding of the Arabian Plate. Hughes Clarke (1988) and Droste and van Steenwinkel (2004) document a base Cretaceous unconformity at the top of the Jubaila Formation overlain progressively by younger units towards the south (Rayda to Habshan formations). In both cases the deepwater facies of the Jubaila Formation are adjacent, if not genetically linked, to the deepwater sediments of the Rayda Formation.

The lower boundary is conformable with the Hanifa to the northwest but unconformable, onlapping on older Sahtan units to the south and east. The Hasirah-4 well (Figure 8.6) illustrates the difficulty of picking this boundary in locations where grainstone facies dominate. Elsewhere, where dolomites dominate the boundary can also be difficult to pick.

Distribution: In Oman the Jubaila occurs only in the extreme western subsurface, being truncated by Kahmah units to the east and south. In the Lekhwair area, it is developed as an ‘aphanitic’ lime-mudstone of possibly deeper water character, which may be a precursor to deposition of the Rayda Formation.

Deposition: The sediment type and microfossil content suggest a low-energy marine setting.

Age: Late Kimmeridgian – Early Tithonian, ca. 152–147 Ma. Considered by Hughes Clarke (1988) to be Early Tithonian in age. Sharland et al. (2001) place their Late Kimmeridgian J70 MFS within the ‘aphanitic’ limestone at the base of the Formation (although, presumably in error, they illustrate it towards the top of the Formation in well Butabul-1). They do not recognise any subsequent Kimmeridgian MFS expressions in Oman (J80 to J100), citing pre-Cretaceous erosion, which is inconsistent with the interpreted ages.

Biostratigraphy:Hughes Clarke (1988) states that although the microfossils are indicative of an age close to that of the Hanifa Formation, palynological evidence is more in keeping with an Early Tithonian age. The Jubaila Formation probably represents the upper part of the Kimmeridgian F475 (Alveosepta jaccardi) Sub-biozone in Oman, and ranges up to the Tithonian F477/9 (Kurnubia jurassica/Crassicollaria spp.) Sub-biozones. It is placed in the upper part of the 4214 (Systematophora cf. areolata) Palynozone.

Hanifa Formation

Authors: Steineke (unpublished, 1937), see Powers (1968).

Introduction

The Hanifa Formation is considered to be one of the major source rocks for Middle East oil (Murris, 1981). Oman was in a marginal setting, where shallow-water, carbonate-dominated sedimentation persisted without the strongly anoxic deposition, which dominates the interior platform area (Droste, 1990).

Type and reference sections: Wadi Hanifa, Saudi Arabia (see Powers, 1968). Oman subsurface reference sections are Butabul-1 in Central Oman (Figure 8.2), Lekhwair-27 in North Oman (Figure 8.4), and Hasirah-4 in Central Oman (Figure 8.6).

Figure 8.4:

Composite electrical logs, lithology and lithological description of the Hanifa/Tuwaiq Mountain formations, Sahtan Group, in well Lekhwair-27, North Oman (Mohammed et al., 1997). See Figure 8.1 for location.

Figure 8.4:

Composite electrical logs, lithology and lithological description of the Hanifa/Tuwaiq Mountain formations, Sahtan Group, in well Lekhwair-27, North Oman (Mohammed et al., 1997). See Figure 8.1 for location.

Lithology: As with the underlying Tuwaiq Mountain Formation and the overlying Jubaila Formation, the Hanifa Formation is a limestone sequence with argillaceous mud-/wackestone below, passing up into porous grain-supported grainstone above (Figures 8.3 and 8.7). It is an ‘aphanitic’ mudstone in Lekhwair (e.g. Lekhwair-27, Figure 8.4) and Dhulaima wells (e.g. Dhulaima-7, Figure 8.5; thin, hard), where the porous grainstones are truncated. If grainstones are absent, identification of the Jubaila/Hanifa boundary is difficult (e.g. see Hasirah-4, Figure 8.6, where the grainstone facies is poorly, if at all, developed).

Figure 8.5:

Composite electrical logs, lithology and lithological description of the Hanifa Formation, Sahtan Group, in well Dhulaima-7, North Oman (Mohammed et al., 1997). See Figure 8.1 for location.

Figure 8.5:

Composite electrical logs, lithology and lithological description of the Hanifa Formation, Sahtan Group, in well Dhulaima-7, North Oman (Mohammed et al., 1997). See Figure 8.1 for location.

Figure 8.6:

Composite electrical logs and lithology of the Jubaila/Hanifa/Tuwaiq Mountain formations, Sahtan Group, in well Hasirah-4, Central Oman (Mohammed et al., 1997). See Figure 8.1 for location.

Figure 8.6:

Composite electrical logs and lithology of the Jubaila/Hanifa/Tuwaiq Mountain formations, Sahtan Group, in well Hasirah-4, Central Oman (Mohammed et al., 1997). See Figure 8.1 for location.

Figure 8.7:

Ditch cuttings from the Sahtan Group: (a) Dolomite from Ramlat-1, Hanifa Formation; (b) Limestonegrainstone from Lekhwair-27, Tuwaiq Mountain Formation; and (c) Limestone-wackestone/mudstone from Yibal-439, Tuwaiq Mountain Formation (scale grid is 1 x 1 mm) (Mohammed et al., 1997).

Figure 8.7:

Ditch cuttings from the Sahtan Group: (a) Dolomite from Ramlat-1, Hanifa Formation; (b) Limestonegrainstone from Lekhwair-27, Tuwaiq Mountain Formation; and (c) Limestone-wackestone/mudstone from Yibal-439, Tuwaiq Mountain Formation (scale grid is 1 x 1 mm) (Mohammed et al., 1997).

Subsurface recognition: The limestone sequence subdivision of Jubaila, Hanifa and Tuwaiq Mountain is reliant on the presence of the grainstone-wackestone/mudstone pairs. In cuttings, predominance of either may be an indication, but realistically only hot-shots for biostratigraphic analysis can confirm the presence of this Formation whilst drilling.

Post-drilling the Formation can be picked on log pattern, combined with biostratigraphy data. The grainstones have a very distinctive bow-shaped Neutron-Density character.

Boundaries: The upper boundary is conformable with the Jubaila Formation. Where the Jubaila Formation is absent, basal Kahmah Group sediments rest unconformably on the Hanifa Formation. The lower boundary with the underlying Tuwaiq Mountain Formation is a very low-angle unconformity (0.1% gradient, according to Rousseau et al., 2006) as the Hanifa onlaps in a south-easterly direction. The Upper Jurassic was a period of overall high sealevel and therefore the unconformity must be partially tectonic in origin. Rousseau et al. (2006) link this unconformity with a period of uplift and erosion documented across the entire eastern edge of the Arabian Plate.

Distribution: The Hanifa is present only in the westernmost subsurface of Interior Oman and is truncated to the east and south.

Deposition: Low- to high-energy shallow-marine settings are indicated by the sediments and a moderately rich microfossil content.

Age: Kimmeridgian, ca. 155.6–152 Ma. It is considered to have a probably early Kimmeridgian age by Hughes Clarke (1988). Interestingly Sharland et al. (2001) illustrate their J60, early Kimmeridgian, MFS in well Butabul-1, associated with the thin Gamma peak at the Jubaila/Hanifa boundary, (i.e. strictly basal Jubaila Formation) but also state that the J60 MFS cannot be recognised in Omani outcrop platform deposits. They place their Middle Oxfordian J50 MFS at the top of a wackestone-‘aphanitic’ limestone package in the lower part of the Formation. Conversely, Rousseau et al. (2006) argue that Oxfordian deposition probably never occurred in Interior Oman and that the Hanifa is indeed Kimmeridgian in age.

Biostratigraphy: The Hanifa Formation represents the base of Sub-biozone F475 (Alveosepta jaccardi) and Palynozone 4214 (Systemaphora cf. areolata). However, this Formation is relatively poorly constrained biostratigraphically (fossil ranges poorly defined).

Tuwaiq Mountain Formation

Authors: Steineke (unpublished, 1937), see Powers (1968).

Introduction

Rousseau et al. (2006) describe the Tuwaiq Mountain Formation as a shallow-marine aggrading carbonate platform system that was first established during underlying Dhruma times. Their stated ‘Late Bathonian – Middle Callovian’ (see age discussion below) depositional time for the Upper Dhruma and Tuwaiq Mountain formations was a period of general transgression over the Arabian Shelf (Al-Husseini, 1997), presumably related to a global sea-level rise.

Type and reference sections: Darb al Hajaz, Saudi Arabia (see Powers, 1968). Oman subsurface reference sections are Butabul-1 in Central Oman (Figure 8.2), Lekhwair-27 in North Oman (Figure 8.4) and Hasirah-4 in Central Oman (Figure 8.6).

Lithology: The Tuwaiq Mountain Formation comprises a single sequence evolving upwards from deep-shelf lime mudstones to shallow-marine porous pack- to grainstones (as in Butabul -1, Figure 8.2). These are slightly, to rarely very, dolomitic, particularly in the upper part.

Subsurface recognition: The transition from mudstones of the basal Hanifa Formation into the packgrainstones of the upper Tuwaiq Mountain may be detected in cuttings whilst drilling, but is dependent on the quality of the cuttings. Post-drilling, the log pattern combined with biostratigraphic analysis is diagnostic. The grainstones have a very distinctive Neutron-Density overlay character.

Boundaries: The Tuwaiq Mountain Formation is underlain almost everywhere by a thin argillaceous limestone of the uppermost Dhruma. Overlain with low-angle unconformity by the Hanifa or, more obviously, unconformably by younger units of the Kahmah Group. Where not eroded in the west, the Tuwaiq Mountain Formation has a constant depositional thickness with a sheet-like depositional geometry. Assuming original constant thickness suggests up to 250 m of erosion of Tuwaiq Mountain and upper Dhruma Formation in the east.

Distribution: The Tuwaiq Mountain carbonates are present only in the western subsurface of Interior Oman; being truncated by younger unconformities to the east and south. They are absent from both Al Huqf and central Al Jabal Al Akhdar outcrop areas. An equivalent is present in the Lower Musandam limestones in Ruus Al Jibal, but this has not been recognised as a separate unit.

Deposition: The sediments of this Formation represent generally shallow-marine carbonate environments with a transition from relative deeper below (containing pelagic lamellibranchs) to shallower above, with algae, corals, foraminifera, etc.

Age: Late Bathonian – Callovian (- ?Oxfordian), ca. 165.5–161.2 (- ?155.6) Ma.

The problems in dating the Hanifa seem to increase as we move down into the Tuwaiq Mountain Formation and major differences in age interpretations occur in the available literature.

Hughes Clarke (1988) suggests an Oxfordian age, in part, supported by correlation with the type-section in Saudi Arabia. Sikkema (1992) considers it likely that Tuwaiq Mountain deposition started in the Late Callovian and in support of this Sharland et al. (2001) place their Middle Callovian J40 MFS approximately at the Tuwaiq Mountain/Dhruma boundary in wells Butabul-1 and Yibal-85. Conversely, Rousseau et al. (2006) group the Dhruma and Tuwaiq Mountain formations within a general Bathonian – Callovian aged episode, placing the Tuwaiq almost exclusively within the Callovian, possibly no younger than Middle Callovian. They argue that no Oxfordian deposits are preserved in Interior Oman. Palynological work in PDO (Jacovides et al., 1998; Woollam et al., 1999) suggests that the Tuwaiq Mountain Formation may range down to the Late Bathonian.

Biostratigraphy: The Tuwaiq Mountain Formation represents Sub-biozone F475 (lower part) and Sub-biozone F473 (Trocholina palastiniensis) and ranges down to Biozone F45 (Pfenderina salernitana, P. trochoidea) at the base. The 4009 (Ctenidodinium cf. ornatum) Palynozone generally characterises this unit, whilst Jacovides et al. (1998) also recorded Late Bajocian – Bathonian ranging dinocysts (Palynozone 4191 assemblages) within the lower Tuwaiq of a number of wells in north Oman. Woollam et al. (1999) used dinocysts to indicate an age somewhere within a general middle Bathonian – Early Callovian range for the Lower Tuwaiq in well Lekhwair-319.

It is beyond the scope of this publication to fully assess the conflicting biostratigraphical interpretations, (e.g. Oxfordian versus Callovian zonal assignments). However, from the available data it does appear that definitive Oxfordian assemblages may be absent in both the foraminiferal and palynological recovery, thereby supporting the age interpretation of Rousseau et al. (2006). All evidence considered, a best estimate, albeit tentative, age for the Tuwaiq Mountain Formation would be a broad Late Bathonian – Callovian age.

Dhruma Formation

Authors: Steineke (unpublished, 1937), see Powers (1968).

Introduction

This unit is seen at outcrop in the Al Huqf and the Al Jabal Al Akhdar. An equivalent sequence is present in the Musandam Limestone in Ruus Al Jibal. It occurs in well-sections in interior Central Oman, but may be absent in South Oman by nondeposition or Cretaceous erosion over palaeohighs.

Rousseau et al. (2006) document depositional geometries evolving through time from a low-angle, homoclinal ramp dipping in a (north) westwards direction, to a pure aggradational, flat-topped platform (upper Dhruma and Tuwaiq Mountain). This overall stratal pattern of the Dhruma-Tuwaiq

Mountain carbonate platform is consistent with the general ‘shallowing-up’ trend inferred in the wells from logs. Aggradation continued in the overlying Tuwaiq Mountain Formation associated with a continued transgression of the shelf. Correlations by Rousseau et al. (2006) indicate a very low-relief system under permanent shallow-marine conditions without any further clastic input.

The porous upper unit is probably continuous with the ‘Uwainat’ reservoir unit from Qatar and the Emirates (Sugden and Standring, 1975) and with the ‘Lower Fadhili’ reservoir unit in Saudi Arabia (Powers, 1968). Rousseau et al. (2006) correlate this porous Uwainat Member throughout Interior Oman.

Type and reference sections: Tuwaiq Mountain scarp, Saudi Arabia (see Powers, 1968). Oman subsurface reference sections are Yibal-85 in North Oman (Figure 8.8), Saih Rawl-27 in North Oman (Figure 8.9) and Zauliyah-5 in Central Oman (Figure 8.10).

Figure 8.8:

Composite electrical logs, lithology and lithological description of the Dhruma Formation, Sahtan Group, in well Yibal-85, North Oman (Mohammed et al., 1997). See Figure 8.1 for location.

Figure 8.8:

Composite electrical logs, lithology and lithological description of the Dhruma Formation, Sahtan Group, in well Yibal-85, North Oman (Mohammed et al., 1997). See Figure 8.1 for location.

Figure 8.9:

Composite electrical logs and lithology of the Dhruma Formation, Sahtan Group, in well Saih Rawl-27, North Oman (Mohammed et al., 1997). See Figure 8.1 for location.

Figure 8.9:

Composite electrical logs and lithology of the Dhruma Formation, Sahtan Group, in well Saih Rawl-27, North Oman (Mohammed et al., 1997). See Figure 8.1 for location.

Figure 8.10:

Composite electrical logs, lithology and lithological description of the Dhruma Formation, Sahtan Group, in well Zauliyah-5, Central Oman (Mohammed et al., 1997). See Figure 8.1 for location.

Figure 8.10:

Composite electrical logs, lithology and lithological description of the Dhruma Formation, Sahtan Group, in well Zauliyah-5, Central Oman (Mohammed et al., 1997). See Figure 8.1 for location.

Lithology: In the reference section in Yibal-85 (Figure 8.8), the Dhruma is composed of mudstones and argillaceous limestones overlain by a slightly dolomitic, porous, grain-supported limestone (Figure 8.11). The argillaceous limestones and thin shales towards the base are associated with relatively higher Gamma values. However, in the greater Lekhwair area the lower part (ca. 50 m) is represented by a very distinctive relatively clean, low Gamma mudstone/wackestone. The upper part is dominantly grain-supported, slightly-dolomitic and porous.

Figure 8.11:

Ditch cuttings from Yibal-439, Dhruma Formation, Sahtan Group: (a) Limestone-packstone/grainstone; and (b) Limestone-wackestone/mudstone (scale grid is 1 x 1 mm) (Mohammed et al., 1997).

Figure 8.11:

Ditch cuttings from Yibal-439, Dhruma Formation, Sahtan Group: (a) Limestone-packstone/grainstone; and (b) Limestone-wackestone/mudstone (scale grid is 1 x 1 mm) (Mohammed et al., 1997).

The Dhruma in Saih Rawl and Barik area wells contains some dolomite (see Saih Rawl-27, Figure 8.9).

Subsurface recognition: Whilst drilling, the interbedding with thin grey and grey-green shales towards the base is diagnostic.

The Rate of Penetration is generally faster in the upper part of the Dhruma. A negative drill break may occur at the upper boundary, followed by a distinctive positive break as the grainstones are penetrated (Yibal-236, Figure 8.12). Post-drilling the paleontological analyses and log correlation will confirm the presence of Dhruma sediments.

Figure 8.12:

Yibal-236 Rate of Penetration log, showing increased penetration rate in Dhruma grainstone (Mohammed et al., 1997). See Figure 8.1 for location.

Figure 8.12:

Yibal-236 Rate of Penetration log, showing increased penetration rate in Dhruma grainstone (Mohammed et al., 1997). See Figure 8.1 for location.

Boundaries: The upper boundary is conformable, marked only by a slight increase in the Gamma log. The lower boundary is distinct and conformable, with limestones resting directly on clastics of the Upper Mafraq. It is likely that the Mafraq-Dhruma transition is diachronous, as is the transition to the underlying Upper Mafraq sandstones (Rousseau et al., 2006). It is often (but not always) reflected by distinct shifts in Gamma and Density-Neutron log patterns.

Distribution: The Dhruma is confined to North and Central Oman and the northern part of South Oman. The Dhruma (and overlying Tuwaiq Mountain) Formation shows a substantial and regular thickening (60 to 550 m) in a (north) westwards direction. The thickest development is in the Lekhwair area.

Deposition: The Dhruma carbonates in Oman represent a shallow-marine depositional environment. The mud-supported and somewhat argillaceous lower and middle units contain mollusc and echinoderm debris, and thin-shelled lamellibranchs of ‘pelagic’ aspect; so a sub-wave-base shelf setting is likely. The grainy textured sediments, particularly those in the upper porous unit, indicate shallow-shoal to intertidal settings.

Age: Late Bajocian – Bathonian, ca. 168–165.5 Ma. Initially the macro- and microfossils from the Dhruma in Oman were thought to indicate a Middle to earliest Late Jurassic age. Hughes Clarke (1988) acknowledges a lack of precision in this interpretation and comparison with the better dated subsurface sequences in Saudi Arabia may have been the main line of evidence for stratigraphic position.

Rousseau et al. (2006) group the Dhruma and Tuwaiq Mountain formations within a general Bathonian – Callovian age episode. Sharland et al. (2001) place three of their MFS surfaces within the Dhruma Formation and illustrate them with respect to well Yibal-85. These are MFS J40 (Middle Callovian), MFS J30 (Early Bathonian) and MFS J20 (Early Bajocian), which they position in the uppermost, middle and lowermost Dhruma Formation respectively. They do acknowledge the difficulty in positioning both MFS J30 and MFS J20 (possibly also J40) in Omani outcrop sections and, in particular, the imprecise dating of the J20 surface in certain areas of the Arabian Plate.

Biostratigraphy: Biozone F45 (Pfenderina salernitana, P. trochoidea). Palynozone 4009 (Ctenidodinium cf. ornatum) has been noted in the upper Dhruma but generally the Formation is charectarised by Palynozone 4191 (Dichadogonyaulax spp.) and even down to 2262 (Sphaeropollenites scabratus). A Bathonian – Callovian age has been inferred from foraminiferal occurrences for the Dhruma of the Haushi area (Dubreuilh et al., 1992a), but Rousseau et al. (2006) argue that the assemblages can be reinterpreted as only Bajocian – Bathonian. They further suggest that the Dhruma-Tuwaiq Mountain boundary roughly approximates to the Bathonian – Callovian boundary.

Rousseau et al. (2005) note a middle Dhruma brachiopod-rich bed of Early Bathonian – Middle Callovian age in Al Jabal Al Akhdar.

Simon Petroleum Technology (1995) assigned Callovian ages to the Dhruma Formation based on micropalaeontology (Trocholina palastiniensis, Posidonia spp. and associated taxa), and palynology (Dichadogonyaulax sellwoodii). In particular in two wells they interpret Early Callovian – Bathonian palynological associations (e.g. Korystocysta gochtii, Wanaea cf. acollaris). Jacovides et al. (1998) note that both D. sellwoodii and K. gochtii range up into the overlying Tuwaiq Mountain Formation (supporting a general Callovian age for that Formation) but that they are particularly characteristic of the Dhruma. They also recorded significant numbers of proximate dinocysts (Valensiella spp., Sentusidinium spp., Escharisphaeridia spp., Durotrigia spp., and Dissiliodinium spp.) and rarer ostracods and foraminifera as evidence to support a Bathonian – Late Bajocian age. Osterloff and Penney (1999) recognise similar gonyaulacid/ctenidodinoid dinocyst assemblages in sidewall cores from two Qarn Nihayda area wells. Woollam et al. (1999) similarly use dinocysts to indicate a Late Bajocian – Bathonian age for the Dhruma in well Lekhwair-319.

As with the Tuwaiq Mountain Formation age assignment, significant uncertainties remain and further work is surely needed, but in the absence of definitive, well documented evidence, a general Late Bajocian – Bathonian age is preferred.

Mafraq Formation

Introduction

The mixed clastic-carbonate Mafraq Formation (Early – Middle Jurassic) is the lowermost unit of the Sahtan Group and comprises a fluvial to shallow-marine succession that onlaps the tilted and eroded Akhdar Group (Permian – Triassic) from the northwest to the southeast.

The base of the sequence is diachronous and comprises offshore, shallow-marine, coastal plain, fluvial and alluvial plain environments. The oldest, Early Jurassic, Mafraq sediments occur in the northwest (basinward) in the vicinity of Lekhwair where it is also thickest (ca. 150 m). In this area the Mafraq Formation can be subdivided into a carbonate Upper Mafraq Member and a clastic Lower Mafraq Member.

The Lower Mafraq is an Early Jurassic continental clastic to shallow-marine unit embracing those deposits laid down during the emergent phase between paralic Akhdar and marine Sahtan deposition. Following Rhaetian to Pliensbachian emergence the Mafraq constitutes the initial Early Jurassic clastic pulse at the onset of the marine transgression that ultimately leads to the fully marine Late Bajocian, Dhruma carbonates. The clastic to carbonate balance is very variable. Ferruginous ooids may be very common in the Upper Mafraq and the distinctive high Gamma, red claystones at the top of the Upper Mafraq may represent extensive soil development.

The Mafraq Formation thins to the southeast, where lithostratigraphic subdivision is difficult. There is an onlapping relationship and platform area sections with thin Mafraq almost certainly represent the youngest Upper Mafraq (based on log correlation with some supporting biostratigraphy). Here the ‘Oman Soil’ has the potential to become a composite unit with the possibility of Late Triassic (Minjur Formation), Early Jurassic (Lower Mafraq) and Middle Jurassic (Upper Mafraq) soils amalgamating into one thick soil package.

The Lower Mafraq is of variable composition but is usually clay-dominated. The lowermost clastic package on top of the carbonates of the Akhdar Group as used in the Oman subsurface is assigned to the Minjur Formation (see Akhdar Group Chapter).

Type and reference sections: Lekhwair-70 in North Oman (Figure 8.13) is proposed as a new subsurface type section. Mafraq-1 (the original type section of Hughes Clarke, 1988) is now interpreted to contain the new Minjur Formation (‘Oman Soil’), overlain by probably only Upper Mafraq. It is used here as an additional reference section (Figure 8.14), together with Saih Rawl-27 in North Oman (Figure 8.15) and Zauliyah-5 in Central Oman (Figure 8.16).

Figure 8.13:

(facing page): Composite electrical logs, lithology and lithological description of the Mafraq Formation, Sahtan Group, in well Lekhwair-70, North Oman (Mohammed et al., 1997). See Figure 8.1 for location.

Figure 8.13:

(facing page): Composite electrical logs, lithology and lithological description of the Mafraq Formation, Sahtan Group, in well Lekhwair-70, North Oman (Mohammed et al., 1997). See Figure 8.1 for location.

Figure 8.14:

Composite electrical logs, lithology and lithological description of the Mafraq Formation, Sahtan Group, in well Mafraq-1, North Oman (Mohammed et al., 1997). See Figure 8.1 for location.

Figure 8.14:

Composite electrical logs, lithology and lithological description of the Mafraq Formation, Sahtan Group, in well Mafraq-1, North Oman (Mohammed et al., 1997). See Figure 8.1 for location.

Figure 8.15:

Composite electrical logs, lithology and lithological description of the Mafraq Formation, Sahtan Group, in well Saih Rawl-27, North Oman (Mohammed et al., 1997). See Figure 8.1 for location.

Figure 8.15:

Composite electrical logs, lithology and lithological description of the Mafraq Formation, Sahtan Group, in well Saih Rawl-27, North Oman (Mohammed et al., 1997). See Figure 8.1 for location.

Figure 8.16:

Composite electrical logs, lithology and lithological description of the Mafraq Formation, Sahtan Group, in well Zauliyah-5, Central Oman (Mohammed et al., 1997). See Figure 8.1 for location.

Figure 8.16:

Composite electrical logs, lithology and lithological description of the Mafraq Formation, Sahtan Group, in well Zauliyah-5, Central Oman (Mohammed et al., 1997). See Figure 8.1 for location.

Lithology: The Lower Mafraq consists of flood-plain to shallow-marine sandstones and shales. The ease of well to well correlation varies and these sands probably represent both amalgamated and single fluvial channel fills.

The Upper Mafraq is more carbonate-dominated, capped by distinctive, mainly red shales which are interpreted to be soils.

On platform areas where the Mafraq cannot be subdivided, it is a thin unit with clastics, subordinate carbonates and soils.

Subsurface recognition: Whilst drilling, the Mafraq is picked on the incoming of the first clastics below the Dhruma limestones (Figure 8.17). In the Lekhwair and adjacent areas the top is usually marked by reddish shales/siltstone (soils) underlying the Dhruma limestones. Elsewhere it is picked at the first occurrence of sandstone. It usually has a positive drill break at the upper boundary. The characteristic lithology of the Mafraq consists of grey limestones, sandstones and shales in varying proportions.

Figure 8.17:

Ditch cuttings from the Mafraq Formation, Sahtan Group: (a) Sandstone from Saih Rawl-17; and (b) Shale from Lekhwair-70 (scale grid is 1 x 1 mm) (Mohammed et al., 1997).

Figure 8.17:

Ditch cuttings from the Mafraq Formation, Sahtan Group: (a) Sandstone from Saih Rawl-17; and (b) Shale from Lekhwair-70 (scale grid is 1 x 1 mm) (Mohammed et al., 1997).

Post-drilling the log patterns may or may not show a clear break at the upper boundary, depending on the lithology below the essentially clastic free Dhruma limestones. There may be a negligible change in Gamma readings between sandstone or carbonate cemented sandstones, but a sharp increase in Gamma readings when entering shale. In many wells there is a coincident Dipmeter break, which is not to be used in isolation. There is also a slight decrease in Resistivity. The lower boundary is marked by an abrupt change to the carbonates of the Akhdar Group or a more subtle change to the Minjur Formation, with its paleosol and lignitic intervals. In the latter case palynology can help pick the top of the Triassic Minjur Formation.

Boundaries: The clastics of the Mafraq Formation may lie unconformably upon the Minjur, Jilh, Sudair or Khuff formations in the Akhdar Group, or on the Haushi Group. Where the Mafraq clastics lie upon clastic levels (this is most often the Minjur and the Sudair), determination of the boundary is assisted by palynological study. Regional mapping shows this boundary to be a low-angle unconformity.

The upper boundary is usually conformable (with the Dhruma Formation) and is placed at the top of the uppermost interval with significant clastic content. Lateral variability of the clastic (mainly thin sand) units and depositional geometries from well-log correlations indicate this boundary is diachronous with onlapping stratal geometries towards the east and southeast.

Distribution: The Mafraq Formation extends over most of North and Central Oman, albeit thinly developed on platform highs. It is absent over most of South Oman.

Deposition: The Lower Mafraq is a mixed continental-marine unit with an overall strong terrestrial character and is virtually restricted to the Lekhwair-Dhulaima area. It contains a significant marine pulse of Early Toarcian age (Jacovides et al., 1998). Rousseau et al. (2006) describe the Lower Mafraq as a wedge of floodplain deposits that onlaps to the east and southeast. Based on reinterpretation of Jacovides et al. (1998) data it appears that Lower Mafraq deposits do not usually extend onto platform areas south of the greater Lekhwair-Dhulaima area. Thin pockets of deposition may occur elsewhere, e.g. possible occurrence in the Al Huwaisah area. In this respect a better understanding of the Lower Mafraq soils has been of key importance, prompting the reassignment of the Lowermost Mafraq as previously used in the subsurface to the Minjur Formation of the Akhdar Group (van Steenwinkel, 2009). In the final Early Toarcian Lower Mafraq package (e.g. the high Gamma shale in Lekhwair-70, Figure 8.13) influxes of marine taxa suggest significant marine influence. The presence of associated amorphous kerogen highlights bottom water anoxia and limited current activity.

The Upper Mafraq is a shallow-marine sequence with significant macro- and microfossil content. Interestingly the dominantly red shales at the top of the Upper Mafraq occasionally yield very low-diversity miospore assemblages with high counts of fungal debris. This supports the interpretation that these may be soil related.

Rousseau et al. (2006) discuss at length the geometries and development of the Lower and Upper Mafraq units, however, Jacovides et al. (1998) suggest that the base Upper Mafraq should be placed at the base of the dolomite unit in their key well Lekhwair-319. This would place the Lower Mafraq Limestone Marker of Rousseau et al. (2006) within the Upper Mafraq and negate their correlation of this unit across the region. In this interpretation a south-easterly directed onlapping geometry still applies but there is also a clear expansion in the Upper Mafraq section, in particular, towards the northwest.

Subdivision: In northwest Oman the Mafraq is very thick and can be subdivided into the Lower and Upper Mafraq (see Lekhwair-70, Figure 8.13).

Age: Early – Middle Jurassic, Late Pliensbachian – Late Bajocian, ca. 187–168 Ma. This is based mainly on reinterpretation of the work of Kharusi (1989) and Jacovides et al. (1998), see discussion below.

Sharland et al. (2001) place their Middle Toarcian J10 MFS towards the top of their Lower Mafraq in well Yibal-85 (at ca. 2,472 m in Figure 8.8) and within a general Mafraq Formation in well Mafraq-1 (at ca. 676 m in Figure 8.14). In the former well PDO interpret their upper Mafraq as Dhruma and the red, high Gamma claystone (their Lower Mafraq) as Upper Mafraq (see Figure 8.8).

Biostratigraphy: The Mafraq Formation spans Biozones F45 (Pfenderina salernitana, P. trochoidea) to F30 (Haurania deserta-Orbitopsella praecursor-Pseudocyclammina liassica) and Palynozones 2262 (Spheropollenites scabratus) and 2242 (Corollina spp.). The Upper Mafraq section may also yield Palynozone 4191 (Dichadogonyaulax spp.).

Rousseau et al. (2006) interpret the report by Roger et al. (1992) of moulds of the ammonite Thambites cf. planus as indicating a Late Bajocian age for the Upper Mafraq in the Al Huqf outcrop area. They mention an Early Pliensbachian age (using Biozone F30 marker Orbitopsella praecursor) for the Lower Mafraq, Lithiotis Limestone in Al Jabal Al Akhdar (from Rabu, 1987).

Work by Jacovides et al. (1998) and subsequent additional work and reinterpretation by Osterloff (in Osterloff and Penney, 1999) provides a large data set, which requires detailed and consistent interpretation. Interpretation of the palynological signatures becomes difficult because of the general paralic nature of the sequence. It has become clear that Palynozones 2262 and 2242 are largely facies dependent. Time-significant zonal boundaries are dependent on the recognition of the more marine sections and floodings, which have more reliable dinocyst and acritarch marker species. The two good tie points are the Upper Mafraq, Late Bajocian Palynozone 4191 (ctenidodinoid dinocyst assemblage) and the Lower Mafraq, Early Toarcian, Palyno-subzone 2262A (sphaeromorph clusters and related taxa).

Carbonates from the Upper Mafraq (Yibal-440, Natih-82) have also yielded the dinocyst Nannoceratopsis gracilis in association with the pollen Callialaspories spp. The latter does not range below the Late Toarcian and these rare occurrences of N. gracilis below definitive Late Bajocian (Palynozone 4191) are thought to be themselves related to Bajocian deposition (not Early Toarcian, or indeed Lower Mafraq as previously interpreted by Jacovides et al., 1998).

The high Gamma shale in the upper part of the Lower Mafraq (Lekhwair-70, Figure 8.13) is characterised by abundant Spheripollenites spp., and significantly by abundant amorphous organic material and relatively frequent prasinophyceaen algae, including sphaeromorph clusters. Such an assemblage is interpreted to be Early Toarcian and corresponds to a worldwide flooding with similar palynological yield. Elements of this assemblage range up into the lower part of the Upper Mafraq. This section equates to the Marrat Formation, elsewhere in Arabia and provides good evidence for the regional Toarcian datum discussed in Al-Husseini (1997). It corresponds to the J10 MFS of Sharland et al. (2001). This Early Toarcian marine event can be correlated around the Lekhwair-Dhulaima area but does not appear to extend onto platform areas to the south and west.

The more terrestrially dominated sections below this Early Toarcian shale probably range down into the Late Pliensbachian.

The interpretations noted above indicate that the correlations by Rousseau et al. (2006), in particular with respect to their Lower Mafraq Limestone Marker, between Lekhwair and Yibal and Natih wells are not correct.

This proposed stratigraphy (here including Minjur Formation) can be summarised as follows:

Formation/UnitPalynological ZoneRelative age
Upper Mafraq4191Late Bajocian
Unzoned *Toarcian – Bajocian
Lower Mafraq2262AEarly Toarcian
Unzoned *Late Pliensbachian – Early Toarcian
(missing section) (Early Pliensbachian – Hettangian)
Minjur2255Rhaetian
Formation/UnitPalynological ZoneRelative age
Upper Mafraq4191Late Bajocian
Unzoned *Toarcian – Bajocian
Lower Mafraq2262AEarly Toarcian
Unzoned *Late Pliensbachian – Early Toarcian
(missing section) (Early Pliensbachian – Hettangian)
Minjur2255Rhaetian
*

Indicates that Palyno-subzone 2262B and Palynozone 2242 should not be used due to facies dependency.

The age dates suggested fit well with the proposed sequence stratigraphic scheme of Al-Husseini (1997) and the broad interpretations of Rousseau et al. (2006). However, although both schemes indicate the absence of Aalenian deposits, this is something that cannot be proven from current Oman well data, i.e. the Upper Mafraq may contain one or more significant hiatus.

Figures & Tables

Figure 8.1:

Location map: Sahtan Group.

Figure 8.1:

Location map: Sahtan Group.

Figure 8.2:

Composite electrical logs, lithology and lithological description of the Jubaila/Hanifa/Tuwaiq Mountain formations, Sahtan Group, in well Butabul-1, Central Oman (Mohammed et al., 1997). See Figure 8.1 for location.

Figure 8.2:

Composite electrical logs, lithology and lithological description of the Jubaila/Hanifa/Tuwaiq Mountain formations, Sahtan Group, in well Butabul-1, Central Oman (Mohammed et al., 1997). See Figure 8.1 for location.

Figure 8.3:

Ditch cuttings from the Sahtan Group: (a) Limestone-mudstone from Ramlat-1, Jubaila Formation; (b) Limestone-mudstone (aphanitic) from Lekhwair-85, Hanifa Formation; and (c) Limestone-packstone/grainstone from Ramlat-1, Hanifa Formation (scale grid is 1 x 1 mm) (Mohammed et al., 1997).

Figure 8.3:

Ditch cuttings from the Sahtan Group: (a) Limestone-mudstone from Ramlat-1, Jubaila Formation; (b) Limestone-mudstone (aphanitic) from Lekhwair-85, Hanifa Formation; and (c) Limestone-packstone/grainstone from Ramlat-1, Hanifa Formation (scale grid is 1 x 1 mm) (Mohammed et al., 1997).

Figure 8.4:

Composite electrical logs, lithology and lithological description of the Hanifa/Tuwaiq Mountain formations, Sahtan Group, in well Lekhwair-27, North Oman (Mohammed et al., 1997). See Figure 8.1 for location.

Figure 8.4:

Composite electrical logs, lithology and lithological description of the Hanifa/Tuwaiq Mountain formations, Sahtan Group, in well Lekhwair-27, North Oman (Mohammed et al., 1997). See Figure 8.1 for location.

Figure 8.5:

Composite electrical logs, lithology and lithological description of the Hanifa Formation, Sahtan Group, in well Dhulaima-7, North Oman (Mohammed et al., 1997). See Figure 8.1 for location.

Figure 8.5:

Composite electrical logs, lithology and lithological description of the Hanifa Formation, Sahtan Group, in well Dhulaima-7, North Oman (Mohammed et al., 1997). See Figure 8.1 for location.

Figure 8.6:

Composite electrical logs and lithology of the Jubaila/Hanifa/Tuwaiq Mountain formations, Sahtan Group, in well Hasirah-4, Central Oman (Mohammed et al., 1997). See Figure 8.1 for location.

Figure 8.6:

Composite electrical logs and lithology of the Jubaila/Hanifa/Tuwaiq Mountain formations, Sahtan Group, in well Hasirah-4, Central Oman (Mohammed et al., 1997). See Figure 8.1 for location.

Figure 8.7:

Ditch cuttings from the Sahtan Group: (a) Dolomite from Ramlat-1, Hanifa Formation; (b) Limestonegrainstone from Lekhwair-27, Tuwaiq Mountain Formation; and (c) Limestone-wackestone/mudstone from Yibal-439, Tuwaiq Mountain Formation (scale grid is 1 x 1 mm) (Mohammed et al., 1997).

Figure 8.7:

Ditch cuttings from the Sahtan Group: (a) Dolomite from Ramlat-1, Hanifa Formation; (b) Limestonegrainstone from Lekhwair-27, Tuwaiq Mountain Formation; and (c) Limestone-wackestone/mudstone from Yibal-439, Tuwaiq Mountain Formation (scale grid is 1 x 1 mm) (Mohammed et al., 1997).

Figure 8.8:

Composite electrical logs, lithology and lithological description of the Dhruma Formation, Sahtan Group, in well Yibal-85, North Oman (Mohammed et al., 1997). See Figure 8.1 for location.

Figure 8.8:

Composite electrical logs, lithology and lithological description of the Dhruma Formation, Sahtan Group, in well Yibal-85, North Oman (Mohammed et al., 1997). See Figure 8.1 for location.

Figure 8.9:

Composite electrical logs and lithology of the Dhruma Formation, Sahtan Group, in well Saih Rawl-27, North Oman (Mohammed et al., 1997). See Figure 8.1 for location.

Figure 8.9:

Composite electrical logs and lithology of the Dhruma Formation, Sahtan Group, in well Saih Rawl-27, North Oman (Mohammed et al., 1997). See Figure 8.1 for location.

Figure 8.10:

Composite electrical logs, lithology and lithological description of the Dhruma Formation, Sahtan Group, in well Zauliyah-5, Central Oman (Mohammed et al., 1997). See Figure 8.1 for location.

Figure 8.10:

Composite electrical logs, lithology and lithological description of the Dhruma Formation, Sahtan Group, in well Zauliyah-5, Central Oman (Mohammed et al., 1997). See Figure 8.1 for location.

Figure 8.11:

Ditch cuttings from Yibal-439, Dhruma Formation, Sahtan Group: (a) Limestone-packstone/grainstone; and (b) Limestone-wackestone/mudstone (scale grid is 1 x 1 mm) (Mohammed et al., 1997).

Figure 8.11:

Ditch cuttings from Yibal-439, Dhruma Formation, Sahtan Group: (a) Limestone-packstone/grainstone; and (b) Limestone-wackestone/mudstone (scale grid is 1 x 1 mm) (Mohammed et al., 1997).

Figure 8.12:

Yibal-236 Rate of Penetration log, showing increased penetration rate in Dhruma grainstone (Mohammed et al., 1997). See Figure 8.1 for location.

Figure 8.12:

Yibal-236 Rate of Penetration log, showing increased penetration rate in Dhruma grainstone (Mohammed et al., 1997). See Figure 8.1 for location.

Figure 8.13:

(facing page): Composite electrical logs, lithology and lithological description of the Mafraq Formation, Sahtan Group, in well Lekhwair-70, North Oman (Mohammed et al., 1997). See Figure 8.1 for location.

Figure 8.13:

(facing page): Composite electrical logs, lithology and lithological description of the Mafraq Formation, Sahtan Group, in well Lekhwair-70, North Oman (Mohammed et al., 1997). See Figure 8.1 for location.

Figure 8.14:

Composite electrical logs, lithology and lithological description of the Mafraq Formation, Sahtan Group, in well Mafraq-1, North Oman (Mohammed et al., 1997). See Figure 8.1 for location.

Figure 8.14:

Composite electrical logs, lithology and lithological description of the Mafraq Formation, Sahtan Group, in well Mafraq-1, North Oman (Mohammed et al., 1997). See Figure 8.1 for location.

Figure 8.15:

Composite electrical logs, lithology and lithological description of the Mafraq Formation, Sahtan Group, in well Saih Rawl-27, North Oman (Mohammed et al., 1997). See Figure 8.1 for location.

Figure 8.15:

Composite electrical logs, lithology and lithological description of the Mafraq Formation, Sahtan Group, in well Saih Rawl-27, North Oman (Mohammed et al., 1997). See Figure 8.1 for location.

Figure 8.16:

Composite electrical logs, lithology and lithological description of the Mafraq Formation, Sahtan Group, in well Zauliyah-5, Central Oman (Mohammed et al., 1997). See Figure 8.1 for location.

Figure 8.16:

Composite electrical logs, lithology and lithological description of the Mafraq Formation, Sahtan Group, in well Zauliyah-5, Central Oman (Mohammed et al., 1997). See Figure 8.1 for location.

Figure 8.17:

Ditch cuttings from the Mafraq Formation, Sahtan Group: (a) Sandstone from Saih Rawl-17; and (b) Shale from Lekhwair-70 (scale grid is 1 x 1 mm) (Mohammed et al., 1997).

Figure 8.17:

Ditch cuttings from the Mafraq Formation, Sahtan Group: (a) Sandstone from Saih Rawl-17; and (b) Shale from Lekhwair-70 (scale grid is 1 x 1 mm) (Mohammed et al., 1997).

GROUPFORMATIONMEMBER
SahtanJubaila 
Hanifa 
Tuwaiq Mountain 
Dhruma 
MafraqUpper Mafraq
Lower Mafraq
GROUPFORMATIONMEMBER
SahtanJubaila 
Hanifa 
Tuwaiq Mountain 
Dhruma 
MafraqUpper Mafraq
Lower Mafraq
ZoneSubzoneMarker speciesRelative ageFormation
4214 Systematophora cf. areolataLate Oxfordian – Early TithonianHanifa – Jubaila
4009 Ctenidodinium cf. ornatumLate Bathonian – Middle OxfordianDhruma – Tuwaiq
4009BCtenidodinium cf. ornatum(Middle Oxfordian)
4009A(downhole decrease S. cf. areolata, increase C. cf. ornatum)(Late Bathonian – Early Oxfordian)
4191 Dichadogonyualax spp.Late Bajocian – ‘middle’ BathonianMafraq – Dhruma
2262 Spheripollenites scabratusEarly Toarcian – Early Bajocian
2262B**Spheripollenites scabratus(Early Toarcian – Early Bajocian)
2262A(absence Callialasporites spp.)(Early Toarcian)
2242** Corollina spp.?Hettangian – PliensbachianMafraq
2255* Bartenia communisCarnian – ? RhaetianMafraq*
2247Vesicaspora schemili(Norian - ?Rhaetian)
2241Partitisporites verrucosus(Carnian – Norian)
ZoneSubzoneMarker speciesRelative ageFormation
4214 Systematophora cf. areolataLate Oxfordian – Early TithonianHanifa – Jubaila
4009 Ctenidodinium cf. ornatumLate Bathonian – Middle OxfordianDhruma – Tuwaiq
4009BCtenidodinium cf. ornatum(Middle Oxfordian)
4009A(downhole decrease S. cf. areolata, increase C. cf. ornatum)(Late Bathonian – Early Oxfordian)
4191 Dichadogonyualax spp.Late Bajocian – ‘middle’ BathonianMafraq – Dhruma
2262 Spheripollenites scabratusEarly Toarcian – Early Bajocian
2262B**Spheripollenites scabratus(Early Toarcian – Early Bajocian)
2262A(absence Callialasporites spp.)(Early Toarcian)
2242** Corollina spp.?Hettangian – PliensbachianMafraq
2255* Bartenia communisCarnian – ? RhaetianMafraq*
2247Vesicaspora schemili(Norian - ?Rhaetian)
2241Partitisporites verrucosus(Carnian – Norian)
*

Now included in the Minjur Formation.

**

Strong facies dependency, see Mafraq Formation, Biostratigraphy discussion.

FormationMicropalaeontological ZonePalynological ZoneRelative age
JubailaF47 (Sub-biozones F479 - F475)4214late Kimmeridgian – Early Tithonian
HanifaF47 (F475)4214Kimmeridgian
Tuwaiq MountainF47-45 (F475, F473, upper F457)4009 - 4191Late Bathonian – Callovian
DhrumaF45 (F457 - F455)(?4009) 4191 - 2262Late Bajocian – Bathonian
Upper MafraqF45 (F455 - ?F453)4191/2262B - 2262B*Toarcian – Late Bajocian
Lower MafraqF302262A - 2242*Late Pliensbachian – Early Toarcian
FormationMicropalaeontological ZonePalynological ZoneRelative age
JubailaF47 (Sub-biozones F479 - F475)4214late Kimmeridgian – Early Tithonian
HanifaF47 (F475)4214Kimmeridgian
Tuwaiq MountainF47-45 (F475, F473, upper F457)4009 - 4191Late Bathonian – Callovian
DhrumaF45 (F457 - F455)(?4009) 4191 - 2262Late Bajocian – Bathonian
Upper MafraqF45 (F455 - ?F453)4191/2262B - 2262B*Toarcian – Late Bajocian
Lower MafraqF302262A - 2242*Late Pliensbachian – Early Toarcian
*

Strong facies dependency, see Mafraq Formation, Biostratigraphy discussion.

Formation/UnitPalynological ZoneRelative age
Upper Mafraq4191Late Bajocian
Unzoned *Toarcian – Bajocian
Lower Mafraq2262AEarly Toarcian
Unzoned *Late Pliensbachian – Early Toarcian
(missing section) (Early Pliensbachian – Hettangian)
Minjur2255Rhaetian
Formation/UnitPalynological ZoneRelative age
Upper Mafraq4191Late Bajocian
Unzoned *Toarcian – Bajocian
Lower Mafraq2262AEarly Toarcian
Unzoned *Late Pliensbachian – Early Toarcian
(missing section) (Early Pliensbachian – Hettangian)
Minjur2255Rhaetian
*

Indicates that Palyno-subzone 2262B and Palynozone 2242 should not be used due to facies dependency.

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