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GROUPFORMATIONMEMBER
  Shu'aibaUpper Shu’aiba
  Lower Shu’aiba
 Mesozoic ClasticsKharaibHawar
KahmahLekhwair 
  Habshan 
  Salil 
  Rayda 
GROUPFORMATIONMEMBER
  Shu'aibaUpper Shu’aiba
  Lower Shu’aiba
 Mesozoic ClasticsKharaibHawar
KahmahLekhwair 
  Habshan 
  Salil 
  Rayda 

Introduction

A global sea level rise in the Early Cretaceous combined with strong subsidence over part of North Oman (Figure 7.1) transgressed the Tithonian unconformity (Figure 7.2). Initially this occurred in a relatively deep-marine basin setting, where the pelagic carbonates and marls of the Rayda Formation were deposited. The Rayda Basin became progressively shallower due to prograding infill by the shelf-derived sediments of the Salil Formation. The last stage of basin fill is represented by the shallow-marine carbonates of the the Habshan Formation, which prograded very rapidly (within approximately 10 My) over the entire Rayda Basin and almost levelled the preexisting topography. The Salil and (partly) Rayda formations can be interpreted as the contemporaneous slope and deepwater equivalents of the outbuilding Habshan/Lekhwair platform (Haan et al., 1990; Droste and van Steenwinkel, 2004; Le Bec, 2004) (Figure 7.2).

Figure 7.1:

Location map: Kahmah Group.

Figure 7.1:

Location map: Kahmah Group.

Figure 7.2:

Schematic cross-section from South to North Oman showing the formations of the Kahmah Group (after Droste and van Steenwinkel, 2004).

Figure 7.2:

Schematic cross-section from South to North Oman showing the formations of the Kahmah Group (after Droste and van Steenwinkel, 2004).

The subsequent ca. 17 My are characterised by extensive carbonate shelf aggradation over a huge, up to 1,000–1,500 km wide, epeiric shelf setting. This epeiric sea was bounded to the north and east by a passive continental margin with the Neo-Tethys Ocean. Prolific carbonate production led to the development of carbonate platforms along the oceanward margin of the epeiric shelf. The exposed Arabian Shield, providing clastic sediments, defined the southwest coastline of the epeiric sea. In between these clastic and carbonate dominated areas was a shifting area with mixed carbonate-clastic sedimentation (Davies et al., 2002). In Oman, overall closer to the higher-energy shelf edge, the upper part of the Kahmah Group is characterised by a succession of cyclic marine carbonates. Many of the shallowing-upward cycles terminated in (local) emergence, exposing the carbonates to a vadose environment, resulting in leaching and localised karstification. Shelf carbonates of the Lekhwair Formation and the overlying Kharaib and Shu’aiba formations cover large areas of Oman. Reservoir properties are excellent throughout this epeiric carbonate stack, but only the Shu’aiba carbonates contain significant hydrocarbon accumulations in structural traps defined by sealing shales of the overlying Nahr Umr Formation (Wasia Group). Additional structural stratigraphic traps are supported by intra-formational seals in the more muddy intra-shelf carbonates that developed during late Shu’aiba times in the intra-shelf Bab Basin bordering the United Arab Emirates (UAE).

The Kahmah Group is essentially the lateral equivalent of the Thamama Group, as that term is used to the west and north and around the Gulf, but differs in the presence of the pelagic facies of the lower Kahmah (Hassan et al., 1975).

The suggestion by Hughes Clarke (1988, citing Murris, 1981) that the evaporitic Arab-Hith units, which underlie and define the base of the Thamama Group may, in part, be age-equivalent to the pelagic units at the base of the Kahmah is not supported by Sharland et al. (2001).

Roger et al. (1997) revised all of the Dhofar Mesozoic geology based on the work of Roger and Platel (1987), which was a detailed study of the outcrop relationships of the Shabon and Hinna members of the Qishn Formation in South Oman. Based on analogy, they related the Qishn Formation with the Kahmah Group as defined by Hughes Clarke (1988), where the Hasheer Member was deemed to be correlative with the Shu’aiba Formation, the Hinna Member with the Kharaib Formation and the Shabon Member with the Lekhwair Formation. The age of the Qishn Formation as given by Roger and Platel (1987) and restated in Roger et al. (1997) ranges from Late Barremian to Aptian. Such an age range implies correlation only with the Upper Kahmah, i.e. the Shu’aiba and Kharaib formations.

The Mesozoic Clastics Formation, deposited along the Eastern Flank in South Oman is herein included within the Kahmah Group. It is roughly time equivalent to the Habshan (Upper) to Shu’aiba formations.

Type and reference sections: Al Jabal Al Akhdar outcrops, named after Wadi Kahmah on the south side of the massif.

Additional reference sections covering the Shu’aiba, Kharaib and Lekhwair formations are Lekhwair-258 (Figure 7.3), Al Huwaisah-2 (Figure 7.4), and Lekhwair-7 (Figure 7.7) in North Oman, Hazar-2 (Figure 7.8) and Hasirah-1 in Central Oman (Figure 7.9), and Jazal-1 in South Oman (Figure 7.10). Reference sections covering the Habshan, Salil and Rayda formations are Dhulaima-4 (Figure 7.12) and Natih-124 in North Oman (Figure 7.13). The reference sections for the Mesozoic Clastics Formation are in South Oman: Dimeet-1 (Figure 7.17), Al Burj-4 (Figure 7.18), Barah-1 (Figure 7.19), and Marmul-287 (Figure 7.20).

Figure 7.3:

Composite electrical logs, lithology and lithological description of the Shu’aiba Formation, Kahmah Group, in well Lekhwair-258, North Oman (Mohammed et al., 1997). See Figure 7.1 for location.

Figure 7.3:

Composite electrical logs, lithology and lithological description of the Shu’aiba Formation, Kahmah Group, in well Lekhwair-258, North Oman (Mohammed et al., 1997). See Figure 7.1 for location.

Figure 7.4:

Composite electrical logs, lithology and lithological description of the Shu’aiba Formation, Kahmah Group, in well Al Huwaisah-2, North Oman (Mohammed et al., 1997). See Figure 7.1 for location.

Figure 7.4:

Composite electrical logs, lithology and lithological description of the Shu’aiba Formation, Kahmah Group, in well Al Huwaisah-2, North Oman (Mohammed et al., 1997). See Figure 7.1 for location.

Lithology: The Kahmah Group comprises a thick carbonate sequence, ranging from deeper marine pelagic porcellanites, cherts and marls in the lower part to shallow-marine shelfal carbonates in the upper part. Partly time-equivalent clastics, ranging from shales to conglomerates, represent the Mesozoic Clastics Formation.

Boundaries: The base is almost everywhere a major sedimentary break, typically from the shallow-marine Sahtan carbonates to pelagic facies of the basal Kahmah. The top is also a sedimentary break with marls/clays of the basal Wasia lying on the top Kahmah carbonates, which locally have an eroded or karstified upper surface. The upper and lower boundaries of the Mesozoic Clastics Formation represent major truncation surfaces.

Distribution: The sequence thins from north to south, mainly related to progressively deeper erosion associated with the overlying base Wasia unconformity. The lower Kahmah (Rayda, Salil and Habshan formations) is limited to areas north of ca. 21°N latitude. The upper Kahmah (Lekhwair, Kharaib and Shu’aiba formations) extends further south, but south of ca. 20°N, it is progressively cut out beneath the basal Wasia, and south of ca. 19°30′N only sporadic thin remnants occur, until a thicker Lower Cretaceous sequence is developed west of Salalah. Well Jazal-1 (Figure 7.10) represents the thickest Kahmah Group subsurface section in South Oman, delineated as undifferentiated Lekhwair/(Kharaib) Formation. A trend of decreasing argillaceous content to the northeast, towards the shelf margin, reduces the potential to split the Upper Kahmah into its constituent formations. In the Jabal Al Akhdar outcrop sections, the Lekhwair, Kharaib and Shu’aiba equivalents are difficult to differentiate and form a single, continuous shallow-marine carbonate unit.

The Mesozoic Clastic Formation is confined to South Oman in an approximately 300 km long NE-SW trending depositional low. This is associated with salt withdrawal along the Eastern Flank of the South Oman Salt Basin.

Subdivision: In subsurface use, the Group is divided into seven formations: (1) Shu’aiba, (2) Kharaib, (3) Lekhwair, (4) Habshan, (5) Salil, (6) Rayda, and (7) Mesozoic Clastics.

Sequence stratigraphy: The Kahmah Group represents the lower part of the AP8 Megasequence of Sharland et al. (2001). Sharland et al. (2001) correlate their MFS K80 to K10 surfaces into the Kahmah Group of Oman (see formational discussions for detail).

Age: Latest Jurassic – Early Cretaceous, Late Tithonian – Late Aptian, ca. 147–113 Ma.

Biostratigraphy: Biozones F56 (Orbitolina(M.)parva) to F51 (Calpionella alpina, C. elliptica) are documented by Sikkema (1991). The older sediments can usually only be analysed using thin-sections, the younger may be analysed with conventional washed samples. These zones are fully defined by Sikkema (1991) but subsequent published and unpublished reports suggest that a major update of these zones is merited. See formational discussions below for detail. The scheme of Sikkema (1991) is summarised as follows:

ZoneSubzoneMarker speciesRelative ageFormation/Member
F56F567Orbitolina (M.) parvaEarly – Late AptianShu’aiba
F563Palorbitolina lenticularis, P. cormyi
F55F557Choffatella decipiens, Salpingoporella dinaricaEarly Barremian – earliest AptianKharaib
F553Salpingoporella muehlbergiiUppermost Lekhwair - Kharaib
F54 Pseudochrysalidina arabicaHauterivian – Early BarremianUppermost Habshan - Lekhwair
F53 Pseudocyclammina lituus, P. cylindricaValanginian – HauterivianRayda - Salil - Habshan
F52F527Calpionella darderi, common calpionellids, common tintinnids
F523common small Calpionella alpinaLate BerriasianRayda - Salil
F51 common large Calpionella alpina, C. elliptica, radiolariaLate Tithonian – Early BerriasianRayda
ZoneSubzoneMarker speciesRelative ageFormation/Member
F56F567Orbitolina (M.) parvaEarly – Late AptianShu’aiba
F563Palorbitolina lenticularis, P. cormyi
F55F557Choffatella decipiens, Salpingoporella dinaricaEarly Barremian – earliest AptianKharaib
F553Salpingoporella muehlbergiiUppermost Lekhwair - Kharaib
F54 Pseudochrysalidina arabicaHauterivian – Early BarremianUppermost Habshan - Lekhwair
F53 Pseudocyclammina lituus, P. cylindricaValanginian – HauterivianRayda - Salil - Habshan
F52F527Calpionella darderi, common calpionellids, common tintinnids
F523common small Calpionella alpinaLate BerriasianRayda - Salil
F51 common large Calpionella alpina, C. elliptica, radiolariaLate Tithonian – Early BerriasianRayda

This scheme should be used with considerable caution, as in places major uncertainties still exist, particularly, in terms of the absolute ranges of the taxa used and a full understanding of their distribution within highly diachronous units e.g. the Rayda to Habshan section.

Nannoplankton analyses have been applied to a lesser degree. No comprehensive zonation has been documented, but this fossil group perhaps provides the greatest potential for calibration of parts of the Kahmah Group to well documented worldwide oceanic events. See formational discussions.

Late Jurassic – Early Cretaceous Palynomorph zones include those of Clarke (1968).

ZoneMarker speciesRelative age
571Ascodinium yibaliiLate Barremian – Aptian
648Gardodinium cerviculumLate Hauterivian – Early Barremian
578Muderongia neocomicaBerriasian – Early Hauterivian
850Hystrichosphaeridium irregulareKimmeridgian – Portlandian
ZoneMarker speciesRelative age
571Ascodinium yibaliiLate Barremian – Aptian
648Gardodinium cerviculumLate Hauterivian – Early Barremian
578Muderongia neocomicaBerriasian – Early Hauterivian
850Hystrichosphaeridium irregulareKimmeridgian – Portlandian

This scheme has effectively fallen into disuse due to the dominance of conventional micropalaeontological based studies in these carbonate sections. Even more so than the micropalaeontological zonation these zones are in need of revision. Recent work suggests that Middle East palynological knowledge has matured to the extent that the discipline can make a significant contribution to our understanding in parts of the Kahmah Group (e.g. Simon Petroleum Technology, 1995; Shaw, 2009).

Shu’aiba Formation

Authors: First defined by Rabanit (unpublished, 1951) and subsequently published by Owen and Nasr (1958). The Formation was redefined by van Bellen et al. (1959/2005), who reverted to the original description of Ranabit.

Introduction

The Shu’aiba Formation is the uppermost unit of the Kahmah Group and consists of an Aptian carbonate complex of some 100 m thick. Carbonate reservoirs in the upper part of the Formation are important oil producers in the greater Lekhwair area of North Oman.

Type and reference sections: Zubair-3 in Southern Iraq, see van Bellen et al. (1959/2005). The reference section in Oman is well Lekhwair-258 (Figure 7.3). This new reference section replaces Al Huwaisah-2 and Lekhwair-7 (Hughes Clarke, 1988), which by comparison have truncated Shu’aiba sections (Figures 7.4 and 7.7).

Additional reference sections are Hazar-2 (Figure 7.8) and Hasirah-1, both in Central Oman (Figure 7.9).

Lithology: The depositional patterns in the Shu’aiba Formation are highly complex, and include open-marine areas, shallow-marine (rudist-rich) shoals and lagoonal (semi-restricted) environments. In general, there is a clear trend towards more open-marine conditions from the ocean facing northeast towards the Lekhwair area in northwest Oman, where deeper water carbonates link up with the more muddy intra-shelf carbonates of the Bab Basin near Oman’s border with the UAE.

The basal part of the Shu’aiba Formation consists of laterally extensive algal (Bacinella/Lithocodium) skeletal wackestones to boundstones (Hughes Clarke, 1988). These are overlain by more chalky carbonates, which can be enriched in organic matter, in some areas reaching source rock quality. The upper part of the Shu’aiba is more variable and contains cleaner foraminiferal and sometimes rudistrich, wacke-packstones and occasionally rudstones. In northwestern Oman (Lekhwair and adjacent areas) the upper part of the Shu’aiba consists of interbedded calcareous clays and limestones. The clays have a variable composition ranging from smectite/illite to kaolinite/chlorite dominated.

These clastics form the basis for dividing the Shu’aiba into Lower and Upper members (Figure 7.5). The Upper Member consists of interbedded calcareous clays and limestones, the Lower Member of clean limestones. The boundary between these members is placed at the base of a tight limestone interval (picked on the Density logs) overlying the more porous limestones of the Lower Shu’aiba. The Upper Member corresponds to what has been defined as the Bab Member in the UAE (Hughes Clarke, 1988; Hassan et al., 1975) and redefined as the Bab Formation (Granier, 2000). In other parts of Oman the calcareous clays are not present and the Shu’aiba is not subdivided into members. However, in several fields (e.g. Al Huwaisah and Yibal) an informal Upper and Lower Shu’aiba subdivision has been applied locally usually with reference to Gamma log profiles. The boundary between these members is usually defined at a high Gamma spike between a generally increasing and decreasing log trend. Note that the Early Aptian Shu’aiba Formation in the Al Huwaisah/Central Oman area actually corresponds to the Lower Shu’aiba Member in Lekhwair and adjacent areas (Bab Basin) (Figure 7.5).

Figure 7.5:

Applied lithostratigraphy of the Aptian Shu’aiba Formation (after Droste, 2003). The Formation overlies the Hawar Member of the Kharaib Formation and is overlain by the Nahr Umr Formation of the Wasia Group. Note informal division of Shu’aiba in Central Oman. Central Oman Shu’aiba correlates to Lower Shu’aiba only, as defined in Northwest Oman.

Figure 7.5:

Applied lithostratigraphy of the Aptian Shu’aiba Formation (after Droste, 2003). The Formation overlies the Hawar Member of the Kharaib Formation and is overlain by the Nahr Umr Formation of the Wasia Group. Note informal division of Shu’aiba in Central Oman. Central Oman Shu’aiba correlates to Lower Shu’aiba only, as defined in Northwest Oman.

In all subsequent discussions the term Upper Shu’aiba (Member) is applied with reference to the ‘Bab Member’ equivalence stated and illustrated above.

Subsurface recognition: Whilst drilling, the Shu’aiba Formation is recognised using ‘hotshots’ to confirm the F56 Biozone of Aptian age (Orbitolina (M.) parva, Palorbitolina lenticularis and Ammobaculites sp. in the Upper Shu’aiba). It is difficult to identify based on lithology alone as the limestones are very similar to the limestones of the Natih and Nahr Umr but they do, however, contain Rudist fragments in the upper parts.

Typically, the occurrence of the algae Bacinella/Lithocodium increases with depth. These are very distinct in cuttings (Figure 7.6). Often 5 m to more than 10 m of pure limestone has to be drilled before confirmation can be given that the Kahmah has been penetrated. Both the top and the base of the Shu’aiba may be marked by a negative drill break (the latter due to the much harder limestones of the Hawar Member at the top of the Kharaib Formation, Figure 7.8). In the Lekhwair and adjacent areas the top Lower Shu’aiba is marked by a positive drillbreak.

Figure 7.6:

Ditch cuttings from Shu’aiba Formation, Kahmah Group: (a and b) limestone-boundstone Bacinella from Lekhwair-1; (scale grid is 1 x 1 mm) (Mohammed et al., 1997).

Figure 7.6:

Ditch cuttings from Shu’aiba Formation, Kahmah Group: (a and b) limestone-boundstone Bacinella from Lekhwair-1; (scale grid is 1 x 1 mm) (Mohammed et al., 1997).

Figure 7.7:

Composite electrical logs, lithology and lithological description of the Shu’aiba/Kharaib/Lekhwair formations, Kahmah Group, in well Lekhwair-7, North Oman (Mohammed et al., 1997). See Figure 7.1 for location.

Figure 7.7:

Composite electrical logs, lithology and lithological description of the Shu’aiba/Kharaib/Lekhwair formations, Kahmah Group, in well Lekhwair-7, North Oman (Mohammed et al., 1997). See Figure 7.1 for location.

Figure 7.8:

Composite electrical logs, lithology and lithological description of the Shu’aiba/Kharaib/Lekhwair formations, Kahmah Group, in well Hazar-2, Central Oman (Mohammed et al., 1997). See Figure 7.1 for location.

Figure 7.8:

Composite electrical logs, lithology and lithological description of the Shu’aiba/Kharaib/Lekhwair formations, Kahmah Group, in well Hazar-2, Central Oman (Mohammed et al., 1997). See Figure 7.1 for location.

Post-drilling the Shu’aiba Formation can easily be picked on log correlation, as it has a clean, blocky Gamma pattern and, if taken, a clear Dipmeter break. Faunal evidence can show the presence of Biozone F56.

Boundaries: The Shu’aiba Formation is separated from the clean carbonates of the underlying Kharaib Formation by an easily recognisable marker bed of tight limestones; the Hawar Member of the Kharaib Formation (Figures 7.3, 7.4 and 7.5). This boundary is conformable.

The Shu’aiba is nearly everywhere overlain by the sealing clays of the Nahr Umr Formation (Figures 7.3, 7.4 and 7.5). This is not the case over parts of the Lekhwair High where the Shu’aiba is unconformably overlain by clays of the Palaeocene Shammar Formation. Younger Cenozoic strata also directly overly the Shu’aiba along the western flank of the Al Huqf High and above salt domes in the eastern Ghaba Salt Basin. The contact with the Nahr Umr is essentially conformable in the central Bab Basin area of northwest Oman, but evidence of erosion and karstification and a clear unconformity indicates a stratigraphic hiatus in other areas. This unconformity is Late Aptian in age and progressively truncates the Shu’aiba and deeper units to the south and east.

Distribution: The Shu’aiba varies in thickness from 130 m in the northwest to 0 m in Central Oman. The regional thinning trend to the south is related to uplift and truncation below the base Nahr Umr unconformity as well as a subtle onlap onto the Al Huqf High. The persistence of the slightly-deeper argillaceous facies through the upper part of the Shu’aiba in the Lekhwair area reflects a northwards transition into what was known as the Bab Member of the Upper Shu’aiba described in Abu Dhabi (Hassan et al., 1975). The Bab Member was subsequently upgraded to the Bab Formation (Granier, 2000).

Deposition: Several facies schemes have been proposed for the Shu’aiba (Witt and Gökdag, 1994; van Buchem et al., 2002; Immenhauser et al., 2004; Boote and Mou, 2003). Droste and van Steenwinkel (2004) proposed a facies scheme, which is tied to distinct platform carbonate architectures that have been recognised from seismic. Summarising this work the Shu’aiba Formation basically represents the establishment of a carbonate platform at the margin of the shelf following the flooding of the exposed Kharaib platform. This carbonate complex prograded towards the shelf interior and deposition ended with regional exposure. Facies analysis shows that during the initial transgression low-angle depositional profiles predominated and a microbial/algal carbonate factory produced the sediments. During the late transgression algal mounds developed a differentiated topography. This topography was filled during the subsequent highstand by progradation from rudist carbonate factories that first established in shallows, shedding large amounts of sediment, laterally prograding into the deeper intrashelf areas. The resultant clinoforms are steep and show strong lateral variations in the amount and direction of progradation. The carbonate platform became subaerially exposed during the following fall in sea level and the remaining depositional lows on the shelf, the ‘Bab’ intra-platform basin, were filled by deposits consisting of alternating offlapping wedges of fine-grained clastics and carbonates. Parallel and laterally continuous clinoform belts infilling the Bab Basin suggest a strong component of alongshore transport in the distribution of the clastics. These clastics provide additional stratigraphic sealing components for hydrocarbon traps in northwest Oman.

Immenhauser et al. (2004) discuss a high-resolution sequence stratigraphic model and a correlation to the framework by van Buchem et al. (2002) for the middle Cretaceous Qishn Formation in the Al Huqf area. The Qishn Formation is the outcrop equivalent of the Lower Shu’aiba and Kharaib formations in the subsurface and is exposed in the Al Huqf and Salalah areas of the Sultanate of Oman as well as in Yemen. Boote and Mou (2003) describe the Shu’aiba sequence stratigraphic architecture for the subsurface in northwestern Oman.

Subdivision: The Shu’aiba Formation can be divided into Lower and Upper members in the Lekhwair and adjacent areas (Figure 7.5).

Age: Early – Late Aptian, ca. 124–113 Ma.

Note that outside the continuous deposition indicated in the Bab Basin a Late Aptian unconformity truncates progressively southwards until at the Al Huqf area the Shu’aiba equivalent sediments (Qishn Formation) are no younger than earliest Aptian (pre-dating the OAE1a event according to Immenhauser et al., 2004).

The Shu’aiba Formation has been intensely studied over the years and age determinations have focused on three main techniques. These are Biostratigraphy, Carbon isotopes and Strontium isotopes. All three are discussed below, with additional comments on Global Mesozoic Oceanic Anoxic Events (OAEs) and Sequence Stratigraphic schemes.

Several publications detail or summarise the main conclusions and issues that arise when attempting to accurately date the Shu’aiba Formation. Simmons, (1994), Witt and Gökdag (1994), Sharland et al. (2001), Davies et al. (2002), van Buchem et al. (2002), Boote and Mou (2003), Immenhauser et al., (2004) and Droste and van Steenwinkel (2004) are amongst the most relevant that contribute to this discussion. Work by Robertson Research International Ltd. (Alex-Sanders, 2002) and Millennia (Packer and Starkie, 2007) provide detailed, reservoir scale, biostratigraphical input.

Biostratigraphy: Studies such as Simmons (1994) and Witt and Gökdag (1994) utilised thin section micropalaeotological and microfacies analysis, whereas the bulk of industrial work in Oman has, to date, concentrated on routine ‘hand specimen’ analysis. Nannofossil work is reported, in summary form, by Witt and Gökdag (1994) and van Buchem et al. (2002). Palynological data is virtually absent in the published domain and has been overshadowed by the other two disciplines. All such studies agree on a general Early to Late Aptian age, but the position of the top of the Early Aptian tends to differ. Gombos et al. (2008) highlight how nannofossil work, and in particular the recognition of the worldwide ‘nannoconid crisis’ and associated events could provide the key to more detailed calibration of the Shu’aiba sections in the region. See full biostratigraphy discussion below.

Carbon isotope stratigraphy: The carbon isotope stratigraphy of the Shu’aiba Formation has been documented by Vahrenkamp (1996), van Buchem et al. (2002) and Immenhauser et al. (1999, 2000, 2004). The carbon isotope data for the lower part of the Shu’aiba consistently show a negative excursion followed by increasing trend towards high δ13C values. This is very similar to the Early Aptian global carbon isotope profile (see Jones and Jenkyns, 2001). However, correlation of isotope trends in shallow-water carbonates, which are likely to have significant and numerous hiatus, variations in sedimentation rates and a strong diagenetic overprint, with the continuous global record from deep-sea sections are questionable. For the upper part of the Lower Shu’aiba and the Upper Shu’aiba, the isotope trends are highly variable both vertically and between closely spaced wells and do not clearly tie to the published curves.

Strontium isotope stratigraphy: Only a few strontium isotope measurements are available for the Shu’aiba. Immenhauser (2004) reports some measurements on rudists from the Qishn Formation, the Shu’aiba/Kharaib outcrop equivalent in the Al Huqf area, and core samples of the Al Huwaisah field. The measurements show a middle Barremian to middle Early Aptian age for the Qishn Formation (the Qishn overlaps only with the lowermost Shu’aiba) and a late Early Aptian age for the Al Huwaisah-2 core in the Lower Shu’aiba.

Oceanic anoxic events: Also of interest are Global Mesozoic Oceanic Anoxic Events (OAEs). In the Shu’aiba Formation, a regionally correlatable interval characterised by a high uranium content (Gamma trend) associated with condensed sedimentation and high organic matter content may be recognised. This has been correlated to the global, earliest Aptian event OAE1a (also know as the ‘Selli’ level, Weissert et al., 1998), which is associated with the beginning of the positive δ13C excursion and the ‘nannoconid crisis’ (Gombon et al., 2008).

Sequence stratigraphy:Sharland et al. (2001) suggest that their K70 MFS is a possible equivalent of the OAE1a event, but place the former within the Hawar Member of the underlying Kharaib Formation. They apply a regional K80 MFS to the Tar (condensed, source rock) Unit of the lower part of the Bab Member in the Shu’aiba Formation of Abu Dhabi, citing a latest Early Aptian age. Their K80 MFS cannot be confidently traced beyond the Bab Basin. Such a condensed, source rock unit would seem to offer a better candidate for OAE1a equivalence, but the younger (late Early Aptian) age determination of Sharland et al. (2001) does not support this.

Van Buchem et al. (2002) and Davies et al. (2002) discuss the range of sequence stratigraphical schemes available and propose alternative MFS positions within the Shu’aiba Formation.

In their work on the Barremian – Early Aptian Qishn Formation, which outcrops in the Al Huqf area, Immenhauser et al. (2004) apply both biostratigraphic and chemostratigraphic techniques. They infer an earliest Aptian age for the Kahraib/Shu’aiba boundary by dating the Hawar Member equivalent in outcrop as earliest Aptian.

Hughes (2005) applies combined foraminiferal, nannoplankton and rudist analysis to the Shu’aiba of the Shaybah field of Saudi Arabia to indicate an Early Aptian age at that locality, but also states an Early and Late Aptian age for deeper marine Bab equivalent rocks to the northeast of the field. The absence of the OAE1a event at Shaybah is attributed to the platform depositional setting.

Biostratigraphy: Biozone F56 split into Sub-biozones F567 (Orbitolina (M.) parva) and F563 (Palorbitolina lenticularis).

Packer and Starkie (2006) represents the most recent and comprehensive biostratigraphical study in northwest Oman, where they analysed 21 wells for thin section and conventional micropalaeontology together with nannoplankton. Their study built on a working zonation developed by Robertson Research International Ltd. biostratigraphers for a horizontal well drilling campaign in the Lekwhair area of North Oman (Alex-Sanders, 2002). This working zonation applied only to routine, or conventional, micropalaeontological analyses, with no thin section or nannofossil input. Both bodies of work add significant detail and refinement to the existing PDO zonation and Packer and Starkie (2006) in particular, provides much needed calibration from nannofossil work. A recent study by Shaw (2009) has added palynological detail to our knowledge of the biological composition of the Shu’aiba.

The existing PDO zonation only provides a crude age calibration. Unravelling the time significance of the known biostratigraphical events within the Shu’aiba highlights the facies dependence of many of the microfaunal components, particularly those recorded in the upper part of the Upper Shu’aiba (where more proximal facies and clinoform progradation dominates). The key to the age calibration and correlation of the Shu’aiba occurs within the lower part of the Upper Shu’aiba (about lower third in the Greater Lekhwair area), where a significant, downhole, deepening event occurs. The combination of increased numbers of planktonic foraminifera and much improved nannoplankton recovery provides key data of age significance. A major downhole influx (abundance/super abundance) of Hedbergella spp. (below top Palorbitolina lenticularis) is associated with a sequence of nannoplankton events, including the so-called, ‘nannoconid crisis’ of earliest Aptian age. In turn these are all associated with a characteristic high Gamma log pattern, which would potentially correlate to the OAE1a event discussed above (Gombos et al., 2008).

The implication proposed here is that the lower portion of the Upper Shu’aiba is of Early Aptian age. Gradstein et al. 2004 (their figure 19.1) place the nannoconid crisis and associated OAE1a event within the basal 2 My of the Aptian Stage. Further work is required to accurately position the ‘nannoconid crisis’ in Omani sections, but results to date seem to suggest a level approximating to the Upper-Lower Shu’aiba boundary (Note: observed in a well where the Lower Shu’aiba exhibits the classic Early Aptian δ13C negative shift and gradual recovery trend). Palynological work by Shaw (2009), supports the conclusion that a potentially significant portion of the Upper Shu’aiba in northwest Oman may be of Early Aptian age. This is at odds with van Buchem et al. (2002) but supports the earlier work of Witt and Gökdag (1994). Note that previous studies (Vahrenkamp, 1996) have suggested that the OAE1a event may equate to the first uphole high Gamma peak in the Lower Shu’aiba, but PDO commissioned work suggests the Upper Shu’aiba position discussed above (e.g. possibly the Gamma peak at ca. 1,205 m in Lekhwair-7, Figure 7.7).

These findings remain preliminary in Oman and need to be the subject of rigorous testing in studies that should include more intensive sampling and a fully integrated approach, involving all three disciplines (routine and thin section micropalaeontology together with nannoplankton and palynology). Full and detailed use of the nannofossil zonation, recognition of the OAE1a event and correlation with potentially significant isotope trends could provide the multidisciplinary ‘golden spike’ to underpin all future Shu’aiba studies.

Clearly the Shu’aiba Formation has generated much study and debate with respect to age calibrations and the veracity of the techniques employed. This is likely to continue. Further detailed work is required.

Kharaib Formation

Authors: The Kharaib Formation was first mentioned by Sugden (unpublished, 1953), but was not published until 1975 by Sugden and Standring, who also distinguished the Hawar Formation. In Oman the Hawar has been included as the Hawar ‘shale’ Member of the Kharaib Formation by Hughes Clarke (1988).

Introduction

The Kharaib is part of the extensive shallow marine shelf system that was established in Early Cretaceous times following the rapid progradation of the Rayda - Salil - Habshan formations (Figure 7.2).

Type and reference sections: The type section is Kharaib-1 in Qatar (Sugden and Standring, 1975). The reference section in Oman is well Lekhwair-7 (Figure 7.7). See also Lekhwair-258 (Figure 7.3) and Al Huwaisah-2 (Figure 7.4) in North Oman. Hazar-2 (Figure 7.8) and Hasirah-1 (Figure 7.9) are reference wells in Central Oman.

Figure 7.9:

Composite electrical logs, lithology and lithological description of the Shu’aiba/Kharaib/Lekhwair formations, Kahmah Group, in well Hasirah-1, Central Oman (Mohammed et al., 1997). See Figure 7.1 for location.

Figure 7.9:

Composite electrical logs, lithology and lithological description of the Shu’aiba/Kharaib/Lekhwair formations, Kahmah Group, in well Hasirah-1, Central Oman (Mohammed et al., 1997). See Figure 7.1 for location.

Lithology: The Kharaib Formation is essentially a clean carbonate unit composed of mudstones, with oncoidal and binding algae in the lower part and packstones/grainstones with common rudists in the upper part. It is capped by a thin, hard limestone unit; the Hawar Member. Lithologically the Formation is very comparable to the overlying Shu’aiba Formation.

The limestones appear chalky and are interbedded with argillaceous limestones. There are generally two beds of chalky limestone and two beds of argillaceous limestone in the formation in the Lekhwair area, but only one of each in Central Oman (Hazar-2 and Hasirah-1, Figures 7.8 and 7.9).

Subsurface recognition: The Kharaib Formation cannot be differentiated on lithology alone at the wellsite, because the limestones are similar to those of the overlying Shu’aiba and the underlying Lekhwair formations.

The Upper part of the Kharaib Formation consists of a tight, hard limestone known as the Hawar Member, associated with a negative drilling break and often high Gamma readings. The Kharaib has generally also a negative drill break at its lower boundary with the underlying Lekhwair Formation.

Hotshot samples may be needed to confirm a Biozone F55 age (although be aware that the upper part of the Lekhwair can also fall within Zone F55).

Post-drilling the Kharaib Formation can be recognised using faunal evidence and the sediments represent part of the F55 (Choffatella decipiens, Salpingoporella dinarica) Biozone (Figure 7.11), combined with offset well Gamma-log correlation.

Figure 7.10:

Composite electrical logs, lithology and lithological description of the Lekhwair/Kharaib formations, Kahmah Group, in well Jazal-1, South Oman (Mohammed et al., 1997). See Figure 7.1 for location.

Figure 7.10:

Composite electrical logs, lithology and lithological description of the Lekhwair/Kharaib formations, Kahmah Group, in well Jazal-1, South Oman (Mohammed et al., 1997). See Figure 7.1 for location.

Figure 7.11:

Fossils from the Kharaib/Lekhwair formations: (a-d) Rare, Choffatella decipiens from the Lower Aptian – Hauterivian Biozone F55; and (e and f) Rare, Salpingoporella dinarica (algae), all from the Lower Aptian – Hauterivian Biozone F55 (Mohammed et al., 1997).

Figure 7.11:

Fossils from the Kharaib/Lekhwair formations: (a-d) Rare, Choffatella decipiens from the Lower Aptian – Hauterivian Biozone F55; and (e and f) Rare, Salpingoporella dinarica (algae), all from the Lower Aptian – Hauterivian Biozone F55 (Mohammed et al., 1997).

Boundaries: Both upper and lower boundaries are conformable. The lower boundary is at a change from slightly argillaceous limestone below to clean limestones above, and the upper boundary is defined by the hard, high Gamma limestone of the Hawar Member below the clean, basal Shu’aiba Formation.

At least one erosional surface associated with subaerial exposure has been observed within the Hawar interval in the Lekhwair field. Shallow-water rudist carbonates underlying this exposure surface, and genetically linked to the underlying Kharaib limestones, are heavily cemented and associated with a high Gamma reading.

Distribution: The Kharaib is widespread and easily correlatable over much of Oman, with facies indicative of a uniform shallow-shelf sea. It is absent by erosion of younger unconformities in the south and east.

Deposition: The sediments and fossils suggest a shallow-marine setting throughout and the depositional setting of the Kharaib Formation is very comparable to that of the Shu’aiba Formation. The rapid progradation of the Habshan Formation created an extensive shallow shelf area. The subsequent transgression of the Kharaib Formation started with a microbial/algal carbonate system followed towards the top by a lateral prograding rudist carbonate system. The Hawar Member capping the Kharaib Formation is particularly argillaceous in offshore UAE and Qatar (Alsharhan and Nairn, 1997), but spectral Gamma-ray logs and XRD analyses indicate that true clay intervals are not present in Oman.

In outcrop the definition of the Hawar Member is different: it corresponds to a decimeter-bedded orbitolinid interval that separates the more massive bedded Shu’aiba and Upper Kharaib limestone beds (van Buchem et al., 2002).

The thickness of the Hawar Member is variable and on a regional scale shows a trend from a few meters in the Greater Lekhwair area in the northwest to some 10–20 m along the eastern edge of the Ghaba Salt Basin. Also towards the Al Hajar Mountains the interval assigned to the Hawar reaches a thickness of up to 25 m (van Buchem et al., 2002). It is a lithologically heterogeneous interval of clay-free limestones, which vary from shallow-marine mudstones to grainstones, locally rich in rudists, to intertidal deposits with wave ripples laminites, mud-cracks, rip up clasts (van Buchem et al., 2002). Towards the Al Huqf High area, the Hawar interval becomes more open marine in character with common corals and echinoids and represents a deeper lagoon setting.

Subdivision: The upper part of the Formation is defined as the Hawar Member (Hughes Clarke, 1988). In the Lekhwair area there are two distinct argillaceous and chalky intervals (Lekhwair-7, Figure 7.7). In north Central Oman there is generally only one argillaceous and one chalky interval (Hazar-2, Hasirah-1; Figures 7.8 and 7.9).

Age: Early Barremian – earliest Aptian ca. 129–124 Ma. Agreement concerning the positions of the K70, K60 and even K50 MFS’s of Sharland et al. (2001) is lacking. Davies et al. (2002) and van Buchem et al. (2002) debate the range of possibilities.

A number of publications and PDO internal reports indicate an Early (earliest) Aptian age for the uppermost Kharaib Formation, Hawar Member (Simmons, 1994; Witt and Gökdag, 1994; Sharland et al., 2001; Davies et al., 2002; van Buchem et al., 2002; Immenhauser et al., 2004). Although some indicate a Late Barremian age for the remainder of the Kharaib the consensus is that the Formation ranges down to the Early Barremian (Simmons, 1994; Davies et al. 2002; van Buchem et al., 2002).

Biostratigraphy: Biozone F55 (Choffatella decipiens, Salpingoporella dinarica) (Figure 7.11) is divisible into Sub-biozones F557 (Choffatella decipiens, Salpingoporella dinarica) and F553 (Salpingoporella muehlbergii). Paleodictyoconus arabicus, the Late Barremian index species of Simmons (1994) has its top occurrence within Sub-biozone F557.

Lekhwair Formation

Authors: First defined by Scherer (unpublished, 1969), see Hassan et al. (1975).

Introduction

The Lekhwair Formation is part of the extensive shallow-marine carbonate shelf system that was established in Early Cretaceous times. The Formation is interpreted as intra-shelf carbonates that developed, in part, contemporaneously with the rapidly prograding Rayda-Salil-Habshan formations, where the Rayda and Salil formations represent respectively basinal and slope sediments and the Habshan Formation the contemporaneous platform edge (Droste and Van Steenwinkel, 2004; Figure 7.2).

Type and reference section: Well Lekhwair-7 in Hughes Clarke (1988) (Figure 7.7). Additional reference sections are Hazar-2 (Figure 7.8) and Hasirah-1 (Figure 7.9), both in Central Oman.

Lithology: The Lekhwair Formation is a sequence of sedimentary cycles involving three main facies: an argillaceous limestone to marl facies with pyrite and some quartz silt; a varyingly chalky algal or fine skeletal wackestone; and a pelletal skeletoidal lime packstone-to-grainstone with common rudists. The argillaceous facies predominates in the cycles in the lower part of the unit, the cycles becoming less argillaceous upwards and laterally to the east. In the south, the base contains quartz sand.

The main difference from the overlying Kharaib and Shu’aiba is the amount of argillaceous limestones and thin shales, notably in the lower part of the Formation. The chalky limestones are only present in the upper part, and are similar to those of the Shu’aiba and Kharaib formations.

Subsurface recognition: The Rate of Penetration log displays a negative drill break at the upper boundary and a positive drillbreak at the lower boundary, when overlying the Habshan Formation. The chalky limestones are soft and generate poor quality cuttings; the argillaceous limestones are harder with better quality cuttings. Hot shots may be needed to confirm F55/54 age (be aware that the Kharaib is also F55). The lithology is distinctive; limestones with some dolomite and thin shales. Minor amounts of dolomite are present in the Lekhwair, Yibal and Al Huwaisah areas, with increased amounts in the Sayh ar Rawl, Barik, Hazar and Haima areas.

Post-drilling the Formation can be recognised using faunal evidence, where the sediments represent Biozone F54 and the lower part of Biozone F55, combined with offset well log correlation.

Boundaries: The lower boundary is a disconformity, with the argillaceous lower Lekhwair lying on Habshan carbonates (a marked seismic discontinuity is noted by Davies et al., 2002) or overstepping older units to the south and east. This surface becomes a comformable, but diachronous boundary in more distal settings (in the Al Hajar Mountains, Le Bec et al., 2002). The upper boundary is transitional into the Kharaib carbonates, which are chalky and non-argillaceous. The top is generally marked by a hard argillaceous limestone.

Distribution: The Lekhwair Formation is present everywhere, except in the south and southeast, where it is eroded at younger unconformities over local highs and the Al Huqf axis. Equivalent sediments reappear to the north and west of Salalah, e.g. Jazal-1 (Figure 7.10). The sequence generally becomes thinner from north to south.

Deposition: The Lekhwair Formation represents shallow, probably restricted marine sediments everywhere, with variations in clay content probably reflecting varied fine clastic supply, but becoming overall cleaner upwards. The sediments are interpreted as the intra-basinal carbonates on a shallow marine, rapidly prograding carbonate shelf, inboard of the platform edge that is represented by the carbonates of the contemporaneous Habshan Formation. Overall progradation of the Salil-Rayda-Habshan-Lekhwair system is followed by the stable aggrading of the Kharaib and Shu’aiba carbonate platforms (Figure 7.2).

Age: General Hauterivian – Early Barremian age, ca. 134–129 Ma. Davies et al. (2002) moved the K50 (Early Barremian) MFS of Sharland et al. (2001) up to the top of the Lekhwair Formation in well Lekhwair-7. Both publications indicate the K40 (Late? Hauterivian) MFS in the lower part of the Formation, both in Lekhwair-7 and in the Wadi Al Muaydin outcrop.

Biostratigraphy: The lower part of Biozone F55 (Choffatella decipiens, Salpingoporella dinarica) and Subbiozone F553 (Salpingoporella muehlbergii) and Biozone F54 (Pseudochrysalidina arabica).

Simon Petroleum Technology (1995) examined seven wells in northeast Oman for micropaleontology (recording Choffatella decipiens and Salpingoporella dinarica), nannofossil (limited data) and palynological (Dicheiropollis etruscus, Systematophora areolata, Muderongia australis, M. tabulata and Oligosphaeridiumamplexum’) content, supporting a Hauterivian – Early Barremian age range.

Habshan Formation

Introduction

The Lower Cretaceous platform started to grow in Central Oman after a major transgression over a structurally collapsed Middle Jurassic carbonate platform. From here, the edge of the platform prograded 250 km to the north in ca. 10 My. This prograding carbonate belt is expressed on seismic by well-defined clinoforms (Haan et al., 1990; Driessen et al., 2006). During this phase, bioclastic and oolitic ‘sands’ dominate the platform edge and these are referred to as the Habshan Formation, whereas the interbasinal carbonates are represented by the Lekhwair Formation.

Type and reference sections: Bab-2 in Abu Dhabi (Hassan et al., 1975). Additional surface reference section is in southeast Al Jabal Al Akhdar along the road to the Saiq Plateau (E57°41′30″, N23°02′00″), where the thickness is ca. 122 m. The Oman subsurface reference section is well Dhulaima-4 (Figure 7.12).

Figure 7.12:

Composite electrical logs, lithology and lithological description of the Habshan/Salil/Rayda formations, Kahmah Group, in well Dhulaima-4, North Oman (Mohammed et al., 1997). See Figure 7.1 for location.

Figure 7.12:

Composite electrical logs, lithology and lithological description of the Habshan/Salil/Rayda formations, Kahmah Group, in well Dhulaima-4, North Oman (Mohammed et al., 1997). See Figure 7.1 for location.

Lithology: The Habsan Formation is a clean carbonate unit of interbedded wacke- to grainstone textures with common ooidal and algal oncoid horizons and common shell debris. The upper section is dominated by grainstone, with excellent reservoir quality (porosities of up to 30% and permeabilities of up to 1,500 mD), the lower section by wackestone (Figure 7.14). Occasionally dolomites are present, which increase in abundance to the east. Metre-scale current bedded units can commonly be developed. There can be thick ooidal grainstones in the upper part, with abundant loose ooids in cuttings (e.g. Dhulaima-4, Figure 7.12).

Subsurface recognition: Whilst drilling the Formation is recognised by the change from argillaceous limestones of the lower part of the Lekhwair to the ooidal grainstones of the Habshan, which show up as abundant loose ooids in the cuttings (Figure 7.14). There is a positive drill break at the upper boundary (Natih-124, Figure 7.13), which is generally consistent over much of the area. There may be a negative drill break at the lower boundary (e.g. Dhulaima-7, Figure 7.15). Generally, there is a higher Rate of Penetration in the upper grainstones. The wackestones of the Habshan are similar to those of the Shu’aiba, Kharaib and Lekhwair and the limestones of the often underlying Sahtan Group. Hotshot sample or on-site analysis may confirm the presence of Biozone F53 fossils.

Figure 7.13:

Composite electrical logs and lithology of the Habshan/Salil/Rayda formations, Kahmah Group, in well Natih-124, North Oman (Mohammed et al., 1997). See Figure 7.1 for location.

Figure 7.13:

Composite electrical logs and lithology of the Habshan/Salil/Rayda formations, Kahmah Group, in well Natih-124, North Oman (Mohammed et al., 1997). See Figure 7.1 for location.

Figure 7.14:

Ditch cuttings from the Habshan Formation, Kahmah Group: (a) dolomite from Musallim-1; (b) limestonegrainstone from Musallim-1; and (c) the Rayda Formation, limestone-mudstone/wackestone from Dhulaima-7 (scale grid is 1 x 1 mm) (Mohammed et al., 1997).

Figure 7.14:

Ditch cuttings from the Habshan Formation, Kahmah Group: (a) dolomite from Musallim-1; (b) limestonegrainstone from Musallim-1; and (c) the Rayda Formation, limestone-mudstone/wackestone from Dhulaima-7 (scale grid is 1 x 1 mm) (Mohammed et al., 1997).

Figure 7.15:

Mud log showing Rate of Penetration in well Dhulaima-7 (North Oman) with negative drill break at the Habshan-Salil boundary (Mohammed et al., 1997). See Figure 7.1 for location.

Figure 7.15:

Mud log showing Rate of Penetration in well Dhulaima-7 (North Oman) with negative drill break at the Habshan-Salil boundary (Mohammed et al., 1997). See Figure 7.1 for location.

Post-drilling the Formation can be picked on log character. The Density-Neutron log character and separation may reflect the porosity changes within the sequence as a result of limestone texture (e.g. grainstones, wackestones). The Gamma readings are consistently low. The lower boundary can be picked on a Gamma break only, which may or may not coincide with sharp break on Density-Neutron log (Dhulaima-4, Figure 7.12).

On seismic the Habshan Formation is clearly recognisable by well-defined clinoforms (Haan et al., 1990; Driessen et al., 2006).

Boundaries: The lower boundary is transitional from the Salil, but the unit may lie disconformably over older units. The upper boundary is relatively sharp with a change from clean carbonates below into argillaceous limestones and marls of the Lekhwair above.

It is overlain everywhere by the Lekhwair. Underlain by the Salil or limestones of the Sahtan Group. The lower boundary with the Salil is difficult to identify in the Al Huwaisah wells due to a significant increase in limestones of the Salil Formation. In the northwest of Central Oman the lower boundary with the Sahtan Group is often difficult to identify due to the similarity of the dolomites in the lower part of the Habshan with those of the Sahtan Group.

Distribution: The Habshan Formation occurs only north of ca. 21°N. It overlies the Salil, but oversteps to the south and east to lie on various levels in the Sahtan Group. To the west and northwest, it thickens and passes laterally into the typical shallow-marine Habshan facies of Abu Dhabi.

Deposition: The sedimentary facies is consistent with a shallow, high-energy, probably tidal-marine setting, interpreted as the platform edge facies, time-equivalent to argillaceous limestones of the Salil Formation that form the slope deposits and the porcellaneous Rayda Formation representing the basinal facies.

Age: Valanginian – Hauterivian, ca. 140–130 Ma.

Microfossils found in the Habshan are indicative of an Early Cretaceous age, approximately Hauterivian in the reference section (Scott, 1990; Simmons and Hart, 1987). Sikkema (1991) and Simmons (1994) both comment on the difficulty of calibrating this (and older) Kahmah Group units, but a general Valanginian – Hauterivian age is supported by the available biological and regional evidence.

Because of its highly diachronous nature, the Habshan system will vary significantly in age in different areas, overall getting younger to the north (possibly even Barremian). In areas where the Habshan Formation is not underlain by Rayda and Salil formations, it is possible that the age ranges down to the Berriasian or even Tithonian as in Abu Dhabi (Alsharhan and Nairn, 1986), although this is not proven by fossil dating in Oman.

Biostratigraphy: Biozone F54 (lower part) (Pseudochrysalidina arabica), F53 (Pseudocylammina lituus) and F52 (upper part as Sub-biozone F527 (Calpionellites darderi and significant numbers of tintinnids). Simon Petroleum Technology (1995) recognised a diverse calpionellid assemblage in the basal Habshan Formation of well Dhulaima-7, with probable Calpionellites darderi and the tintinnid Tintinnopsella longa (e.g. Sub-biozone F527). The nannofossil Nannoconus steinmannii steinmannii was also recorded in abundance. Both fossil groups indicate an Early Valanginian age.

Salil Formation

Authors: Scherer (unpublished, 1969), see Hassan et al. (1975).

Introduction

The Salil Formation represents the slope facies of a rapidly prograding carbonate system that transgressed the Arabian Plate in the Early Cretaceous (Figure 7.2). The underlying Rayda Formation represents the contemporaneous deep-water equivalents, with the overlying Habshan and Lekhwair formations corresponding to respectively the platform edge and intra-platform carbonate facies (Droste and van Steenwinkel, 2004).

The best development of the Salil/Rayda formations is in western North Oman in the Lekhwair and adjacent areas. In the Al Huwaisah area the presence of the Rayda is questionable; the entire sequence is here assigned to the Salil and consists of limestone with lesser amounts of dolomite and marl.

Type and reference section: Southeast Al Jabal Al Akhdar, alongside the road to the Saiq Plateau (E57°41′30″, N23°02′00″). Additional reference section is Dhulaima-4 (Figure 7.12).

Lithology: A sequence of alternating thin limestones, argillaceous limestones and marls. The upper surfaces of some limestone beds are reddened. Limestone beds may show evidence of graded bedding.

Subsurface recognition: Whilst drilling, the Salil Formation is difficult to identify. It is usually a more marly argillaceous sequence than over- and underlying sequences, grading to cleaner limestones at the base. It has a positive drill break at the upper boundary (Dhulaima-7, Figure 7.15) of variable intensity e.g. there is an imperceptible change in Natih-124 (Figure 7.13). Hotshot samples may reveal that the sequence contains abundant tintinnids.

Post-drilling the Formation can be picked on faunal evidence; it comprises the F52/51 Biozones. The log character shows wide separation on Density-Neutron log and the top of the Salil Formation can be picked on a Gamma break only (increase). A break on the Density-Neutron log generally occurs above the Gamma pick (Dhulaima-4, Figure 7.12).

Boundaries: Both lower and upper boundaries are conformable and transitional. The upper boundary is picked at the base of a continuous clean carbonate, shown in the subsurface by Gamma-ray log character and in outcrop as the base of a massive weathering cliff, which forms the overlying Habshan Formation.

Distribution: The Salil Formation occurs over the same area as the Rayda Formation, but tends to overstep it to the southeast, reflecting the progradational nature of the Lower Kahmah succession. Absent south of approximately 22°N.

Deposition: Pelagic fossils in the marls and lower limestones indicate a sub-wavebase marine setting. The upward increase of shallow water debris, partly as turbidite-like beds, corresponds to the rapid progradation of a carbonate platform slope succession, progressively shallowing upwards.

Age: Late Berriasian – Valanginian, ca. 142–136 Ma. Sharland et al. (2001) correlate their MFS K30 and K20 surfaces into the Salil Formation.

Biostratigraphy: Biozone F53 (lower part) and F52 (to ?F51). Biozone F52 can be subdivided into Early Valanginian Sub-biozone F527 (Calpionellites darderi and significant numbers of tintinnids) and Late Berriasian Sub-biozone F523 (common small Calpionella alpina). Simmons (1994) again highlights the difficulty in calibrating this part of the Lower Kahmah Group (albeit only from outcrop work in the Oman Mountains) but agrees with Sikkema (1991) in assigning a dominantly Late Berriasian – Valanginian age to the Salil Formation. Calpionellites darderi is an Early Valanginian marker species (Sharland et al., 2001), which is probably associated with the K30 MFS, placed at ca. 1,990 m in Dhulaima-4 by Sharland et al. (2001). They position the Late Berriasian K20 MFS at ca. 2,118 m in the same well.

Simon Petroleum Technology (1995) supports this age assignment, having recorded the nannoplankton Nannoconus steinmannii steinmannii and N. steinmannii minor, the dinocysts Perisseiasphaeridium insolitum, Tubotuberella apatela, ?Batioladinium varigranosum, Muderongia microperforata and Tehamadinium daveyi together with the microfauna Calpionellites darderi, Calpionella alpina and Tintinnopsella longa.

Rayda Formation

Authors: Witt (unpublished, 1971), see Hassan et al. (1975).

Introduction

The pelagic carbonates and marls of the Rayda Formation represent deep marine deposition following basin subsidence and drowning by a global sea level rise in the Early Cretaceous (Figure 7.2).

Type and reference section: Southeast Al Jabal Al Akhdar, alongside the road to the Saiq Plateau (E57°41′30″, N23°02′00″). Additional reference section is Dhulaima-4 (Figure 7.12).

Lithology: The Rayda Formation is a sequence of thin-bedded, dense, porcellanitic limestones (lime mudstones) and marls with fine argillaceous partings and/or levels of thin cherts. At the base, a variable bed of crinoidal debris may locally be present with synsedimentary fracture fills (‘neptunian dykes’) and very locally a ‘bone-bed’ with concentrated fish-teeth, belemnites, ammonites and bone fragments is found at outcrop in Al Jabal Al Akhdar. The lower part of the unit may be variably reddened, particularly along the argillaceous partings. Horizontal burrows are present on bedding planes.

Subsurface recognition: Whilst drilling the Rayda can be recognised using ‘hotshot’ samples to determine the presence of Biozones F52/51 and the presence of radiolaria (thin-sections generally required). The limestones are cleaner than those of the Salil Formation. The upper boundary of the Rayda Formation is characterised by a positive drilling break (Lekhwair-85, Figure 7.16) and a decrease in Gamma log. Post-drilling the faunal evidence can be used to delineate the Formation. The presence of Biozones F52/51 and first occurrence of radiolaria is diagnostic.

Figure 7.16:

Mud log showing Rate of Penetration in well Lekhwair-85 (North Oman) with positive drill break at the Salil-Rayda boundary (Mohammed et al., 1997). See Figure 7.1 for location.

Figure 7.16:

Mud log showing Rate of Penetration in well Lekhwair-85 (North Oman) with positive drill break at the Salil-Rayda boundary (Mohammed et al., 1997). See Figure 7.1 for location.

Boundaries: The lower boundary is a sharp contact with the carbonates of the Sahtan Group, stepping down from the Jubaila Formation in the western subsurface to the Dhruma Formation in the Al Jabal Al Akhdar outcrops. The upper boundary is transitional, with the Rayda thin-bedded limestones passing upwards into alternating thin limestones and marls of the Salil Formation.

Distribution: The Rayda Formation occurs only in northern Oman, north of ca. 22°N. It thins and dies out to the west and southeast. It probably continues into the similar lithologies at the base of the upper Musandam Group in Ruus al Jibal (Hudson and Chatton, 1959; Ricateau and Riché, 1980).

Deposition: The Rayda Formation sediments are characterised by deep marine pelagic fossils (radiolaria, tintinnids, crinoids, belemnites) and sediment surface ichnofossils. The depositional environment is well below wave-base, with slow sedimentation. The deep marine sediments are interpreted as the basinal facies associated with the rapidly prograding Lower Kahmah shelf carbonates (Droste and Van Steenwinkel, 2004).

Subdivision: An upper and lower division has been distinguished in the outcrop type section based on a facies change (Haan et al., 1990). The lower unit of light grey-weathering, thin-bedded, lime mudstones with laminar and nodular cherts grades into an upper unit of medium- to thin-bedded, olive to dark grey-weathering mud-/wackestones. This has been interpreted as the onset of regression following initial drowning of the basin (Corbin and Mabillard, 1984).

Age: Latest Tithonian – Early Valanginian, ca. 147–139 Ma.

On the basis of the microfossil (calpionellid association) and macrofossil (ammonite) content, the Rayda Formation has been assigned to the uppermost Tithonian – Berriasian in Al Jabal Al Akhdar (Rousseau et al., 2005). Sharland et al. (2001) place their Early Berriasian K10 MFS within shales near the base of the Rayda Formation in well Dhulaima-4 and in the Wadi Al Muaydin outcrop. Interestingly they also illustrate a potential mid-Tithonian J110 MFS in Wadi Al Muaydin (their figure 4.53).

Biostratigraphy: Biozones F52 (lower part) and F51 (common large Calpionella alpina, C. elliptica and the presence of radiolaria).

Simon Petroleum Technology (1995) recorded a basal Rayda Formation assemblage with Crassicollaria parvula and numerous Calpionella alpina, which they interpret to be Late Tithonian – Early Berriasian in age.

Mesozoic Clastics Formation

Authors: This unit has not been published in external literature to date. It is herein proposed as a new formation.

Introduction

Internally, within PDO the Mesozoic Clastics Formation was previously loosely defined as the Mesozoic Clastics Group. The unit refers to a sequence of continental clastics encountered along the Eastern flank of the South Oman Salt Basin. The spatial-temporal genetic relationship of the Formation has been fairly unconstrained: it is bounded by the latest Aptian – Albian Nahr Umr Formation above, and the Upper Permian – Early Triassic Khuff Formation below. Because of its ‘often barren nature’, conventional biostratigraphic age dating remained broadly within a Jurassic to Lower Cretaceous range. The Upper

Cretaceous is discounted by the existing age constraint given by the overlying Nahr Umr Fm (latest Aptian at base).

From more recent work, based on variable palynology recovery, the age of the unit may be better constrained, but it is evident that the Formation could have a complex, long ranging depositional history. Osterloff (2000a) favours a Late Barremian to early Late Aptian age. The data presented, is however, not conclusive in this respect and the age may in fact range to Hauterivian or older. In a strict time correlative sense these clastic rocks are potentially age-equivalent to the Habshan, Lekhwair, Kharaib and even Shu’aiba formations. The bulk probably equates to the Lekhwair and Kharaib formations, and in particular with the more argillaceous Lekhwair Formation, e.g. in South Oman the Lekhwair Formation has sand recorded at its base.

The Triassic – Jurassic clastics of the Mafraq Formation occupy an older stratigraphic position, confined to North Oman, but also unconformably overlying the Khuff Formation, becoming younger to the south. It is possible that Mafraq and Mesozoic Clastics represent preserved end-members of an original time transgressive sequence of clastics that developed on top of the Khuff unconformity, with the older clastics eroded or non-deposited in the south and preserved in the north, and the younger clastics only developed in the south, preserved in salt withdrawal lows at the edge of the South Oman salt Basin.

Type and reference sections: Dimeet-1 (Figure 7.17). Additional reference sections are Al Burj-4 (Figure 7.18), Barah-1 (Figure 7.19), and Marmul-287 (Figure 7.20). All wells are in South Oman.

Figure 7.17:

Composite electrical logs, lithology and lithological description of the Mesozoic Clastics Formation, Kahmah Group, in well Dimeet-1, South Oman (Mohammed et al., 1997). See Figure 7.1 for location.

Figure 7.17:

Composite electrical logs, lithology and lithological description of the Mesozoic Clastics Formation, Kahmah Group, in well Dimeet-1, South Oman (Mohammed et al., 1997). See Figure 7.1 for location.

Figure 7.18:

Composite electrical logs, lithology and lithological description of the Mesozoic Clastics Formation, Kahmah Group, in well Al-Burj-4, South Oman (Mohammed et al., 1997). See Figure 7.1 for location.

Figure 7.18:

Composite electrical logs, lithology and lithological description of the Mesozoic Clastics Formation, Kahmah Group, in well Al-Burj-4, South Oman (Mohammed et al., 1997). See Figure 7.1 for location.

Figure 7.19:

Composite electrical logs, lithology and lithological description of the Mesozoic Clastics Formation, Kahmah Group, in well Barah-1, South Oman (Mohammed et al., 1997). See Figure 7.1 for location.

Figure 7.19:

Composite electrical logs, lithology and lithological description of the Mesozoic Clastics Formation, Kahmah Group, in well Barah-1, South Oman (Mohammed et al., 1997). See Figure 7.1 for location.

Lithology: The Mesozoic Clastics Formation is an interbedded sequence of sandstones (locally pebbly and argillaceous), siltstones, shales and conglomerates. The basal part (2–40 m) of the Mesozoic Clastics consists of poorly sorted, very fine to very coarse, angular to sub-rounded, chert-free sandstones (Basal A sands of Osterloff, 2000a, Figures 7.17 to 17.20, 17.21c and 7.22). Above this lower unit the clastics are chert-rich sandstones and conglomerates with greenish grey siltstones and red/purple shales. It can be confused with the Gharif Formation, from which it differs by the presence of chert, (except in the basal sandstones). The shales of the Mesozoic Clastics are similar in colour to the Khuff and Gharif shales. Khuff shales can be micaceous but it is generally impossible to differentiate between them.

Figure 7.20:

Composite electrical logs, lithology and lithological description of the Mesozoic Clastics Formation, Kahmah Group, in well Marmul-287, South Oman (Mohammed et al., 1997). See Figure 7.1 for location.

Figure 7.20:

Composite electrical logs, lithology and lithological description of the Mesozoic Clastics Formation, Kahmah Group, in well Marmul-287, South Oman (Mohammed et al., 1997). See Figure 7.1 for location.

Figure 7.21:

Ditch cuttings from Mesozoic Clastics Formation, Kahmah Group: (a) sandstone from Marmul-287; and (b and c) sandstone from Dimeet-1 (scale grid is 1 x 1 mm) (Mohammed et al., 1997).

Figure 7.21:

Ditch cuttings from Mesozoic Clastics Formation, Kahmah Group: (a) sandstone from Marmul-287; and (b and c) sandstone from Dimeet-1 (scale grid is 1 x 1 mm) (Mohammed et al., 1997).

Figure 7.22:

Mud log showing Rate of Penetration (ROP) in well Dimeet-1 (South Oman) with positive and negative drill breaks bounding the Basal sandstones of the Mesozoic Clastics Formation (Mohammed et al., 1997). See Figure 7.1 for location.

Figure 7.22:

Mud log showing Rate of Penetration (ROP) in well Dimeet-1 (South Oman) with positive and negative drill breaks bounding the Basal sandstones of the Mesozoic Clastics Formation (Mohammed et al., 1997). See Figure 7.1 for location.

Subsurface recognition: Whilst drilling the presence of often chert rich sandstones and red/purple siltstones/shales are diagnostic. The siltstones/shales sometimes contain fragments of chert otherwise they are similar to other formations.

The upper and lower boundaries of the Formation are marked by positive and negative drill breaks, respectively. The basal sand unit is generally marked by a positive drill break (Dimeet-1, Figure 7.22). The coarser clastics are clearly expressed on Gamma, Sonic, Density and Neutron logs.

Post-drilling the Mesozoic Clastics Formation is recognised by detailed lithological analysis supported by petrographic and biostratigraphic work, which can differentiate it from the Lower Cretaceous (Kahmah) and Permian (Khuff and Gharif formations). The very variable log character hampers intra-formational correlation.

Boundaries: It is overlain everywhere by the Nahr Umr Formation. The upper boundary is generally placed at the first appearance of either a red, sandy silty claystone (which sometimes contains chert fragments) or poorly sorted sandstone with a large chert component. Generally overlies the Khuff Formation but does in places overlie the Gharif Formation especially in the Marmul area (Al Lamki, 1997; Figure 7.23).

Figure 7.23:

Cross-section in Marmul field showing the Mesozoic Clastics Formation below the Nahr Umr Formation and above formations of Palaeozoic age (Al Lamki, 1997).

Figure 7.23:

Cross-section in Marmul field showing the Mesozoic Clastics Formation below the Nahr Umr Formation and above formations of Palaeozoic age (Al Lamki, 1997).

Distribution: The unit is confined to South Oman in an approximately 300 km long NE-trending depositional low. This is associated with salt withdrawal along the Eastern Flank of the South Oman Salt Basin.

Deposition: The Mesozoic Clastics are continental deposits with thick units of coarse-clastic channel fills including conglomerates with high clay matrix content. The lack of sorting of the sandstones, their irregularity of Dipmeter readings and ?lateral discontinuity suggests rapid (proximal) deposition. The varicoloured claystones are predominantly red-brown, red-purple with locally mottled green hues. The wide variation in colours and the mottled nature of the claystones supports the interpretation of paleosols in a mid-lower alluvial plain setting. Rare lignitic sediments have been reported from core, but conditions seem not suitably anaerobic enough to develop wider paralic, coal-forming conditions, indicating perhaps a more arid rather than humid environment.

The presence of terrestrially derived miospores and absence of marine microplankton and microfaunal evidence confirms the continental setting. However, towards the northern and southern fringes of the main Mesozoic Clastics depocentre, in-situ marine indicators have been noted, although in some instances marine evidence could be the result of caving from the overlying shallow marine Nahr Umr Formation in ditch cuttings.

Subdivision: Except for the basal, chert free, sands no further subdivisions have been recognised.

Age: Hauterivian to Early – mid Late Aptian, ca. 134–117 Ma.

Dated by palynological recovery, mainly terrestrially derived sporomorphs. The evidence is both patchy and, in places, open to interpretation. Future work may extend the age limits of this Formation.

Biostratigraphy: The larger portion of the Formation is barren and unless sidewall core or core recovery is available, cavings from the highly productive Nahr Umr Formation can mask in-situ, ditch-derived, assemblages. Osterloff (2000a) summarises and reinterprets results from several contractor and internal PDO studies. He indicates that the evidence is both ‘piece-meal’ and in need of further interrogation.

The biostratigraphic age range is based primarily on terrestial sporomorphs. Meaningful marine (palynological) taxa are rare and significant, in-situ, microfaunal recovery is virtually absent. Miospore assemblages tend to be dominated by simple smooth trilete spores (Cyathidites/Deltoidospora) or, to a lesser extent, the pollen Classopollis/Corollina gp. and Spheripollenites spp. Other elements, usually in small numbers, include ornamented trilete spores such as representatives of the genera Trilobosporites/Appendicisporites, Cicatricosisporites and Aequitriradites plus the pollen Callialasporites spp. (including C. dampieri and C. trilobatus). The marker pollen grain Dicheiropollis etruscus, occurs in at least three wells, notably in high numbers at the top of the Mesozoic Clastics section in well Barah-1 (Figure 7.19).

Osterloff (2000a) proposed a Late Barremian to Early – mid Late Aptian age, based on a limited number of wells with angiosperm (flowering plant) pollen and/or supporting dinoflagellate cyst and micropalaeontological evidence. However, the general assemblages noted above, and in particular the Barah-1 section with common/abundant Dicheiropollis etruscus point to a potentially older Early Cretacaeous age. The Barah-1 section (Figure 7.19) may even be restricted to Hauterivian and older in age.

Well Angudan-1, in the southern Kahmah fringe area of the Eastern Flank in South Oman (Osterloff, 2001a) yielded marine dinocyst assemblages, which include Muderongis simplex, Systematophora cf. areolata and Subtilispharea spp. The co-occurrence of these forms would restrict the age of the section to Hauterivian – Barremian, and again possibly even just Hauterivian.

Clearly the Mesozoic Clastics Formation is a complex set of sediments, with respect to both spatial and temporal distribution. It will remain difficult to date and future age limit revisions are to be expected.

Figures & Tables

Figure 7.1:

Location map: Kahmah Group.

Figure 7.1:

Location map: Kahmah Group.

Figure 7.2:

Schematic cross-section from South to North Oman showing the formations of the Kahmah Group (after Droste and van Steenwinkel, 2004).

Figure 7.2:

Schematic cross-section from South to North Oman showing the formations of the Kahmah Group (after Droste and van Steenwinkel, 2004).

Figure 7.3:

Composite electrical logs, lithology and lithological description of the Shu’aiba Formation, Kahmah Group, in well Lekhwair-258, North Oman (Mohammed et al., 1997). See Figure 7.1 for location.

Figure 7.3:

Composite electrical logs, lithology and lithological description of the Shu’aiba Formation, Kahmah Group, in well Lekhwair-258, North Oman (Mohammed et al., 1997). See Figure 7.1 for location.

Figure 7.4:

Composite electrical logs, lithology and lithological description of the Shu’aiba Formation, Kahmah Group, in well Al Huwaisah-2, North Oman (Mohammed et al., 1997). See Figure 7.1 for location.

Figure 7.4:

Composite electrical logs, lithology and lithological description of the Shu’aiba Formation, Kahmah Group, in well Al Huwaisah-2, North Oman (Mohammed et al., 1997). See Figure 7.1 for location.

Figure 7.5:

Applied lithostratigraphy of the Aptian Shu’aiba Formation (after Droste, 2003). The Formation overlies the Hawar Member of the Kharaib Formation and is overlain by the Nahr Umr Formation of the Wasia Group. Note informal division of Shu’aiba in Central Oman. Central Oman Shu’aiba correlates to Lower Shu’aiba only, as defined in Northwest Oman.

Figure 7.5:

Applied lithostratigraphy of the Aptian Shu’aiba Formation (after Droste, 2003). The Formation overlies the Hawar Member of the Kharaib Formation and is overlain by the Nahr Umr Formation of the Wasia Group. Note informal division of Shu’aiba in Central Oman. Central Oman Shu’aiba correlates to Lower Shu’aiba only, as defined in Northwest Oman.

Figure 7.6:

Ditch cuttings from Shu’aiba Formation, Kahmah Group: (a and b) limestone-boundstone Bacinella from Lekhwair-1; (scale grid is 1 x 1 mm) (Mohammed et al., 1997).

Figure 7.6:

Ditch cuttings from Shu’aiba Formation, Kahmah Group: (a and b) limestone-boundstone Bacinella from Lekhwair-1; (scale grid is 1 x 1 mm) (Mohammed et al., 1997).

Figure 7.7:

Composite electrical logs, lithology and lithological description of the Shu’aiba/Kharaib/Lekhwair formations, Kahmah Group, in well Lekhwair-7, North Oman (Mohammed et al., 1997). See Figure 7.1 for location.

Figure 7.7:

Composite electrical logs, lithology and lithological description of the Shu’aiba/Kharaib/Lekhwair formations, Kahmah Group, in well Lekhwair-7, North Oman (Mohammed et al., 1997). See Figure 7.1 for location.

Figure 7.8:

Composite electrical logs, lithology and lithological description of the Shu’aiba/Kharaib/Lekhwair formations, Kahmah Group, in well Hazar-2, Central Oman (Mohammed et al., 1997). See Figure 7.1 for location.

Figure 7.8:

Composite electrical logs, lithology and lithological description of the Shu’aiba/Kharaib/Lekhwair formations, Kahmah Group, in well Hazar-2, Central Oman (Mohammed et al., 1997). See Figure 7.1 for location.

Figure 7.9:

Composite electrical logs, lithology and lithological description of the Shu’aiba/Kharaib/Lekhwair formations, Kahmah Group, in well Hasirah-1, Central Oman (Mohammed et al., 1997). See Figure 7.1 for location.

Figure 7.9:

Composite electrical logs, lithology and lithological description of the Shu’aiba/Kharaib/Lekhwair formations, Kahmah Group, in well Hasirah-1, Central Oman (Mohammed et al., 1997). See Figure 7.1 for location.

Figure 7.10:

Composite electrical logs, lithology and lithological description of the Lekhwair/Kharaib formations, Kahmah Group, in well Jazal-1, South Oman (Mohammed et al., 1997). See Figure 7.1 for location.

Figure 7.10:

Composite electrical logs, lithology and lithological description of the Lekhwair/Kharaib formations, Kahmah Group, in well Jazal-1, South Oman (Mohammed et al., 1997). See Figure 7.1 for location.

Figure 7.11:

Fossils from the Kharaib/Lekhwair formations: (a-d) Rare, Choffatella decipiens from the Lower Aptian – Hauterivian Biozone F55; and (e and f) Rare, Salpingoporella dinarica (algae), all from the Lower Aptian – Hauterivian Biozone F55 (Mohammed et al., 1997).

Figure 7.11:

Fossils from the Kharaib/Lekhwair formations: (a-d) Rare, Choffatella decipiens from the Lower Aptian – Hauterivian Biozone F55; and (e and f) Rare, Salpingoporella dinarica (algae), all from the Lower Aptian – Hauterivian Biozone F55 (Mohammed et al., 1997).

Figure 7.12:

Composite electrical logs, lithology and lithological description of the Habshan/Salil/Rayda formations, Kahmah Group, in well Dhulaima-4, North Oman (Mohammed et al., 1997). See Figure 7.1 for location.

Figure 7.12:

Composite electrical logs, lithology and lithological description of the Habshan/Salil/Rayda formations, Kahmah Group, in well Dhulaima-4, North Oman (Mohammed et al., 1997). See Figure 7.1 for location.

Figure 7.13:

Composite electrical logs and lithology of the Habshan/Salil/Rayda formations, Kahmah Group, in well Natih-124, North Oman (Mohammed et al., 1997). See Figure 7.1 for location.

Figure 7.13:

Composite electrical logs and lithology of the Habshan/Salil/Rayda formations, Kahmah Group, in well Natih-124, North Oman (Mohammed et al., 1997). See Figure 7.1 for location.

Figure 7.14:

Ditch cuttings from the Habshan Formation, Kahmah Group: (a) dolomite from Musallim-1; (b) limestonegrainstone from Musallim-1; and (c) the Rayda Formation, limestone-mudstone/wackestone from Dhulaima-7 (scale grid is 1 x 1 mm) (Mohammed et al., 1997).

Figure 7.14:

Ditch cuttings from the Habshan Formation, Kahmah Group: (a) dolomite from Musallim-1; (b) limestonegrainstone from Musallim-1; and (c) the Rayda Formation, limestone-mudstone/wackestone from Dhulaima-7 (scale grid is 1 x 1 mm) (Mohammed et al., 1997).

Figure 7.15:

Mud log showing Rate of Penetration in well Dhulaima-7 (North Oman) with negative drill break at the Habshan-Salil boundary (Mohammed et al., 1997). See Figure 7.1 for location.

Figure 7.15:

Mud log showing Rate of Penetration in well Dhulaima-7 (North Oman) with negative drill break at the Habshan-Salil boundary (Mohammed et al., 1997). See Figure 7.1 for location.

Figure 7.16:

Mud log showing Rate of Penetration in well Lekhwair-85 (North Oman) with positive drill break at the Salil-Rayda boundary (Mohammed et al., 1997). See Figure 7.1 for location.

Figure 7.16:

Mud log showing Rate of Penetration in well Lekhwair-85 (North Oman) with positive drill break at the Salil-Rayda boundary (Mohammed et al., 1997). See Figure 7.1 for location.

Figure 7.17:

Composite electrical logs, lithology and lithological description of the Mesozoic Clastics Formation, Kahmah Group, in well Dimeet-1, South Oman (Mohammed et al., 1997). See Figure 7.1 for location.

Figure 7.17:

Composite electrical logs, lithology and lithological description of the Mesozoic Clastics Formation, Kahmah Group, in well Dimeet-1, South Oman (Mohammed et al., 1997). See Figure 7.1 for location.

Figure 7.18:

Composite electrical logs, lithology and lithological description of the Mesozoic Clastics Formation, Kahmah Group, in well Al-Burj-4, South Oman (Mohammed et al., 1997). See Figure 7.1 for location.

Figure 7.18:

Composite electrical logs, lithology and lithological description of the Mesozoic Clastics Formation, Kahmah Group, in well Al-Burj-4, South Oman (Mohammed et al., 1997). See Figure 7.1 for location.

Figure 7.19:

Composite electrical logs, lithology and lithological description of the Mesozoic Clastics Formation, Kahmah Group, in well Barah-1, South Oman (Mohammed et al., 1997). See Figure 7.1 for location.

Figure 7.19:

Composite electrical logs, lithology and lithological description of the Mesozoic Clastics Formation, Kahmah Group, in well Barah-1, South Oman (Mohammed et al., 1997). See Figure 7.1 for location.

Figure 7.20:

Composite electrical logs, lithology and lithological description of the Mesozoic Clastics Formation, Kahmah Group, in well Marmul-287, South Oman (Mohammed et al., 1997). See Figure 7.1 for location.

Figure 7.20:

Composite electrical logs, lithology and lithological description of the Mesozoic Clastics Formation, Kahmah Group, in well Marmul-287, South Oman (Mohammed et al., 1997). See Figure 7.1 for location.

Figure 7.21:

Ditch cuttings from Mesozoic Clastics Formation, Kahmah Group: (a) sandstone from Marmul-287; and (b and c) sandstone from Dimeet-1 (scale grid is 1 x 1 mm) (Mohammed et al., 1997).

Figure 7.21:

Ditch cuttings from Mesozoic Clastics Formation, Kahmah Group: (a) sandstone from Marmul-287; and (b and c) sandstone from Dimeet-1 (scale grid is 1 x 1 mm) (Mohammed et al., 1997).

Figure 7.22:

Mud log showing Rate of Penetration (ROP) in well Dimeet-1 (South Oman) with positive and negative drill breaks bounding the Basal sandstones of the Mesozoic Clastics Formation (Mohammed et al., 1997). See Figure 7.1 for location.

Figure 7.22:

Mud log showing Rate of Penetration (ROP) in well Dimeet-1 (South Oman) with positive and negative drill breaks bounding the Basal sandstones of the Mesozoic Clastics Formation (Mohammed et al., 1997). See Figure 7.1 for location.

Figure 7.23:

Cross-section in Marmul field showing the Mesozoic Clastics Formation below the Nahr Umr Formation and above formations of Palaeozoic age (Al Lamki, 1997).

Figure 7.23:

Cross-section in Marmul field showing the Mesozoic Clastics Formation below the Nahr Umr Formation and above formations of Palaeozoic age (Al Lamki, 1997).

GROUPFORMATIONMEMBER
  Shu'aibaUpper Shu’aiba
  Lower Shu’aiba
 Mesozoic ClasticsKharaibHawar
KahmahLekhwair 
  Habshan 
  Salil 
  Rayda 
GROUPFORMATIONMEMBER
  Shu'aibaUpper Shu’aiba
  Lower Shu’aiba
 Mesozoic ClasticsKharaibHawar
KahmahLekhwair 
  Habshan 
  Salil 
  Rayda 
ZoneSubzoneMarker speciesRelative ageFormation/Member
F56F567Orbitolina (M.) parvaEarly – Late AptianShu’aiba
F563Palorbitolina lenticularis, P. cormyi
F55F557Choffatella decipiens, Salpingoporella dinaricaEarly Barremian – earliest AptianKharaib
F553Salpingoporella muehlbergiiUppermost Lekhwair - Kharaib
F54 Pseudochrysalidina arabicaHauterivian – Early BarremianUppermost Habshan - Lekhwair
F53 Pseudocyclammina lituus, P. cylindricaValanginian – HauterivianRayda - Salil - Habshan
F52F527Calpionella darderi, common calpionellids, common tintinnids
F523common small Calpionella alpinaLate BerriasianRayda - Salil
F51 common large Calpionella alpina, C. elliptica, radiolariaLate Tithonian – Early BerriasianRayda
ZoneSubzoneMarker speciesRelative ageFormation/Member
F56F567Orbitolina (M.) parvaEarly – Late AptianShu’aiba
F563Palorbitolina lenticularis, P. cormyi
F55F557Choffatella decipiens, Salpingoporella dinaricaEarly Barremian – earliest AptianKharaib
F553Salpingoporella muehlbergiiUppermost Lekhwair - Kharaib
F54 Pseudochrysalidina arabicaHauterivian – Early BarremianUppermost Habshan - Lekhwair
F53 Pseudocyclammina lituus, P. cylindricaValanginian – HauterivianRayda - Salil - Habshan
F52F527Calpionella darderi, common calpionellids, common tintinnids
F523common small Calpionella alpinaLate BerriasianRayda - Salil
F51 common large Calpionella alpina, C. elliptica, radiolariaLate Tithonian – Early BerriasianRayda
ZoneMarker speciesRelative age
571Ascodinium yibaliiLate Barremian – Aptian
648Gardodinium cerviculumLate Hauterivian – Early Barremian
578Muderongia neocomicaBerriasian – Early Hauterivian
850Hystrichosphaeridium irregulareKimmeridgian – Portlandian
ZoneMarker speciesRelative age
571Ascodinium yibaliiLate Barremian – Aptian
648Gardodinium cerviculumLate Hauterivian – Early Barremian
578Muderongia neocomicaBerriasian – Early Hauterivian
850Hystrichosphaeridium irregulareKimmeridgian – Portlandian

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