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GROUPFORMATIONMEMBER
WasiaNatih*Natih A
Natih B
Natih C
Natih D
Natih E
Natih F
Natih G
Nahr UmrUpper Nahr Umr
Marker Limestone
Lower Nahr Umr
GROUPFORMATIONMEMBER
WasiaNatih*Natih A
Natih B
Natih C
Natih D
Natih E
Natih F
Natih G
Nahr UmrUpper Nahr Umr
Marker Limestone
Lower Nahr Umr
*

Natih ‘letter’ members are informal.

Authors: Steineke (unpublished, 1938) as Wasia Formation, see van Bellen et al. (1959/2005) and Powers (1968). Elevated to Group level in Owen and Nasr (1958).

Introduction

The Wasia Group is widely used throughout the Gulf area to cover the package of sediments between two regional hiatus/disconformities; i.e. those between the Shu’aiba and Nahr Umr, and a more marked break beneath the Upper Cretaceous, usually shaly, Aruma Group sediments. The Wasia is divided into several units over most of the Gulf area, but in Oman only the lowest of those formations, the Nahr Umr, is maintained as a characteristic argillaceous basal unit to the Group, whereas the upper, mainly carbonate, sequence is assigned to a single unit, the Natih Formation.

Following uplift along eastern Oman, probably associated with a Neo-Tethys rift shoulder, the exposed area was eroded, successively cutting deeper towards the southeast. Oman, at that time, was geographically located in the humid equatorial belt and erosion was associated with karstification of the underlying Shu’aiba carbonates. There followed a major supply of siliciclastics into the northern Arabian Gulf areas (Beydoun, 1995), where (reservoir) sandstones were deposited. These are represented in Oman by the distal shale/marl deposits of the the Nahr Umr Formation. The (up to 150 m) thick Nahr Umr shales provide a world-class regional seal for the hydrocarbons in the Shu’aiba and older formations. Closer to the shelf-margin settings, towards the Neo-Tethys Ocean in the northeast (the area of the present-day Al Hajar Mountains) and south east (Masirah Trough and Dhofar regions, see Enclosure 2a), the Nahr Umr sequence consists of marly and argillaceous carbonates. Cyclic carbonates of the Natih Formation overlie the Nahr Umr sediments, some of which have argillaceous source rock bearing intervals. Sedimentation was terminated by a regional emergence causing erosion and leaching of the Natih, and locally older carbonates (see also Figure 7.2).

Type and reference sections: The type section is not specified, but implied to be in southern Iraq and Kuwait (van Bellen et al., 1959/2005). Reference sections occur in Oman in southern Al Jabal Al Akhdar wadis, e.g. Wadi Al Muaydin (Figure 6.1, see Homewood et al., 2008).

Figure 6.1:

Location map: Wasia Group.

Figure 6.1:

Location map: Wasia Group.

Distribution: Occurs throughout Oman, showing a uniform thinning towards the southeast, e.g. onto the Al Huqf axis and the Eastern Flank high. In the extreme south and east can locally be absent by overstep of Cenozoic units.

Sequence Stratigraphy: Upper part of Megasequence AP8 (Sharland et al., 2001). Sharland et al. (2001) correlate their MFS K140 to K90 surfaces into the Wasia Group in Oman (see formational discussions).

Age: Early – Late Cretaceous, latest Aptian – Middle Turonian, ca. 113–91 Ma.

Biostratigraphy: Biozone F62 (Dohaia planata, Dictyoconella minima) to F57, Sub-biozones F573/571 (Cytherelloidea IRR29/Orbitolina texana, Spineloberis IRK20), are recognised. The zonation and ages are based on Sikkema (1991). Packer (2004) revised Sub-biozones F575 and F573 and defined Sub-biozone 571. Potential Middle Turonian Natih Formation and latest Aptian Nahr Umr Formation ages are not reflected in the current zonation. See formational discussions.

ZoneSubzoneMarker speciesRelative age*Formation/Unit
F62 Dohaia planata, Dictyoconella minimaLate Cenomanian - Early TuronianNatih A and B Members
F61 Praealveolina cretacea cretacea, Trocholina lenticularisMiddle to Late CenomanianNatih C and D Members
  Orbitolina concava, Hemicyclammina whiteiAlbian – Early CenomanianNahr Umr Formation – Natih E Member
 F579Orbitolina concava, Hemicyclammina whitei(Early Cenomanian)Natih E Member
F57F577Hedbergella washitensis(Late Albian – Early Cenomanian)Natih E (base) – G Members
F575Trocholina altispira, Pseudocyclammina hedbergi(Middle – Late Albian) 
 F573**Orbitolina texana(Early – Middle Albian)Nahr Umr Formation
 F571**Spineloberis IRK20, Cytherelloidea IRR29(Early Albian) 
ZoneSubzoneMarker speciesRelative age*Formation/Unit
F62 Dohaia planata, Dictyoconella minimaLate Cenomanian - Early TuronianNatih A and B Members
F61 Praealveolina cretacea cretacea, Trocholina lenticularisMiddle to Late CenomanianNatih C and D Members
  Orbitolina concava, Hemicyclammina whiteiAlbian – Early CenomanianNahr Umr Formation – Natih E Member
 F579Orbitolina concava, Hemicyclammina whitei(Early Cenomanian)Natih E Member
F57F577Hedbergella washitensis(Late Albian – Early Cenomanian)Natih E (base) – G Members
F575Trocholina altispira, Pseudocyclammina hedbergi(Middle – Late Albian) 
 F573**Orbitolina texana(Early – Middle Albian)Nahr Umr Formation
 F571**Spineloberis IRK20, Cytherelloidea IRR29(Early Albian) 
*

Provisional based on Sikkema (1991), many uncertainties remain.

**

Subzones are in part coeval and facies restricted – see Nahr Umr text for discussion.

Natih Formation

Authors: Gigon (unpublished, 1967), see Hughes Clarke (1988).

Introduction

The Natih Formation is recognised across most of the Arabian Gulf. It forms the interior part of an extensive carbonate platform that covered the eastern margin of the Arabian Shield during the mid Cretaceous (see Figure 7.2).

The Formation consists of a number of repetitive sedimentary cycles ranging from several tens to 150 m in thickness and these have been used to subdivide seven informal members designated by the letters А–G from top to base (Hughes Clarke, 1988; Scott, 1990). Philip et al. (1995) proposed a correlation of these informal members to the outcrop sections in the Oman Mountains.

Each member represents a sedimentary cycle: from basal, generally thin, variably argillaceous mud-supported units followed by thick shallowing-upward carbonates ending with shell-rich or pelletal-skeletal, grain-supported units. Superimposed on these cycles two to five, third-order, generally shoaling-upward sequences have been distinguished (Scott, 1990; van Buchem et al., 2002; Droste and van Steenwinkel, 2004; Homewood et al., 2008). The Natih Formation records the transition of the Arabian Gulf from a tectonically stable shelf margin to a flexural basin, subsiding in response to the westward obduction of an accretionary prism (Semail/Hawasina).

Type and reference sections: Fahud-3, drilled as Fahud North-3 in North Oman (Figures 6.1 and 6.2). The surface reference section is in Wadi Al Muaydin, with a thickness of 344 m. Additional reference sections are Suwaihat-1 in Central Oman (Figure 6.3), and Nimr-1 in South Oman (Figure 6.4).

Figure 6.2:

Composite electrical logs, lithology and lithological description of the Natih Formation, Wasia Group, in well Fahud-3, North Oman (Mohammed et al., 1997). See Figure 6.1 for location.

Figure 6.2:

Composite electrical logs, lithology and lithological description of the Natih Formation, Wasia Group, in well Fahud-3, North Oman (Mohammed et al., 1997). See Figure 6.1 for location.

Figure 6.3:

Composite electrical logs, lithology and lithological description of the Natih Formation, Wasia Group, in well Suwaihat-1, Central Oman (Mohammed et al., 1997). See Figure 6.1 for location.

Figure 6.3:

Composite electrical logs, lithology and lithological description of the Natih Formation, Wasia Group, in well Suwaihat-1, Central Oman (Mohammed et al., 1997). See Figure 6.1 for location.

Figure 6.4:

Composite electrical logs, lithology and lithological description of the Natih Formation, Wasia Group, in well Nimr-1, South Oman (Mohammed et al., 1997). See Figure 6.1 for location.

Figure 6.4:

Composite electrical logs, lithology and lithological description of the Natih Formation, Wasia Group, in well Nimr-1, South Oman (Mohammed et al., 1997). See Figure 6.1 for location.

Lithology: An interbedded sequence of chalky limestones and calcareous shales subdivided into members, each comprising an upper limestone overlying a basal shale (Figures 6.5. and 6.6). The shales are distinctive, varicoloured olive brown and green, which is unlike other shales but identical to the Nahr Umr in general fauna and lithology. Source-rock facies (bituminous limestones) occur in the B and E members in the North and north Central Oman (van Buchem et al., 1996; also Homewood et al., 2008).

Figure 6.5:

Ditch cuttings from source rock Natih A and B Members, Wasia Group: (a) limestone-wackestone from Habiba-2; (b) limestone-wackestone from Fahud-7; and (c) limestone-wackestone/mudstone from Barik-10 (scale grid is 1 x 1 mm) (Mohammed et al., 1997).

Figure 6.5:

Ditch cuttings from source rock Natih A and B Members, Wasia Group: (a) limestone-wackestone from Habiba-2; (b) limestone-wackestone from Fahud-7; and (c) limestone-wackestone/mudstone from Barik-10 (scale grid is 1 x 1 mm) (Mohammed et al., 1997).

Figure 6.6:

Ditch cuttings from the Wasia Group: (a) shale, Natih D, from Habiba-2; (b) limestone-wackestone, Natih E source rock, from Habiba-2; and (c) shale, Nahr Umr, from Barik-10 (scale grid is 1 x 1 mm) (Mohammed et al., 1997).

Figure 6.6:

Ditch cuttings from the Wasia Group: (a) shale, Natih D, from Habiba-2; (b) limestone-wackestone, Natih E source rock, from Habiba-2; and (c) shale, Nahr Umr, from Barik-10 (scale grid is 1 x 1 mm) (Mohammed et al., 1997).

Subsurface recognition: Whilst drilling the limestones are generally recognised by negative drillbreaks. The source rock intervals are generally a variegated brown colour (Figures 6.5 and 6.6) and quite distinctive in cuttings. The near top Natih E is identified by the top occurrence of Orbitolina concava (Figure 6.7), a species which is easily identified in cuttings. The varicoloured olive brown and green shale colouration (Figure 6.6) is also distinctive. The Natih limestones are off-white/light grey, clean and chalky in contrast to the vuggy, highly fossiliferous and pale yellow brown limestones of the Umm er Radhuma Formation and the frequently more argillaceous limestones of the Arada Member of the Fiqa Formation. However, they may still be difficult to separate. The shales and limestones are similar to the Nahr Umr and are impossible to differentiate at the wellsite.

Figure 6.7:

Fossils from the Natih E Member and Nahr Umr Formation: (a-c) Common, abundant and sometimes loose, Orbitolina concava from the Albian – Lower Cenomanian Biozone F57; (d and e) Common and sometimes loose, Hemicyclammina whitei from the Albian – Lower Cenomanian Biozone F57; and (f) Common and sometimes loose, Trocholina altispira from the Albian – Lower Cenomanian Biozone F57 (Mohammed et al., 1997).

Figure 6.7:

Fossils from the Natih E Member and Nahr Umr Formation: (a-c) Common, abundant and sometimes loose, Orbitolina concava from the Albian – Lower Cenomanian Biozone F57; (d and e) Common and sometimes loose, Hemicyclammina whitei from the Albian – Lower Cenomanian Biozone F57; and (f) Common and sometimes loose, Trocholina altispira from the Albian – Lower Cenomanian Biozone F57 (Mohammed et al., 1997).

The post-drilling recognition can be based on log correlation (with expanded gamma scale if logged through casing). The source-rock intervals have high Gamma, low velocity and high Resistivity. The top of an individual Natih member is routinely picked at the top of the limestone of the limestone/shale pair clearly visible on the gamma log. The A, B, C and D members are faunally quite distinct from the E, F and G members.

Boundaries: The lower boundary is usually conformable and transitional.

The upper contact of the Wasia Group is marked by a regional hiatus characterised by intense erosion, karstification and, in places deep incision of the Natih Formation (Grélaud et al., 2006) as well as in the coeval Mishrif Formation across most of the eastern Arabian Gulf (Alsharhan and Nairn, 1993; Alsharhan, 1995). Current datings suggest a likely Mid-Late Turonian (possibly into Early Coniacian) age for this downcutting event. In Oman, the shaly units of the Aruma Group disconformably overlie the Natih carbonates, which, over structural highs and to the south and east, have varying amounts of the upper cycles truncated.

Distribution: The Natih Formation occurs throughout Oman, although truncated strongly to completely over local positive features, e.g. absent in the Lekhwair area. The lowermost two members; Natih F to Natih G are considered equivalent to the Mauddud Formation, whilst the overlying members Natih E to Natih A are the lateral equivalent to the large scale shallowing upwards succession identified within the Shilaif (Khatiyah) and Mishrif formations (van Buchem et al., 1996).

Deposition: In the Oman area the depositional setting of the Natih Formation alternated between an extremely broad and flat, submerged-to-emergent platform setting (G, F, D and C members), and moderately deeper-water, intra-shelf basins, with anoxic conditions allowing deposition of source rocks (E, B and A members) (van Buchem et al., 1996, 2002; Homewood et al., 2008). Droste and van Steenwinkel (2004) provide a detailed seismostratigraphic underpinned model for the development of the Natih intra-shelf carbonates. Further studies focussing on the Oman outcrops were published by Schwab et al. (2005), building a seismic model based on outcrop data, and Grélaud et al. (2006) documenting incision surfaces in the Natih. The outcrop sedimentology in Oman is extensively documented by Homewood et al. (2008).

Subdivision: The Natih Formation is subdivided into informal members A to G, each comprising an upper limestone overlying a basal shale. In North and Central Oman the top Natih E is equivalent to the Natih E Carbonate. However, in South Oman a thick shale package is developed above the thick carbonate and referred to as the Natih E Shale.

Age: Late Albian to Early – (locally) Middle Turonian, ca. 104–91 Ma. Sharland et al. (2001) place their MFS K140 to K110 surfaces within the Natih Formation. See Homewood et al. (2008) for a brief discussion of ages (their figure 5). Bulot et al. (in preparation) place the Albian – Cenomanian boundary towards the base of Natih E. Jacovides and Varol (2000), however, extend the Upper Albian as high as Natih C, where they update the microfaunal zonation of Sikkema (1991) and propose new datings, based primarily on nannofossil data. Further work is therefore required to reconcile ammonite, microfaunal, and nannofossil data.

Ammonite findings discussed in Kennedy and Simmons (1991) and van Buchem et al. (1996) indicate an intra-Natih A, Early Turonian age from the deeper-water settings of the western Adam Foothills (near the southern limit of the Al Hajar Mountains in the Afar area). Bulot et al. (in preparation) similarly place the Cenomanian – Turonian boundary within Natih A, at Wadi Qusaybah. Note, however, that Sharland et al. (2001) argue for a revision of earlier work and propose an Early Turonian age for upper Natih B (their MFS K140, surface 8e of van Buchem et al., 1996).

Packer and Zucchi (2002) used combined nannofossil and microfaunal data to interpret Early (?) and Middle Turonian ages in marly limestones and calcareous claystones of the uppermost Natih Formation in two wells in North Oman. These comprise thin, less than 10-m-thick, locally developed latest Natih sections. These units are probably time equivalent to Large Scale Sequence IV of van Buchem et al. (2002), which comprises deeper water, transgressive deposits in the Tanuf to Nakhr area (Al Hajar Mountains). Homewood et al. (2008) recognise a thickness of up to ca. 30 m of this Sequence IV in Fahud wells, whereas in Jabal Qusaybah (Afar area) and also in sections from Jabal Tanuf to Wadi Al Muaydin (Al Hajar Mountains) they split this unit further to interpret an uppermost Natih Sequence V (grainstone association, channel complex).

Biostratigraphy: Biozones F62 (Dohaia planata, Dictyoconella minima) to F57 (Orbitolina concava), Subbiozone F577 (Hedbergella washitensis) (see Figures 6.7 and 6.8). See also Group section and additional discussion under age section. Clear uncertainties remain concerning some of the detailed age calibrations applied to the Formation.

Figure 6.8:

Fossils from the Natih (A-D) Formation: (a and b) Rare and sometimes loose, Praealveolina cretacea from the Lower Turonian – Middle Cenomanian Biozones F62-F61; and (c and d) Rare and sometimes loose, (with thin section) Nezzazata simplex from the Lower Turonian – Middle Cenomanian Biozones F62-F61 (Mohammed et al., 1997).

Figure 6.8:

Fossils from the Natih (A-D) Formation: (a and b) Rare and sometimes loose, Praealveolina cretacea from the Lower Turonian – Middle Cenomanian Biozones F62-F61; and (c and d) Rare and sometimes loose, (with thin section) Nezzazata simplex from the Lower Turonian – Middle Cenomanian Biozones F62-F61 (Mohammed et al., 1997).

Nahr Umr Formation

Authors: Glynn Jones (unpublished, 1948), see van Bellen et al. (1959/2005).

Introduction

A regional drop in sea level associated with subaerial exposure and influx of fine-grained clastics terminates extensive carbonate deposition (Shu’aiba Formation) on the Arabian Platform in the early Late Aptian. Clastic deposition dominated the whole shelf during most of the Albian. In Oman the Nahr Umr shales represent this clastic wedge, which provides a world class seal responsible for a large proportion of the retained hydrocarbons in the Oman subsurface.

Carbonates were only present along the shelf margin. The Al Hassanat Formation in the Saih Hatat area of northern Oman is a platform-margin carbonate succession that has been interpreted as correlative with the Nahr Umr Formation (Immenhauser et al., 2000; Immenhauser et al., 2001).

Type and reference sections: Nahr Umr-2 in Iraq (see van Bellen et al., 1959/2005). The reference section in Oman is Maqhoul-1 in North Oman (Figure 6.9). Additional reference sections are Baqlah-1 in North Oman (Figure 6.10) and Kulan-1 in Central Oman (Figure 6.11).

Figure 6.9:

Composite electrical logs, lithology and lithological description of the Nahr Umr Formation, Wasia Group, in well Maqhoul-1, North Oman (Mohammed et al., 1997). See Figure 6.1 for location.

Figure 6.9:

Composite electrical logs, lithology and lithological description of the Nahr Umr Formation, Wasia Group, in well Maqhoul-1, North Oman (Mohammed et al., 1997). See Figure 6.1 for location.

Figure 6.10:

Composite electrical logs, lithology and lithological description of the Nahr Umr Formation, Wasia Group, in well Baqlah-1, North Oman (Mohammed et al., 1997). See Figure 6.1 for location.

Figure 6.10:

Composite electrical logs, lithology and lithological description of the Nahr Umr Formation, Wasia Group, in well Baqlah-1, North Oman (Mohammed et al., 1997). See Figure 6.1 for location.

Figure 6.11:

Composite electrical logs, lithology and lithological description of the Nahr Umr Formation, Wasia Group, in well Kulan-1, Central Oman (Mohammed et al., 1997). See Figure 6.1 for location.

Figure 6.11:

Composite electrical logs, lithology and lithological description of the Nahr Umr Formation, Wasia Group, in well Kulan-1, Central Oman (Mohammed et al., 1997). See Figure 6.1 for location.

Lithology: The Nahr Umr Formation consists of varyingly calcareous shales, marls and some argillaceous limestones. All units are highly fossiliferous with abundant foraminifera, notably orbitulinids. A widely-traceable, thin, clean ‘Marker’ limestone lies within the lower 30 m of the Formation, but does not extend into South Oman. Towards the south and in the Al Huqf area outcrops, the Nahr Umr thins significantly (e.g. Nimr-1, Figure 6.5) and becomes more silt to sand prone (often with associated chamosite content as burrow fill or ooid grains). To the east of the Eastern Flank/Al Huqf highs and in the extreme south of Oman, thicker limestone/claystone sections indicate a shift to a more proximal, platform edge, setting (e.g. Kulan-1, Figure 6.11).

There is no visible difference between the Natih, Nahr Umr and the Shu’aiba limestones. Shales are varicoloured, generally shades of olive brown and greyish green and are indistinguishable from the Natih shales. They can be distinguished from Gharif shales (over which the Nahr Umr sometimes lies, see Nimr-1, Figure 6.4) by colour and being recovered in a slightly more fissile/elongate state than the more massive, amorphous and reddish brown Upper and Middle Gharif shales.

Subsurface recognition: Whilst drilling the Nahr Umr is generally impossible to separate from Natih shales unless the Nahr Umr is very thick (all Natih shales are thin). The low calcimetry readings are characteristic.

The Nahr Umr can often be sandy (sandstone laminae or as disseminated sand grains) at the base. Glauconite, chamositic ooid and chamositic burrow fill associations are recorded in Central and South Oman. The top of the Nahr Umr Marker limestone is generally marked by a negative drillbreak.

Boundaries: The upper boundary is usually conformable and transitional up into the lower carbonates of the Natih Formation.

The Formation conformably overlies Upper Shu’aiba (Bab Member clinoforms) infill of the central Bab Basin in northwest Oman. Elsewhere, the base Nahr Umr is an unconformity corresponding to a regional Arabian Platform intra-Late Aptian hiatus. This unconformity expands further southwards and onto structural highs (Eastern Flank) where it progressively truncates older stratigraphic units of the Kahmah, Mesozoic Clastics, Sahtan, Akhdar, Haushi or Mahatta Humaid groups.

The Aptian – Albian boundary corresponds to a global second-order sequence boundary and a major sea-level drop. Widespread erosion on basin edges, associated with that sea-level drop, resulted in a hiatus of variable span, usually intra-Late Aptian, but occasionally down into the Early Aptian.

Distribution: The Nahr Umr is widespread over Oman, representing a global Cretaceous sea-level rise, and transgresses across the subcrop of many older units to the south and east, thinning in the same directions. The Formation is generally much thinner, with greater sand content in South and Central Oman, with the exception of Kulan-1 (Figure 6.11) and other platform edge wells to the east and south of regional highs.

Deposition: The Nahr Umr Formation is a laterally extensive unit of shallow-marine, calcareous shales, grading into lime mudstones toward the north. The Formation condenses, becomes more sandy and is of shallower depositional aspect to the south. Packer (2004), in contrast to Witt and Gökdag (1994), suggests that most of the southerly wells do contain an Early Albian section and that the base is not necessarily diachronous. However, it is entirely possible that the onlap onto the southern part of the Eastern Flank did not occur until the Middle – Late Albian (e.g. Marmul area, as stated by Witt and Gökdag, 1994, and wells to the south where direct Early Albian evidence is lacking). Thicker, mixed silt/shale-carbonates sections are seen to the east (including offshore) and extreme south of Oman.

In sequence stratigraphic terms the basal clays of the Nahr Umr Formation represent the transgressive systems tract of the flooding culminating in the maximum flooding represented by the Marker Limestone. Immenhauser et al. (1999) used evidence from stable isotopes, brackish fluid inclusions, and petrographic interpretations of root structures to suggest repeated subaerial exposure in the Nahr Umr of northern Oman, but this has been disputed by Davies et al. (2002).

Subdivision: In North Oman the Formation is subdivided into Lower and Upper members on the basis of a thin marker limestone - the Nahr Umr Marker Limestone. This marker limestone, absent from South Oman, is often important as the last downhole reference point for wells targeting the Shu’aiba Formation.

Age: Latest Aptian – Late Albian, ca. 113–104 Ma. The Marker Limestone is considered to represent the MFS K90 surface of Sharland et al. (2001), whereas they position surface MFS K100 within the limestones of the Upper Nahr Umr.

Immenhauser et al. (1999) argue for a Late Aptian age at the base of the Nahr Umr (restated in Immenhauser and Matthews, 2004). Packer (2004) supports this conclusion. Again the feeling is that further work is required to confirm such an age interpretation. Note that Vahrenkamp (2008) applying carbon-isotope stratigraphy places the boundary between the Shu’aiba and Nahr Umr formations within the latest Aptian. Biostratigraphy: Sub-biozones F575 (Trocholina altispira, Pseudocyclammina hedbergi) – F573/571 (Orbitolina texana/Spineloberis IRK20, Cytherelloidea IRR29). Packer (2004) carried out a study of 33 well sections from throughout Oman, in which he reviewed and updated the existing Sikkema (1991) zonation. He integrated ostracod, benthic foraminiferal and rare planktonic foraminiferal data to essentially split the Nahr Umr into Late – Middle (F575) and Early Albian (F571) (see Group discussion). It should be noted that the study utilised conventional micropalaeontological, ‘hand specimen’ sample analysis and the need to further exploit the stratigraphical potential of the orbitulinids (mesobitulinids) via thin section study was not realised.

One of the key events used to identify the Early Albian is the last occurence of the ostracod Spineloberis IRK20, as previously noted by Witt and Gökdag (1994). Sub-biozone F573 of Sikkema (1991) is essentially coeval with the new Sub-biozone F571 and its top was redefined as a downhole influx of Mesorbitulina spp. towards the base of the Late – Middle Albian section. The Sub-biozones F573 and F571 represent different depositional biofacies within the Early Albian and the full range from ‘Mesorbitulina spp. dominated’, through ‘mixed’ to ‘ostracod dominated’ assemblages is evident.

Concerns remain about the true range of certain marker species and the use of facies-dependent taxa (ostracods and benthic foraminifers). Further study utilising nannofossils, dinoflagellate cysts and thin section micropalaeontology may prove worthwhile.

Figures & Tables

Figure 6.1:

Location map: Wasia Group.

Figure 6.1:

Location map: Wasia Group.

Figure 6.2:

Composite electrical logs, lithology and lithological description of the Natih Formation, Wasia Group, in well Fahud-3, North Oman (Mohammed et al., 1997). See Figure 6.1 for location.

Figure 6.2:

Composite electrical logs, lithology and lithological description of the Natih Formation, Wasia Group, in well Fahud-3, North Oman (Mohammed et al., 1997). See Figure 6.1 for location.

Figure 6.3:

Composite electrical logs, lithology and lithological description of the Natih Formation, Wasia Group, in well Suwaihat-1, Central Oman (Mohammed et al., 1997). See Figure 6.1 for location.

Figure 6.3:

Composite electrical logs, lithology and lithological description of the Natih Formation, Wasia Group, in well Suwaihat-1, Central Oman (Mohammed et al., 1997). See Figure 6.1 for location.

Figure 6.4:

Composite electrical logs, lithology and lithological description of the Natih Formation, Wasia Group, in well Nimr-1, South Oman (Mohammed et al., 1997). See Figure 6.1 for location.

Figure 6.4:

Composite electrical logs, lithology and lithological description of the Natih Formation, Wasia Group, in well Nimr-1, South Oman (Mohammed et al., 1997). See Figure 6.1 for location.

Figure 6.5:

Ditch cuttings from source rock Natih A and B Members, Wasia Group: (a) limestone-wackestone from Habiba-2; (b) limestone-wackestone from Fahud-7; and (c) limestone-wackestone/mudstone from Barik-10 (scale grid is 1 x 1 mm) (Mohammed et al., 1997).

Figure 6.5:

Ditch cuttings from source rock Natih A and B Members, Wasia Group: (a) limestone-wackestone from Habiba-2; (b) limestone-wackestone from Fahud-7; and (c) limestone-wackestone/mudstone from Barik-10 (scale grid is 1 x 1 mm) (Mohammed et al., 1997).

Figure 6.6:

Ditch cuttings from the Wasia Group: (a) shale, Natih D, from Habiba-2; (b) limestone-wackestone, Natih E source rock, from Habiba-2; and (c) shale, Nahr Umr, from Barik-10 (scale grid is 1 x 1 mm) (Mohammed et al., 1997).

Figure 6.6:

Ditch cuttings from the Wasia Group: (a) shale, Natih D, from Habiba-2; (b) limestone-wackestone, Natih E source rock, from Habiba-2; and (c) shale, Nahr Umr, from Barik-10 (scale grid is 1 x 1 mm) (Mohammed et al., 1997).

Figure 6.7:

Fossils from the Natih E Member and Nahr Umr Formation: (a-c) Common, abundant and sometimes loose, Orbitolina concava from the Albian – Lower Cenomanian Biozone F57; (d and e) Common and sometimes loose, Hemicyclammina whitei from the Albian – Lower Cenomanian Biozone F57; and (f) Common and sometimes loose, Trocholina altispira from the Albian – Lower Cenomanian Biozone F57 (Mohammed et al., 1997).

Figure 6.7:

Fossils from the Natih E Member and Nahr Umr Formation: (a-c) Common, abundant and sometimes loose, Orbitolina concava from the Albian – Lower Cenomanian Biozone F57; (d and e) Common and sometimes loose, Hemicyclammina whitei from the Albian – Lower Cenomanian Biozone F57; and (f) Common and sometimes loose, Trocholina altispira from the Albian – Lower Cenomanian Biozone F57 (Mohammed et al., 1997).

Figure 6.8:

Fossils from the Natih (A-D) Formation: (a and b) Rare and sometimes loose, Praealveolina cretacea from the Lower Turonian – Middle Cenomanian Biozones F62-F61; and (c and d) Rare and sometimes loose, (with thin section) Nezzazata simplex from the Lower Turonian – Middle Cenomanian Biozones F62-F61 (Mohammed et al., 1997).

Figure 6.8:

Fossils from the Natih (A-D) Formation: (a and b) Rare and sometimes loose, Praealveolina cretacea from the Lower Turonian – Middle Cenomanian Biozones F62-F61; and (c and d) Rare and sometimes loose, (with thin section) Nezzazata simplex from the Lower Turonian – Middle Cenomanian Biozones F62-F61 (Mohammed et al., 1997).

Figure 6.9:

Composite electrical logs, lithology and lithological description of the Nahr Umr Formation, Wasia Group, in well Maqhoul-1, North Oman (Mohammed et al., 1997). See Figure 6.1 for location.

Figure 6.9:

Composite electrical logs, lithology and lithological description of the Nahr Umr Formation, Wasia Group, in well Maqhoul-1, North Oman (Mohammed et al., 1997). See Figure 6.1 for location.

Figure 6.10:

Composite electrical logs, lithology and lithological description of the Nahr Umr Formation, Wasia Group, in well Baqlah-1, North Oman (Mohammed et al., 1997). See Figure 6.1 for location.

Figure 6.10:

Composite electrical logs, lithology and lithological description of the Nahr Umr Formation, Wasia Group, in well Baqlah-1, North Oman (Mohammed et al., 1997). See Figure 6.1 for location.

Figure 6.11:

Composite electrical logs, lithology and lithological description of the Nahr Umr Formation, Wasia Group, in well Kulan-1, Central Oman (Mohammed et al., 1997). See Figure 6.1 for location.

Figure 6.11:

Composite electrical logs, lithology and lithological description of the Nahr Umr Formation, Wasia Group, in well Kulan-1, Central Oman (Mohammed et al., 1997). See Figure 6.1 for location.

GROUPFORMATIONMEMBER
WasiaNatih*Natih A
Natih B
Natih C
Natih D
Natih E
Natih F
Natih G
Nahr UmrUpper Nahr Umr
Marker Limestone
Lower Nahr Umr
GROUPFORMATIONMEMBER
WasiaNatih*Natih A
Natih B
Natih C
Natih D
Natih E
Natih F
Natih G
Nahr UmrUpper Nahr Umr
Marker Limestone
Lower Nahr Umr
*

Natih ‘letter’ members are informal.

ZoneSubzoneMarker speciesRelative age*Formation/Unit
F62 Dohaia planata, Dictyoconella minimaLate Cenomanian - Early TuronianNatih A and B Members
F61 Praealveolina cretacea cretacea, Trocholina lenticularisMiddle to Late CenomanianNatih C and D Members
  Orbitolina concava, Hemicyclammina whiteiAlbian – Early CenomanianNahr Umr Formation – Natih E Member
 F579Orbitolina concava, Hemicyclammina whitei(Early Cenomanian)Natih E Member
F57F577Hedbergella washitensis(Late Albian – Early Cenomanian)Natih E (base) – G Members
F575Trocholina altispira, Pseudocyclammina hedbergi(Middle – Late Albian) 
 F573**Orbitolina texana(Early – Middle Albian)Nahr Umr Formation
 F571**Spineloberis IRK20, Cytherelloidea IRR29(Early Albian) 
ZoneSubzoneMarker speciesRelative age*Formation/Unit
F62 Dohaia planata, Dictyoconella minimaLate Cenomanian - Early TuronianNatih A and B Members
F61 Praealveolina cretacea cretacea, Trocholina lenticularisMiddle to Late CenomanianNatih C and D Members
  Orbitolina concava, Hemicyclammina whiteiAlbian – Early CenomanianNahr Umr Formation – Natih E Member
 F579Orbitolina concava, Hemicyclammina whitei(Early Cenomanian)Natih E Member
F57F577Hedbergella washitensis(Late Albian – Early Cenomanian)Natih E (base) – G Members
F575Trocholina altispira, Pseudocyclammina hedbergi(Middle – Late Albian) 
 F573**Orbitolina texana(Early – Middle Albian)Nahr Umr Formation
 F571**Spineloberis IRK20, Cytherelloidea IRR29(Early Albian) 
*

Provisional based on Sikkema (1991), many uncertainties remain.

**

Subzones are in part coeval and facies restricted – see Nahr Umr text for discussion.

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