Skip to Main Content
GROUPFORMATIONMEMBERSEQUENCE‘SUB-UNIT’/SEQUENCE CODING
AraDhahaban A6A6 Carbonate/Shale (A6C)
A6 Salt + Anhydrite (A6E)
Al Noor A6 Intrasalt Clastics (A6CL)
A5A5 Carbonate (A5C)
A5 Salt + Anhydrite (A5E)
AthelThuleilatA4*A4 Athel Shale (A4AS)
Al ShomouA4 Athel Silicilyte (A4Sil)
ThamoudA4 Athel Carbonate (A4AC)
U A4 U-Carbonate (A4UC)
A4 U-Shale (A4US)
A4 Carbonate (A4C)
A4 Salt + Anhydrite (A4E)
Birba A3A3 Carbonate (A3C)
A3 Salt + Anhydrite (A3E)
A2A2 Carbonate (A2C)
A2 Salt + Anhydrite (A2E)
A1A1 Carbonate (A1C)
A1 Salt + Anhydrite (A1E)
A0A0 Basal Carbonate + Shale (A0C)
GROUPFORMATIONMEMBERSEQUENCE‘SUB-UNIT’/SEQUENCE CODING
AraDhahaban A6A6 Carbonate/Shale (A6C)
A6 Salt + Anhydrite (A6E)
Al Noor A6 Intrasalt Clastics (A6CL)
A5A5 Carbonate (A5C)
A5 Salt + Anhydrite (A5E)
AthelThuleilatA4*A4 Athel Shale (A4AS)
Al ShomouA4 Athel Silicilyte (A4Sil)
ThamoudA4 Athel Carbonate (A4AC)
U A4 U-Carbonate (A4UC)
A4 U-Shale (A4US)
A4 Carbonate (A4C)
A4 Salt + Anhydrite (A4E)
Birba A3A3 Carbonate (A3C)
A3 Salt + Anhydrite (A3E)
A2A2 Carbonate (A2C)
A2 Salt + Anhydrite (A2E)
A1A1 Carbonate (A1C)
A1 Salt + Anhydrite (A1E)
A0A0 Basal Carbonate + Shale (A0C)
*

No relative stratigraphical position implied. Some key uncertainties remain concerning the geographical and temporal relationships of some units within the A4 sequence.

Authors: Parish (unpublished, 1960), published by Hughes Clarke (1988) as the Ara Formation. Elevated to Group level by Teyssen (1990).

Introduction

The Ara Group comprises sediments laid down in a transpressional/transtensional basin setting related to the final convergence of East and West Gondwana (Linskaill and Teyssen, 1991; Immerz et al., 2000; Grotzinger et al., 2002; van Den Berg et al., 2008). Basins in Oman include the South Oman Salt Basin, the Ghaba Salt Basin and the Fahud Salt Basin. They comprise a roughly north/northeast – south/southwest trending system of restricted basins assumed to be coeval with the Hormuz Series salts of the Arabian Peninsula and southwest Iran (Al-Husseini and Husseini, 1990; Al-Husseini et al., 2003).

The stratigraphy and sedimentology of Ara Group rocks is discussed in a number of publications (Mattes et al., 1984; Mattes, 1985; Teyssen, 1990; Mattes and Conway Morris, 1990; Schröder, 2000; Peters et al., 2003; Al-Husseini et al., 2003; Al Siyabi, 2005; Amthor et al., 2005; Schröder et al., 2003, 2004, 2005). Age constraints based on chemostratigraphy (Burns and Matter, 1993; Schröder, 2000), biostratigraphy (Mattes and Conway Morris, 1990; Conway Morris et al., 1990; Brasier, 1999; Fike et al., 2006; Fike, 2007) and geochronology (Amthor et al., 2003; Bowring et al., 2007) confirm a terminal Neoproterozoic (latest Ediacaran) to earliest Cambrian age for the Ara Group, i.e. it straddles the Neoproterozoic – Cambrian boundary.

The sequence of carbonates, siliciclastics and evaporites reflects changes in depth and salinity of water in the original depositional basin. The evaporites and carbonates are evidence for a series of basins restricted from the sea, most likely in an overall long-term, evaporitic-drawdown, sub-sealevel setting. Thick salt deposits formed during periods of low sealevel and maximum evaporitic drawdown. The carbonates were deposited on a range of shelf, platform or carbonate ramp settings. The Ara encompasses one of the most important hydrocarbon systems in Oman, with self-contained reservoir, seal (Schoenherr et al., 2007a) and charge (Grantham et al., 1988; Visser, 1991; Terken et al., 2001; Schoenherr et al., 2007b; Grosjean et al., 2009). For an overview of the exploration history of this important system, see Al Siyabi (2005). It is assumed to be an important source for hydrocarbons in younger, overlying fields (Terken et al., 2001). However, distinct source rocks within Ara carbonates have not been identified yet. It is assumed that deeper parts of the basin were at least periodically anaerobic, resulting in the preservation of substantial amounts of organic material and the formation of hydrocarbon source rocks (see also Schröder and Grotzinger, 2007).

Ara sedimentation started with the rapid deposition of the carbonates and evaporites of the Birba Formation, already differentiated in both shallow shelf and deeper-basin facies. Deposition was associated with marked volcanic activity.

The evaporitic mode of the basin changed around the Neoproterozoic – Cambrian boundary associated with the onset of limited (fine) clastic sedimentation of the Athel and U formations, which both contain shale. The Athel Formation also contains silicilytes (a sequence of laminated cherts), whose occurrence is localised, and whose origin is enigmatic but may well be related to key events at the Neoproterozoic – Cambrian boundary (Amthor et al., 2005; Schröder and Grotzinger, 2007).

Evaporite and, in a more limited sense also carbonate sedimentation, then resumed with a clear change to a more uniform basin setting with the Al Noor and Dhahaban formations. This was associated with increasing clastic influence compared to the Birba Formation, sourced from basin margins in the southwest and west. Where the Athel and U formations are absent, the change can be detected by the sharp transition from clean to argillaceous/silty salt. Pure clastic sedimentation subsequently occurs with the deposition of the Nimr Group.

Type and reference sections: Ara Group: Shamah-1 in South Oman (Figure 16.2). Herein replaces original type well Birba-1 (Hughes Clarke 1988). Following Romine et al. (2008) the uppermost, fine clastic, section of the Ara in Birba-1 is re-interpreted as Nimr Group.

Figure 16.1:

Location map: Ara Group.

Figure 16.1:

Location map: Ara Group.

Figure 16.2:

Composite electrical logs, lithology and lithological description of the Ara Group in well Shamah-1, South Oman. See Figure 16.1 for location.

Figure 16.2:

Composite electrical logs, lithology and lithological description of the Ara Group in well Shamah-1, South Oman. See Figure 16.1 for location.

Selecting a type well for the Group, which has all formations and units represented is not possible. As such Shamah-1 is one of the most representative wells. A combination of the following formational type wells, all in South Oman, provides the most appropriate overview of the Group.

Figure 16.3:

Composite electrical logs, lithology and lithological description of the Dhahaban Formation, Ara Group, in well Dhahaban South-1, South Oman (Mohammed et al., 1997). See Figure 16.1 for location.

Figure 16.3:

Composite electrical logs, lithology and lithological description of the Dhahaban Formation, Ara Group, in well Dhahaban South-1, South Oman (Mohammed et al., 1997). See Figure 16.1 for location.

Lithology: The Ara Group is dominated by a cyclic arrangement of evaporites (mainly halite, common anhydrite, rarely sylvite) and carbonates (mostly dolomites), with associated siliciclastics and rare interbedded volcanic ash beds. The carbonates often ‘float’ isolated in the salt (hence the term ‘stringers’), they can attain considerable thickness (50–200 m) and can be traced (on seismic sections) over large areas within the salt. The carbonates are of (restricted) marine origin and have been deposited on platform, rimmed shelf or carbonate-ramp settings.

Carbonate depositional facies include microbial boundstones, intraclast-peloid-ooid grainstone-packstones, and mudstones. Microbial facies dominate, and display a variety of textures that conform to systematic variations in water depth and inferred accommodation regimes. Platform interior facies consist of peritidal stromatolites with pustular, smooth, and tufted textures. These pass laterally into thrombolite sheet and mound facies, which then pass downslope into offshore mudstones that interfinger with crinkly laminites in the most distal settings. Crinkly laminites are widespread in basinal settings and result from accumulation of both pelagic and benthic microbial organic debris. These basinal microbialites form one reservoir type whose performance deteriorates in proportion to the influx of shelf-derived muds. Other reservoirs are developed principally in shelf interior to shelf margin microbialites and associated grainstones.

Ara evaporites comprise halite, anhydrite, and minor amounts of the higher salts.

Anhydrite is the only major sulphate mineral present in the Ara Group today, but it replaced primary gypsum. Each carbonate ‘stringer’ is commonly bound above and below by relatively thin anhydritic layers acting as transitions into continuous halite sections. An increase in brine concentration led to the prograding depositional succession carbonate - sulphate - halite (partly including higher salts), while the retrograding succession (decreasing brine concentration) generates a reverse halite - sulfate - carbonate series (Schröder, 2000). Three distinct anhydrite facies associations can be distinguished (Mattes et al., 1984; Schröder, 2000).

  • (1) ‘Floor Anhydrite’: intervals of the retrograding succession, sandwiched between underlying salt and overlying carbonate deposits, with sharp upper and lower contacts, laminated or contain depositional/dissolution breccias and graded beds.

  • (2) ‘Roof Anhydrite’: often thicker, massive intervals of the prograding succession with nodular character, which occur at top of each carbonate cycle and below the next overlying salt.

  • (3) ‘Chicken-wire Anhydrite’: associated with dolomite in the upper parts of each carbonate cycle, occurs in variety of forms, but most common as nodular or ‘chicken-wire’ mosaics.

Volumetrically the most significant evaporite phase in the South Oman Salt Basin is halite. Halite precipitation is typically rapid (10+ metres per thousand years), initially infilling the basinal areas and later forming regional extensive units that entomb the shallow-water carbonates. The original halite thickness is hard to estimate due to post-depositional diapirism and possible extensive dissolution, but halite deposits can be many tens to hundreds of metres thick (Bowring et al., 2007).

Massive beds of higher salts have been interpreted from open-hole logs (Mattes et al., 1984; Faulkner, 1996). Higher salts may be disseminated in halite layers. XRD analyses of some bulk-rock halite samples have demonstrated the presence of sylvite (up to 10 vol.%), polyhalite, as well as traces of bloedite and/or mirabilite. The two latter minerals form cements between halite crystals, and they usually occur at the contact between halite and a floor anhydrite.

Potash salt precipitation occurs when salinity is greater than 50%. To reach such high salinity levels a strong evaporitic drawdown is required. Low relative humidity, below 35%, is needed to form and preserve potassic salts. Hence, the high salts usually form from very shallow, highly concentrated brines during phases of final basin desiccation. Settings arid enough to precipitate potassic and magnesian salts are rare and occur either at very high elevations and/or within mountain range shadows developed in hot and arid regions. Potash salts are typically associated with the deepest part of the basin, in the regions of maximum subsidence (Faulkner, 1996), but have also been encountered in isolated intra-salt sub-basins.

The Athel Formation contains locally a succession of several hundred metres of laminated cherts (silicilytes) that are bounded above by the organic-rich Thuleilat shale and below by the organic-rich shales of the U Formation (Amthor et al., 2005).

Boundaries: The Ara Group lies on the uniform dolomite sequence of the Buah Formation, locally with an anhydrite layer at the base. Existing δ34S isotope data suggests that the contact between the two may be a condensed section or unconformity, which is also recognised locally on seismic. This boundary approximately coincides with the down hole, essentially A0 level, shift from δ34S values of over 35-40‰ to 20–25‰ in the Nafun Group (see Figure 15.1 and Fike and Grotzinger, 2008). Note, however, that this shift is not seen in the type well Shamah-1 (Figure 16.2), where interpreted Buah Formation retains a high δ34S value. Similar conflicts occur in other wells and these have yet to be satisfactorily resolved. The top of the Ara Group is formed by a laterally variable unit made up of fine dolomites, silts or clays. This unit separates the halite sequences from the overlying rock units and is distinguished as the Dhahaban Formation.

It is unconformably overlain by the Nimr, Mahatta Humaid or Haushi groups. Near the western margin of the South Oman Salt Basin the transition is gradual and potentially conformable, with respect to a clastic influenced Dhahaban Formation overlain by Nimr Group fines (Karim facies).

Distribution: In well penetrations, the evaporitic Ara Group is highly variable in thickness, facies and internal sequence.

This variability reflects genetic patterns of lateral facies variations, further complicated by large-scale halokinetic movements and loss of salt by dissolution. Salt movements took place during the Palaeozoic and Mesozoic and markedly affected the depositional patterns and structures in the overlying Haima to Akhdar units (Heward, 1990). The present distribution of the Ara Salt does not represent the original outline of the evaporitic basins, but is a result of Palaeozoic tectonism and of continuing salt movement and loss.

The Ara Group is a lateral equivalent of, and may even have been genetically continuous with, the Hormuz ‘series’ of the Persian Gulf and southern Iran, and the ‘saline series’ of the Salt Range in Pakistan. The age inferred for the Hormuz from the Iranian diapirs and their setting in Iran (Infra-Cambrian to Mid-Cambrian; Kent, 1979; Al-Husseini and Husseini, 1990) is in keeping with the inferred age for the Ara surface diapirs in the Ghaba Basin (Peters et al., 2003). These show many similarities to the Hormuz diapirs in terms of the the range of lithologies brought to surface entrained in the salt (in particular the laminated dolomites), but differ in the minor content of igneous material in the Oman diapirs. The rather heterogeneous Dhahaban Formation, the shallowest carbonate of the Ara Group, probably owes part of its variability to its stratigraphic position in the top of the salt, complicated by salt dissolution. Thick continuous carbonate successions below the Haima penetrated in the Eastern flank of the South Oman Salt Basin have been loosely referred to as ‘remnant Ara’, even though no salt is present.

The Ara Group is in part time equivalent to the Fara Formation in the Al Jabal Al Akhdar (Bowring et al., 2007). The Fara Formation consists of a succession of laminated cherts and volcaniclastics (Rabu, 1988; McCarron, 2000; Bowring et al., 2007). Mapping in the Al Huqf outcrop area has revealed a succession of carbonates in an approximate equivalent stratigraphic position to the subsurface Ara Group. It is difficult to define a reliable correlation with the subsurface in the absence of any reliable age dates, but further isotope correlation work may provide further insight (e.g. the Sarab Formation of Nicholas and Gold, in preparation).

Deposition: The bromine geochemistry of the evaporites indicate a largely marine origin for the evaporite brines (Schreiber and Schröder, 2001; Schröder et al., 2003, 2004, 2005; Schröder and Grotzinger, 2007). Integration of these data with evaporite facies analysis and with the stratigraphic relationships between Ara carbonates and evaporites suggests that the Ara salt was deposited from shallow-marine brines in a physiographically deep, partially desiccated basin (deep basin - shallow water model). Halite accumulates during sea level lowstands and forms basin fill or onlap geometries. The presence of interbedded halite, potassium salts and red beds suggests that the halite was deposited in a very shallow sea or a series of highly saline lakes.

The Ara Group sequences represent a period of relatively stable arid climatic conditions in a tectonically active basin. Strong subsidence allowed for the accommodation of thick evaporites, while tectonic barriers simultaneously provided the necessary restricted conditions. The carbonate stringers represent more normal marine conditions during the transgressive and highstand system tracts of individual sequences, either as interludes between or lying marginal to the hypersaline water bodies. The Ara evaporites blanket the carbonate platforms (post A2C only), which requires prolonged and extensive hypersaline conditions fed by continual barrier seepage in a ‘mega-halite’ setting. Some of the fine clastic intercalations have been plausibly interpreted as aeolian sediments, deposited either into a hypersaline basin or over a desiccated, dry evaporite surface. Additionally clastics are likely to have been shed from the western margin of the South Oman Salt Basin.

Subdivision: The Ara Group can be subdivided (from top to bottom) into the Dhahaban, Al Noor, Athel/U and Birba formations. Numerous factors, primarily associated with tectonics and halokinesis can make it extremely difficult to consistently differentiate and correlate formations, let alone members or sequence units within the South Oman Salt Basin. In particular the relationship of the Athel Formation to the other units has yet to be fully and confidently resolved.

Sequence stratigraphy: The Ara Group comprises at least six third-order evaporite carbonate sequences (A1 to A6) in addition to a basal pre-evaporitic carbonate (A0) (Schröder, 2000), and all are part of the AP1 Megasequence of Sharland et al. (2001). Sharland et al. (2001) place their Cm10 MFS (revised in Sharland et al., 2004) at the base of the limestones in the A4C unit.

An ‘idealised’ Ara evaporite-carbonate sequence can be defined based on sequence boundaries. A sequence boundary is placed between evaporites above and carbonates below, defining a sequence that comprises an evaporite and carbonate depositional cycle. Evaporites occur within the lower parts of the sequences, precipitated in the lowstand systems tract (lowstand basin fill gypsum and halite) following initial drawdown. Potash salts may form in instances of extreme basin restriction. Carbonates, and some associated gypsum layers, occur in the upper parts of sequences, mostly deposited on platforms in the basin centre and around the basin margin, and these represent the transgressive and highstand systems tracts associated with basin flooding.

Sequence boundaries could be developed on the carbonate platforms surrounding the salt basin. During base-level lowstands the margins of the salt basin would be exposed and a palaeokarst could develop. However, direct evidence for this is lacking and no top stringer karst has been observed in salt encased carbonates. The sequence boundary within the basin will be a correlative conformity at the base of the evaporites, below salt-pan gypsum (anhydrite) and halite.

Ara Group carbonates occupy positions within the basin (in addition to flank-related deposits) and vary in thickness from 50–200 m. Ara carbonate platforms formed during transgressive to highstand accommodation conditions, superimposed upon a progressive, long-term accommodation increase, which forced platforms within each cycle to occupy progressively less area (i.e. a back-stepping system). Older platforms (e.g. A1C and A2C) are laterally more extensive. Intermediate age platforms (e.g. A3C) are more differentiated with respect to shelf margin and slope-to-basin facies. Younger platforms are thinner, often dominated by transgressive system tract deeper-water facies (e.g. A4C), and the A5C consists of numerous smaller, more isolated pinnacle reefs. The differentiation of the two upper sequences A5 and A6 is often problematic and is further hampered by dissolution at the top of the salt.

Thick evaporite sequences such as the Ara salt require pre-existing depressions to allow accumulation. Because evaporite accumulation rates are typically very rapid (10+ metres per thousand years), tectonically-induced subsidence probably remained significant during Ara salt deposition.

Depending on the climatic and palaeogeographic conditions, evaporites may occupy a place in each of the systems tracts of a depositional sequence. However, thick halite-dominated successions such as the Ara Group can only develop during sea-level lowstands due to evaporitic drawdown.

Carbonates and thick, basin-centre halites do not form at the same time in a basin. Shallow-water carbonate sedimentation ceases before basinal evaporite precipitation begins, hence the existence of ‘floor’ and ‘roof’ anhydrites. Salt deposits do not pass laterally into carbonates in the Ara system.

The platform carbonates have a complex internal architecture of deep to shallow-water facies (Grotzinger, 2000; Grotzinger and Amthor, 2002; Al Siyabi, 2005), and are generally completely sealed by thick sequences of mostly shallow-water evaporites (although stacked carbonates occur locally, such as on the Birba Platform). Ara evaporites subsequently blanket these carbonate platforms, providing top and base seals for the hydrocarbons contained within the carbonates.

Age: Latest Ediacaran – earliest Cambrian, ca. 547–538 Ma. Until the late 1990s the entire Ara Group was placed in the terminal Proterozoic, mainly based on biostratigraphy (the presence of Cloudina and probable Namacalathus in the Birba Formation) (Conway Morris et al., 1990) and chemostratigraphy (Burns and Matter, 1993; Braiser et al., 2000; McCarron, 2000). Work since then involving significant carbon isotopic data, biostratigraphy and U-Pb geochronology (volcanic tuffs) has placed the upper part of the Ara in the ‘Early’ Cambrian (Amthor et al., 2003; Al-Husseini et al., 2003; Bowring et al., 2007; see Figure 15.1). Bowring et al. (2007) speculate on the average duration of a Carbonate-Evaporite Cycle, stating 1.2 to 1.3 My. They base this on four U-Pb tuff ages from the Birba and basal U formations, which are indeed relatively well age constrained. However, it may be premature to extrapolate such time-scale conclusions into the more complex Al Noor and Dhahaban formations with their potentially significantly different evolution (see below). The combined evidence indicates that the Ara Group is latest Ediacaran to earliest Cambrian in age, ca. 547–538 Ma. Taking all evidence into account a probable minimum duration for the whole of the Ara Group is in the region of 9 million years.

Biostratigraphy: Ara core material has provided reliable evidence of body fossils, though diversity is limited. Most commonly, Cloudina and, to a lesser extent, probable Namacalathus occur in close association with thrombolite build-up facies. Cloudina also occurs as skeletal debris in associated grainstones and packstones. Both body fossils primarily occur in, or associated with, thrombolites of the A2 and A3 sequences, whereas Cloudina is also present in A1 sections.

The extinction of Cloudina and Namacalathus has been taken to coincide with the top of the A3 of the Birba Formation, which in turn has been taken to correspond to the Neoproterozoic – Cambrian boundary (Amthor et al., 2003).

Mattes and Conway Morris (1990) discuss a range of Ara micropalaeontological occurrences, notably siliceous cyst-like spheres, acritarchs, general spherical bodies (?cyanobacterial), micritic bush-like masses (?also cyanobacterial) and Cloudina. Subsequent investigations have only singled out Cloudina as the most reliable marker fossil. In terms of acritarch recovery several palynological investigations have proved disappointing in terms of finding stratigraphically significant forms (e.g. Butterfield, 2001: expanded on in Butterfield and Grotzinger, in preparation). Productive samples tend to be dominated by amorphous organic material and/or simple, smooth, long-ranging acritarchs. Rare, ornamented acritarchs have proved difficult to assign to known taxa and have not yet helped refine the age of the Group.

Dhahaban Formation

Authors: Teyssen (unpublished, 1990).

Introduction

The Dhahaban Formation comprises all carbonates and fine clastics ‘sandwiched’ between the Nimr Group or younger Haima Supergroup clastics and the Al Noor salt (Figure 16.4). Where appropriate, the base of the Dhahaban is defined at the base of the clastics (generally red shales)/top of the evaporite sequence. A purely carbonate top salt section, could however, equally be assigned to the A5 sequence and inconsistencies in interpretation remain.

Figure 16.4:

Composite electrical logs, lithology and lithological description of the Dhahaban Formation, Ara Group, in well Al Noor-2, South Oman (Mohammed et al., 1997). See Figure 16.1 for location.

Figure 16.4:

Composite electrical logs, lithology and lithological description of the Dhahaban Formation, Ara Group, in well Al Noor-2, South Oman (Mohammed et al., 1997). See Figure 16.1 for location.

The Dhahaban Formation represents the last phase of carbonate/evaporite deposition in the South Oman Salt Basin. Nimr-‘type’ continental clastic sedimentation already starts in the Ara Group. This is most likely related to the onset of a major tectonic phase culminating in the Angudan unconformity associated with thrusting along the Western flanks of the salt basins (Linskaill and Teyssen, 1991). On a larger scale this is related to the final locking of East and West Gondwana, (the East African Orogeny) as described by Immerz et al. (2000), Koopman et al. (2007) and van den Berg et al. (2008). The Dhahaban Formation represents the last evaporite-carbonate phase of the Oman salt basins. With the onset of clastic sedimentation, salt tectonism as well as the onset of regional thrusting along the Western margins of the basins, it is not surprising to find complex stratigraphic relationships. These range from endmember evaporite-carbonate cycle sedimentation to a complex mix of clastics, carbonates and evaporites possibly including salt dissolution products.

Warren (2009) summarises an alternative model for the Dhahaban Formation, much more influenced by surface and subsurface salt dissolution. He argues that the nature of the Formation has been determined in response to salt-edge dissolution and resultant infill. Such dissolution continues to this day having commenced at the time the Dhahaban first formed. This helps to explain the diverse internal textures (e.g. ‘classic’ collapse breccias, which were previously interpreted to be depositional), variable wireline log signatures, the lateral facies complexity and the variation in underlying and overlying stratigraphical units.

Type and reference sections: Dhahaban S-1 in South Oman (Figure 16.3). Additional type section is Al Noor-2 in South Oman (Figure 16.4).

Lithology: The Dhahaban Formation consists predominantly of light grey, stromatolitic dolomites (Figure 16.5) with lesser limestones and shales (A6C), overlying anhydrite and halite (A6E). Red-brown continental clastics, shallow-marine carbonates (occasionally stromatolitic) and mudstones including (rare) anoxic shales (see Dhahaban S-1, Figure 16.3) are all variably developed.

Figure 16.5:

Ditch cuttings from the Ara Group: (a) Dolomite from the Dhahaban Formation in Dhahaban South-1; (b) Salt from the Al Noor Formation in Dasimi-1H2; and (c) Salt from the Al Noor Formation in Al Noor-1H2 (scale grid is 1 x 1 mm) (Mohammed et al., 1997).

Figure 16.5:

Ditch cuttings from the Ara Group: (a) Dolomite from the Dhahaban Formation in Dhahaban South-1; (b) Salt from the Al Noor Formation in Dasimi-1H2; and (c) Salt from the Al Noor Formation in Al Noor-1H2 (scale grid is 1 x 1 mm) (Mohammed et al., 1997).

The shales of the Dhahaban are generally reddish and similar to those of the Nimr and the Al Noor Salt. Organic-rich, thick source rock intervals are more common in the Thuleilat Member of the Athel Formation.

Subsurface recognition: The carbonates are similar to the other carbonates of the Huqf Supergroup. Locally it may be possible (rare) to identify a relict laminated stromatolitic texture to help differentiate from Mesozoic carbonates.

More often than not, the change from clastics of the Haima to carbonates of the Dhahaban Formation should be dramatic and easy to recognise. However, in areas where clastics are juxtaposed the change may not be as easy to identify e.g. the red shales/siltstones of the Dhahaban Formation are similar in appearance to those of the lower part of the overlying Haima or Nimr.

There may be a negative drill break at the upper boundary and a positive drill break at the lower boundary with the Al Noor Salt. Generally, the Dhahaban has a much lower Rate of Penetration than under-and overlying formations.

Post drilling the Formation can be recognised by its generally low Gamma response in carbonates and variable, higher Gamma in the clastics.

It has a high Density and wide positive separation on Density-Neutron logs, when it consists of pure dolomite.

Boundaries: The Dhahaban Formation is generally unconformably overlain by Haushi, Nimr or Haima clastics.

Normally overlying the Al Noor Salt, the Dhahaban can locally cut down to the Athel or even Birba formations. The exact nature of the basal boundary is still a subject of debate (e.g. unconformable, dissolution surface, locally conformable) but a stratigraphical break is generally assumed.

Distribution: The Dhahaban is currently confined to South and south Central Oman, but this assumption is constrained by a very limited number of well penetrations in the Ghaba and Fahud Salt basins. A small number of North Oman wells partially penetrate carbonates below the Haima/Nimr but these cannot be definitively assigned to the Dhahaban.

Deposition: Deposition occurred in extensive shallow-marine settings. Deposition is to a large part influenced by salt dissolution, halokinesis and tectonic activity. Warren (2009) believes that the nature of the Formation relates significantly to salt-edge dissolution and resultant infill. This goes some way to explain why the Dhahaban Formation is highly variable in thickness and in its lithological makeup, i.e. without systematic depositional trends related to either the underlying salt or the overlying Nimr or Haima. The thick intercalated red-brown, dominantly fine-grained siliciclastic deposits are likely to be of continental origin, i.e. erosional products from a major orogeny-associated mountain belt and foreland in East Africa. Runoff and erosion products from those mountains may have flowed northward (foreland parallel) into the basins of Oman. These incoming clastics finally swamp and terminate the Huqf evaporite-carbonate sequences with the deposition of the Nimr Group.

Subdivision: Sequence subdivisions are based on lithology, labelled A6C for carbonates and shales and A6E for evaporites.

Sequence Stratigraphy: The Dhahaban comprises part of the A6 sequence of the Ara Group.

Age: Earliest Cambrian, ca. 539–538 Ma, loosely based on the extrapolation of carbonate-evaporite cycle durations by Bowring et al. (2007). The dates of Bowring et al. (2007) provide good age constraint for the Birba and basal U formations but ages applied to later units are much more speculative.

Biostratigraphy: The Dhahaban has yielded no age diagnostic fossils (see general Group discussion). Butterfield and Grotzinger (in preparation) note the presence of leiospheres and large diameter filaments, neither of which add to our stratigraphical understanding.

Al Noor Formation

Authors: Teyssen (unpublished, 1990).

Introduction

Recognition of the Al Noor Formation depends partly on distinguishing the underlying U Formation and/or the shales and cherts (silicilytes) of the Al Shomou Member (of the Athel Formation). The Al

Noor Formation is characterised by the presence of ‘dirty’ salt with clastics that start to prograde further out into the basin. The distribution of the Al Noor sediments does not seem to be controlled by older structural basin trends like the underlying formations of this Group. This indicates a more uniform and, in view of the Al Noor thickness, possibly increasing subsidence across the basin(s). Work by Schramm (2009), supported and built on by Warren (2009), indicates that the salts are dominantly recycled and halokinetic in nature.

Type and reference sections: Al Noor-1 in South Oman (Figure 16.6). Additional subsurface reference sections are Suwaihat-5H2 in Central Oman (Figure 16.7) and Birba-1 in South Oman (Figure 16.8).

Figure 16.6:

Composite electrical logs, lithology and lithological description of the Al Noor Formation, Ara Group, in well Al Noor-1, South Oman. See Figure 16.1 for location.

Figure 16.6:

Composite electrical logs, lithology and lithological description of the Al Noor Formation, Ara Group, in well Al Noor-1, South Oman. See Figure 16.1 for location.

Figure 16.7:

Composite electrical logs, lithology and lithological description of the Al Noor Formation, Ara Group, in well Suwaihat-5H2, Central Oman (Mohammed et al., 1997). See Figure 16.1 for location.

Figure 16.7:

Composite electrical logs, lithology and lithological description of the Al Noor Formation, Ara Group, in well Suwaihat-5H2, Central Oman (Mohammed et al., 1997). See Figure 16.1 for location.

Figure 16.8:

Composite electrical logs, lithology and lithological description of the Al Noor Formation, Ara Group, in well Birba-1, South Oman (Mohammed et al., 1997). See Figure 16.1 for location.

Figure 16.8:

Composite electrical logs, lithology and lithological description of the Al Noor Formation, Ara Group, in well Birba-1, South Oman (Mohammed et al., 1997). See Figure 16.1 for location.

Lithology: The Al Noor Formation is a thick salt sequence, including potash salts (Figure 16.5), and argillaceous salt, with few carbonate (A5C) and/or anhydrite stringers (Figure 16.9) and significant saliferous shale/red siltstone intercalations.

Figure 16.9:

Ditch cuttings from the Ara Group: (a) Anhydrite from the Al Noor Formation in Al Shomou-1; and (b and c) Organic shale from the Thuleilat Member in Athel-1 (scale grid is 1 x 1 mm) (Mohammed et al., 1997).

Figure 16.9:

Ditch cuttings from the Ara Group: (a) Anhydrite from the Al Noor Formation in Al Shomou-1; and (b and c) Organic shale from the Thuleilat Member in Athel-1 (scale grid is 1 x 1 mm) (Mohammed et al., 1997).

It has fewer carbonates than the Birba Formation, from which it is distinguished by the presence of argillaceous/silty salt and saliferous shale/siltstone.

Subsurface recognition: Whilst drilling the Al Noor Formation can be recognised by a positive drill break on entering the salt. Because of possible poor cuttings recovery in the salt (dependent on mud type) the change in Rate of Penetration is likely to be the first indication of salt penetration (see Birba-1, Figure 16.8). Further clues come from an increase in chloride content in unsaturated mud, the distinct taste and appearance of the salt and the red shales and siltstones, thinly interbedded with the salt. These shales and siltstones are similar to those observed in the Dhahaban Formation and the Nimr Group.

Post drilling the recognition is based on log patterns:

Clean Na rich salt: low Gamma and Density, wide negative separation on Density-Neutron, Resistivity off scale.

Clean K rich salt: high Gamma, low Density, wide negative separation on Density-Neutron, variable/erratic Resistivity.

Dirty salt/saliferous shales: higher Gamma and smaller negative separation on Density-Neutron.

The Al Noor evaporites may be distinguished from the Birba Formation evaporites by its intercalated clastics. Initial results (Schramm, 2009) suggest that bromine concentrations in the Al Noor halites are generally lower than in Birba halites. This is most likely due to subsequent recycling. Also in contrast to the more uniformly low concentrations in the Al Noor the carbonate associated Birba halites tend to exhibit primary depositional/evaporation trends in their bromine concentrations, i.e. limited recycling.

Carbonates of the Al Noor Formation show a return to positive values of δ13C (2.5‰ to 3.5‰) compared to the underlying U Formation (A4C) (see also Figure 15.1).

Boundaries: The Al Noor Formation is separated from the Birba salt by the U Formation (characterised by a distinctive uranium anomaly) and/or Athel silicilytes and shales. If neither of these is present, the Al Noor salt sequence cannot be separated from the Birba salt sequence other than by the presence of clastics in the Al Noor. Although there is an absence of red clastics in the Birba this feature cannot be relied upon to differentiate between the two, as a clean Al Noor sequence can look identical to a clean Birba sequence. Stratigraphically both the U Formation and the Athel Formation occur sandwiched between the carbonates/evaporites of the Birba Formation and Al Noor Formation.

The upper boundary is the base of the Dhahaban Formation or younger units.

Distribution: The Al Noor Formation occurs in South and Central Oman, but is poorly constrained by the very limited number of data points in the Fahud and Ghaba Salt basins.

Deposition: The carbonates are characterised by quite varied and distinct facies assemblages comprising small, isolated patch and pinnacle reefs (?), thin (<60 m) deeper water (sapropelic) laminites of limited extent as well as, interpreted, ponded deep-water stringers (120–210 m thick and of small lateral extent, e.g. 5–10 sq km). The basinal facies are comprised of low-energy and turbiditic carbonate mudstones. Very thick salt was deposited basin-wide, interbedded with anhydrite and intra-salt clastics. There was increased subsidence across the South Oman Salt Basin, with maximum subsidence towards the west-northwest as indicated by the large amounts of fine intrasalt clastics interbedded with the salt. Periods of intense evaporitic drawdown are represented by occasional potash salts forming in the rapidly subsiding basin centre (Faulkner, 1996).

Salt bromine data indicate salt recycling in more laterally extensive connected basins (Schröder, 2000 and Schramm, 2009).

Warren (2009) suggests that the Al Noor halite association is different in character from the underlying Birba Formation lithofacies in that it is characterised by halokinesis and an intimate interaction between quartzose sediment loading (Ara clastics) and salt flow. By contrast the Birba comprises alternating episodes of evaporite salts and evaporitic carbonates, little affected by halokinesis.

Subdivision: Sequence subdivisions again depend on lithology, labelling A6CL for intrasalt clastics, A5C for carbonates and shales and A5E for evaporites.

Sequence Stratigraphy: The Al Noor Formation comprises the A5 depositional sequence and the evaporitic (intrasalt clastics) lower part of the A6 sequence.

Age: Earliest Cambrian, ca. 540–539 Ma. The Al Noor Formation is younger than the radiometrically dated A4C stringer. This age can be supported by broad extrapolation of available U-Pb ages (Bowring et al., 2007).

Biostratigraphy: No fossils are known from the Al Noor Formation (see general Group discussion).

Athel Formation*

Authors: The Athel Formation was originally defined in Suana (unpublished, 1987), to cover the Ara Group on the eastern flank. It was redefined in Teyssen (1990) to include only the middle part of Suana’s Athel Formation.

Introduction

The Athel Formation represents an exceptional phase in the development of the Ara Group, without evaporites but instead with rather localised relatively deep-water deposition of laminated cherts (silicilytes of the Al Shomou Member) on top of organic-rich shales of the U Formation and overlain by organic-rich shales of the Thuleilat Member. The origin of the silicilytes is enigmatic (Amthor et al., 2005).

Light oil has been produced from the silicilytes of the Al Noor field in South Oman since 2000. Very similar laminated cherts have been described by Rabu (1988, ‘chert noir’) in the Fara Formation in the Jabal Al Akhdar. These are associated with volcaniclastics, which overlap in time with the Birba and U formations of the Ara Group. Contemporaneous shallow-water deposition is represented by platform carbonates.

Type and reference sections: Athel-1 in South Oman (Figures 16.10 and 16.14). Additional subsurface reference sections are Thuleilat-2 (Figure 16.11), Al Noor-4 (Figure 16.12), Marmul NW-7 (Figure 16.13) and Thamoud-6 (Figure 16.15, Thamoud Member only), all in South Oman.

Figure 16.10:

Composite electrical logs, lithology and lithological description of the Athel Formation, Ara Group, in well Athel-1, South Oman (Mohammed et al., 1997). See Figure 16.1 for location.

Figure 16.10:

Composite electrical logs, lithology and lithological description of the Athel Formation, Ara Group, in well Athel-1, South Oman (Mohammed et al., 1997). See Figure 16.1 for location.

Figure 16.11:

(facing page): Composite electrical logs, lithology and lithological description of the Thuleilat Member, Athel Formation, Ara Group, in well Thuleilat-2, South Oman (Mohammed et al., 1997). See Figure 16.1 for location.

Figure 16.11:

(facing page): Composite electrical logs, lithology and lithological description of the Thuleilat Member, Athel Formation, Ara Group, in well Thuleilat-2, South Oman (Mohammed et al., 1997). See Figure 16.1 for location.

Figure 16.12:

Composite electrical logs, lithology and lithological description of the Thuleilat Member, Athel Formation, Ara Group, in well Al Noor-4, South Oman (Mohammed et al., 1997). See Figure 16.1 for location.

Figure 16.12:

Composite electrical logs, lithology and lithological description of the Thuleilat Member, Athel Formation, Ara Group, in well Al Noor-4, South Oman (Mohammed et al., 1997). See Figure 16.1 for location.

Figure 16.13:

Composite electrical logs, lithology and lithological description of the Al Shomou Member, Athel Formation, Ara Group, in well Marmul NW-7, South Oman (Mohammed et al., 1997). See Figure 16.1 for location.

Figure 16.13:

Composite electrical logs, lithology and lithological description of the Al Shomou Member, Athel Formation, Ara Group, in well Marmul NW-7, South Oman (Mohammed et al., 1997). See Figure 16.1 for location.

Figure 16.14:

(facing page): Composite electrical logs, lithology and lithological description of the Al Shomou Member, Athel Formation, Ara Group, in well Athel-1, South Oman (Mohammed et al., 1997). See Figure 16.1 for location.

Figure 16.14:

(facing page): Composite electrical logs, lithology and lithological description of the Al Shomou Member, Athel Formation, Ara Group, in well Athel-1, South Oman (Mohammed et al., 1997). See Figure 16.1 for location.

Figure 16.15:

Composite electrical logs, lithology and lithological description of the Thamoud Member, Athel Formation, Ara Group, in well Thamoud-6, South Oman (Mohammed et al., 1997). See Figure 16.1 for location.

Figure 16.15:

Composite electrical logs, lithology and lithological description of the Thamoud Member, Athel Formation, Ara Group, in well Thamoud-6, South Oman (Mohammed et al., 1997). See Figure 16.1 for location.

Lithology: The Athel Formation is a series of shales, carbonates and laminated cherts, (the silicilytes of the Al Shomou Member). The shales of the Thuleilat Member are organic-rich, with excellent sourcerock quality, often separated into a lower and upper unit by a carbonate marker horizon of variable thickness. This carbonate is generally regionally correlatable, but may be locally absent and/or difficult to identify (e.g. Al Noor-4, Figure 16.12).

The shales show high concentrations of uranium, with comparable levels to those seen in the U Formation (see Al Noor-4, Figure 16.12).

Subsurface recognition: Whilst drilling the recognition of this Formation can be based on the lithology change (usually from salt or dolomites) to low Density, dark grey organic shales (Figure 16.9). The Thuleilat Member shale is usually overlain by Al Noor salts. There may be a carbonate section between the Thuleilat proper and the Al Noor salts and this has been interpreted as an Al Noor ‘stringer’ (A5C). On the Eastern flank the Thuleilat Member can be overlain by the Dhahaban and younger formations.

The dark grey shales are lithologically identical to the U shales.

The upper and lower boundaries of the Carbonate Marker Horizon (when present in the Thuleilat Member) are identified by negative and positive drill breaks, respectively.

The transition from the Thuleilat to the underlying Al Shomou is identified by the incoming of the silicilytes, which are laminated, harder, more silty and result in more sharply edged ditch fragments than the shales.

Post drilling the Formation is identified by the very high Gamma of the Thuleilat shales and the lower densities and narrower positive (shale) Density-Neutron separation in the upper unit than in the lower. The Al Shomou Member also exhibits characteristic log patterns (see below).

Boundaries: The Athel Formation is overlain by the Al Noor Formation in the centre of the South Oman Salt Basin, and by the Dhahaban or younger formations on the Eastern Flank. It overlies the organic-rich shales of the U Formation.

Distribution: The Athel Formation is confined to small areas in South Oman, possibly deeper basinal settings.

Deposition: The lower Al Shomou silicilyte deposits are interpreted to represent deposition in (possibly) deep anoxic basinal settings with minimal background siliciclastic input (Amthor et al., 2005; Schröder and Grotzinger, 2007). The upper Al Shomou silicilytes and subsequent deposition of organic-rich Thuleilat shales record an increase of clastic material, still in a presumed deeper anoxic basinal setting. The carbonates (Thamoud Member) are interpreted to represent the time-equivalent platform or basin margin carbonate deposition.

Subdivision: The Athel Formation comprises three members (Faulkner, 1995; Amthor et al., 2005): the Thuleilat Member shale, the Al Shomou Member silicilyte, and the Thamoud Member carbonate. The Thamoud Member is only distinguished along the Eastern Margin of the South Oman Salt Basin and therefore interpreted as the carbonate platform equivalent of the other two basinal deposits. The Thamoud carbonates are not found in association with the Thuleilat shale or the Al Shomou silicilytes. The Al Shomou Member has been subdivided into two parts by a regionally correlatable marker that separates the lower Gamma silicilytes of the lower Al Shomou from the higher Gamma silicilytes of the upper Al Shomou (Faulkner, 1995). A regionally extensive carbonate marker separates the lower Thuleilat shale from the upper Thuleilat shale. The three members are further described and discussed separately below.

Sequence Stratigraphy: The Athel Formation is interpreted to represent the upper part of the A4 depositional sequence. The silicilytes of the Al Shomou Member were deposited during times of high relative sea level, but only in areas that correspond to depressions within the basin. These may have been loci for anoxic conditions, which are thought to be a prerequisite for silicilyte deposition (Amthor et al., 2005).

The basal part of the upper Thuleilat shale then records a regional transgression onto the carbonate platform highs along the Eastern Flank (Faulkner, 1995). Carbonate deposition was later to resume on the platform highs, within a highstand system tract. A major sequence boundary separates the Athel Formation from the overlying evaporites of the Al Noor Formation.

Age: Earliest Cambrian, ca. 542–540 Ma. The Athel Formation is correlated with the upper A4 sequence assuming the U-marker in the U Formation and in the A4C carbonate is time equivalent (Amthor et al., 2003, 2005). However, because of its position in a basinal setting there is still some uncertainty concerning exact stratigraphic relationships. The age is based on extrapolation from the 541 Ma U-Pb date from the basal A4C (U) carbonate (Bowring et al., 2007). Given the uncertainties remaining in terms of the correlatablity of key U and Athel Formation units only a broad age range of 542–540 Ma is applied to the overall A4 sequence covering both the U and Athel formations, i.e. from the base of the A4 Salt and Anhydrite (A4E) to the top of the A4 sequence (e.g. top Thamoud, top U carbonate or top A4 stringer).

Radiometric U-Pb age dates put the laminated cherts in the lower Fara Formation (Al Hajar Mountains) around 548 Ma, which is older than the dated A4 carbonate (541 Ma) in the South Oman Salt Basin (Bowring, 2007).

Biostratigraphy: No fossils are known from this Formation. Palynological recovery is totally dominated by abundant amorphous organic material (see general Group discussion).

Thuleilat Member

Author: Faulkner (unpublished, 1995).

Introduction

The Thuleilat comprises a series of organic-rich shales separated into upper and lower units by a carbonate marker horizon.

Type and reference sections: Athel-1 in South Oman (Figure 16.10). Additional reference sections are Thuleilat-2 (Figure 16.11) and Al Noor-4 (Figure 16.12), both in South Oman.

Lithology: The Thuleilat Member consists of organic shales, often separated into a lower and upper unit by a carbonate marker horizon of variable thickness but, although generally regionally correlatable, it may be locally absent and/or difficult to identify (e.g. Al Noor-4, Figure 16.12). The shales show high concentrations of uranium, with comparable levels to those seen in the U Formation.

Subsurface recognition: The lithology change (usually from salts or dolomites) to low density, dark grey organic shales can easily be identified at wellsite (Figure 16.9).

The dark grey shales are lithologically identical to the U shales and the dark grey organic shales of the overlying Dhahaban. The non-organic shales of the Dhahaban are reddish in colour.

Post drilling this Member is typified by the very high Gamma shales and generally characteristic log patterns.

Boundaries: The Thuleilat Member is overlain by the Al Noor Formation in basinal settings and by younger formations on the Eastern Flank. It overlies the Al Shomou Member.

Distribution: The Member is restricted to South Oman.

Deposition: The shales are interpreted to represent off-platform basinal settings. The carbonates in the sequence might be derived from erosion of the surrounding carbonate platforms.

Subdivision: Sometimes a twofold subdivision is possible, based on the presence of a separating carbonate unit.

Al Shomou Member

Author: Faulkner (unpublished, 1995).

Introduction

The Al Shomou is an enigmatic unit of laminated, organic-rich cherts, called silicilytes, in the South Oman Salt Basin. Amthor et al. (2005) provides a good introduction to this unique body of rock.

Type and reference sections: Marmul NW-7 in South Oman (Figure 16.13). An additional subsurface reference section is Athel-1 in South Oman (Figure 16.14).

Lithology: The Al Shomou Member consists of distinctively laminated grey-brown cherts (silicilytes). The silicilytes are typically organic-rich, finely laminated and consists of 80–90% microcrystalline quartz with a crystal-size of 2–3 microns (Amthor et al., 2005).

Subsurface recognition: Whilst drilling, the distinctively banded grey-brown silicilytes (Figure 16.16) are distinguishable in ditch cuttings at the wellsite.

Figure 16.16:

Ditch cuttings from the Ara Group: (a) Silicilyte from the Athel Formation in Al Shomou-1; (b) Silicilyte from the Athel Formation in Athel-1; and (c) Organic shale from the U Formation in Athel-1 (scale grid is 1 x 1 mm) (Mohammed et al., 1997).

Figure 16.16:

Ditch cuttings from the Ara Group: (a) Silicilyte from the Athel Formation in Al Shomou-1; (b) Silicilyte from the Athel Formation in Athel-1; and (c) Organic shale from the U Formation in Athel-1 (scale grid is 1 x 1 mm) (Mohammed et al., 1997).

The change from high organic shales of the Thuleilat Member or from clastics of younger sequences should be clear and relatively easy to spot (the silicilytes are laminated, cherty, harder and the cuttings are sharper edged than the organic-rich shales of adjacent sequences).

A Gamma spectrometer can be used at the wellsite to measure the U, K and Th radiation from ditch cuttings. The organic-rich Thuleilat shales show very high spectral Gamma log values compared to the silicilytes (with lower log values). The trends are similar (but not the absolute API values) to those observed on wireline logs.

There may be a positive drill break at the upper boundary, but more commonly there is no distinct change.

Post drilling the log patterns aid recognition. Overall, the Al Shomou silicilytes show Gamma log values significantly lower than the high Gamma of the Thuleilat and U formations.

The upper Al Shomou has a relatively high Gamma response in comparison to the lower Al Shomou. The top of the lower unit is picked at the base of the last (downhole) very high (ca. >150 API) Gamma peak.

There is none to very little separation on the Density-Neutron (sand/silt like separation).

Boundaries: It is conformably overlain by the organic-rich shales of the Thuleilat Member and in a very few instances unconformably by the Al Noor Salt or clastics of the Haima and Haushi. The Al Shomou Member overlies organic shales of the U Formation.

Distribution: Confined to South Oman, primarily in the vicinity of the Al Noor and Al Shomou fields and in a narrow portion of the Eastern Flank (see figure 3 of Amthor et al., 2005).

Deposition:Amthor et al. (2005) argue for an origin in an anoxic basin setting, possibly re-crystallised from biochemically precipitated silica gel in silica-saturated seawater (see also Schröder and Grotzinger, 2007).

The localised occurrence near faults suggests deposition in fault controlled depositional low areas. The origin of the silica is unknown. Potential sources are volcanic material, hydrothermal action or silica weathering, all of which are known to occur in the South Oman Salt Basin at the time of deposition of the Al Shomou silicilytes. Warren (2009) suggests a model of silicilyte formation relating to a halokinetically-induced association of sea bottom anoxia and organic enrichment within depressions on the deep sea floor that were occupied by anoxic density-stratified brine lakes.

Subdivision: This Member is separated into two units (lower and upper). The upper silicilyte unit is characterised by higher Gamma values than those observed in the lower unit. This intra-Al Shomou Gamma-log break is regionally correlatable; as a ‘marker’ it may be traced as far as the Eastern Flank into the carbonates of the Thamoud Member (Thamoud-6, Figure 16.15), according to Faulkner (1995).

Thamoud Member

Author: Faulkner (unpublished, 1995).

Introduction

The Thamoud Member is interpreted as the carbonate platform equivalent of the Thuleilat and Al Shomou members. However, the Thamoud carbonates are not found in association with the Thuleilat shale or the Al Shomou silicilytes.

Type section: Thamoud-6 in South Oman (Figure 16.15).

Lithology: The Thamoud Member consists of light grey dolomites with thin subordinate limestones. There are anhydrite and shale intercalations in the upper part. It is lithologically similar to the U and Birba carbonates.

Subsurface recognition: This Member cannot be differentiated at the wellsite.

It is lithologically similar to the U carbonates, but without any uranium anomaly, and the Dhahaban carbonates, although a Dhahaban/Thamoud contact has not yet been observed.

Post drilling the Thamoud can be recognised on Gamma log pattern.

Boundaries: It overlies the U Formation, which is characterised by the high uranium content and distinctive log character. The upper boundary is usually uncertain in the eastern flank setting, where multiple carbonates can be stacked without intervening salt. Such carbonate packages are overlain by Nimr of younger formations.

Distribution: The Thamoud has not been found in association with the Thuleilat Shale or the Al Shomou silicilyte (in wells to date). It may be the lateral facies (carbonate platform) equivalent of these members. Restricted to date to the Eastern Flank (Faulkner, 1995) and a small east-west trending area in Central Oman. Only a handful of well penetrations have been noted and the Member is in need of further study and clarification.

Deposition: The Thamoud Member is tentatively interpreted as the carbonate rim of an open-marine basin.

U Formation

Authors: First described by Mohammed et al. (unpublished, 1997).

Introduction

The U Formation separates the Al Noor Formation from the Birba Formation. U Formation carbonates (A4UC and A4C) contain both a strong negative carbon isotope (δ13C) anomaly, ranging to –5‰, (see Figure 15.1) as well as a positive uranium anomaly (U carbonate).

Type and reference sections: Al Noor-1 in South Oman (Figure 16.17). An additional subsurface reference section is Thuleilat-2 in South Oman (Figure 16.18).

Figure 16.17:

Composite electrical logs, lithology and lithological description of the U Formation, Ara Group, in well Al Noor-1, South Oman (Mohammed et al., 1997). See Figure 16.1 for location.

Figure 16.17:

Composite electrical logs, lithology and lithological description of the U Formation, Ara Group, in well Al Noor-1, South Oman (Mohammed et al., 1997). See Figure 16.1 for location.

Figure 16.18:

Composite electrical logs, lithology and lithological description of the U Formation, Ara Group, in well Thuleilat-2, South Oman (Mohammed et al., 1997). See Figure 16.1 for location.

Figure 16.18:

Composite electrical logs, lithology and lithological description of the U Formation, Ara Group, in well Thuleilat-2, South Oman (Mohammed et al., 1997). See Figure 16.1 for location.

Lithology: The U Formation comprises carbonates (light grey dolomites) and organic, uranium-rich grey shales (Figures 16.16 and 16.19). It shows a very distinctive high Gamma log character compared to the Birba carbonates.

Figure 16.19:

Core photograph of (a) base A4C stringer carbonate, (b) high Gamma ‘tuffaceous mudstone’ and (c) A4 anhydrite, in well Birba-4.

Figure 16.19:

Core photograph of (a) base A4C stringer carbonate, (b) high Gamma ‘tuffaceous mudstone’ and (c) A4 anhydrite, in well Birba-4.

As with other ‘stringer’ carbonates the A4 carbonate stringer is overlain and underlain by thin anhydrite units. These are generally between 10 m to 20 m thick. There is a decimetre scale (ca. 30 cm thick) high Gamma unit at the base of the carbonate at the contact with the lower anhydrite (see Figure 16.19). This has been taken by many to be the volcanic ash bed, which provides the ca. 541.0 Ma dating of Bowring et al. (2007). The bed produces a very high and distinctive Gamma-ray peak (e.g. Shamah-1, Figures 16.2 and 16.21; Thamoud-6, Figures 16.15 and 16.24). In some wells, logs suggest that the high gamma peak occurs within the anhydrite section, but core material indicates that the lithologies above the bed are actually anhydrite-plugged carbonates. The high gamma lithology is a dark grey to variegated mudstone (Figure 16.19), often anhydritic and disrupted. It probably represents, at least in part, an anhydrite dissolution unit, which incorporates potential volcanic material, but it has commonly been called the ‘base A4C tuff’.

Subsurface recognition: Difficult whilst drilling as logs are necessary for precise identification, combined with the stratigraphic position relative to known drilled formations.

Post drilling, logs patterns show the very distinctive and correlatable high (API values) Gamma unit. The uranium anomaly, which characterises the unit, is seen on spectral Gamma logs. Uranium concentrations in the U (A4C) carbonate range from 0.902 to 4.001 ppm, significantly above the background of stratigraphically lower and higher Ara carbonates, which range from 0.358 to 0.912 ppm. The average for the U (A4C) carbonate (2.043 ppm) is three times higher than the average for other Ara carbonates (0.650 ppm). This 3-fold increase in uranium for the carbonate is prominently displayed in the Gamma curves, resulting in unambiguous stratigraphic correlation of the A4 carbonate. The uranium concentration correlates well with similarly unique carbon isotope (δ13C) values, with higher uranium content corresponding to greater depletion in δ13C. Carbonates of the U Formation are characterized by a marked, up-hole, negative excursion where δ13C values decrease by 6–7‰ to δ13C readings as low as –5 ‰ over a vertical distance of approximately 4 metres near the base of the A4 carbonate. The remainder of the U carbonate unit then records a gradual but continuous increase in δ13C values to approximately –2‰. Post A4 carbonates return to positive values of +2.5‰ to +3.5‰ (Amthor et al., 2005). Recognition that the U carbonate contains both a strong negative carbon isotope anomaly, as well as a positive uranium anomaly, permits confident correlation of this stratigraphic interval to the eastern margin of the basin, where the thick basin-centre evaporites are absent, see for example Thamoud-6 (Figure 16.24, Amthor et al., 2003, 2005).

Boundaries: The U Formation lies unconformably on the Birba Formation. It is generally overlain by the Athel Formation (shale/silicilytes) and the Al Noor salt sequence.

Distribution: The U Formation is confined to South Oman, specifically to the Birba area and further north. It has not been seen further south in the Harweel area.

Deposition: Evaporite deposition of the Birba Formation terminated with the transgression of the U Formation. The transgressions caused an in-building of carbonate platforms, trying to keep-up with sea-level rise. The U Formation records a clear increase of fine clastics into the basin, showing a proximal to distal grading from silts to (organic-rich) argillaceous sediments. Sedimentation is probably associated with volcanics shed into the basins (volcanics have not been encountered in the overlying Al Noor Formation). The nature of the sediments suggests deposition in a restricted basinal environment.

Figure 16.20:

Ditch cuttings from the Ara Group: (a) Dolomite from the U Formation in Birba-1; and (b) Dolomite from the Birba Formation in Areeq-1 (scale grid is 1 x 1 mm) (Mohammed et al., 1997).

Figure 16.20:

Ditch cuttings from the Ara Group: (a) Dolomite from the U Formation in Birba-1; and (b) Dolomite from the Birba Formation in Areeq-1 (scale grid is 1 x 1 mm) (Mohammed et al., 1997).

Sequence stratigraphy: The U Formation belongs to the A4 depositional sequence of the Ara Group, representing a transgressive to highstand systems tract. The transgressive U shale deposition terminates all evaporite deposition north of the Birba shelf edge, e.g. in the deeper basin. The platform carbonates are thinner (60–80 m) than A2 and A3 carbonates. They are dominated by transgressive deeper-water facies and are only well-developed in the greater Birba Area behind the Birba shelf edge. A4 highstand deposits are concentrated around the basin margin and include shallow-water carbonates, as well as sabkha and lagoonal evaporites.

The A4 carbonate distribution documents a back stepping of carbonate production from the southern domain (Dhahaban-Harweel) towards Birba and the eastern margin of the basin.

Sharland et al. (2001) place their MFS Cm10 at the base of the A4 stringer (A4C) in well Birba-1.

Age: Latest Ediacaran – earliest Cambrian, ca. 542–540 Ma. The U Formation has been dated from a volcanic tuff associated with the base of the A4C stringer carbonate at 541 Ma in well Birba-5 (Bowring et al., 2007). This age has been correlated to the U shale, assuming that the Uranium marker in both the U shale and A4C carbonate represents a timeline. This updated age effectively shifts the Ediacaran – Cambrian boundary, also associated with the negative carbon excursion, to ca. 541 Ma from the previous 542 Ma (see Gradstein et al. 2004 and Bowring et al., 2007). By definition the lower U Formation, A4E sequence, becomes latest Ediacaran in age.

Biostratigraphy:Amthor et al. (2003) report an absence of Cloudina and Namacalathus from this and subsequent Ara sequences.

Birba Formation

Author: Teyssen (unpublished, 1990).

Introduction

The Birba Formation is the lowermost evaporite-carbonate dominated sequence of the Ara Group.

Type and reference sections: Shamah-1 in South Oman (Figure 16.21). Additional subsurface reference sections are Miqrat-1 in North Oman (Figure 16.22) and Budour NE-2 (Figure 16.23) and Thamoud-6 (Figure 16.24), both in South Oman.

Figure 16.21:

Composite electrical logs, lithology and lithological description of the Birba Formation, Ara Group, in well Shamah-1, South Oman (Mohammed et al., 1997). See Figure 16.1 for location.

Figure 16.21:

Composite electrical logs, lithology and lithological description of the Birba Formation, Ara Group, in well Shamah-1, South Oman (Mohammed et al., 1997). See Figure 16.1 for location.

Figure 16.22:

Composite electrical logs, lithology and lithological description of the Birba Formation, Ara Group, in well Miqrat-1H1, North Oman (Mohammed et al., 1997). See Figure 16.1 for location.

Figure 16.22:

Composite electrical logs, lithology and lithological description of the Birba Formation, Ara Group, in well Miqrat-1H1, North Oman (Mohammed et al., 1997). See Figure 16.1 for location.

Figure 16.23:

Composite electrical logs, lithology and lithological description of the Birba Formation, Ara Group, in well Budour NE-2, South Oman (Mohammed et al., 1997). See Figure 16.1 for location.

Figure 16.23:

Composite electrical logs, lithology and lithological description of the Birba Formation, Ara Group, in well Budour NE-2, South Oman (Mohammed et al., 1997). See Figure 16.1 for location.

Figure 16.24:

Composite electrical logs, lithology and lithological description of the Birba Formation, Ara Group, in well Thamoud-6, South Oman (Mohammed et al., 1997). See Figure 16.1 for location.

Figure 16.24:

Composite electrical logs, lithology and lithological description of the Birba Formation, Ara Group, in well Thamoud-6, South Oman (Mohammed et al., 1997). See Figure 16.1 for location.

Lithology: The Birba Formation is a sequence of salts, dolomites, anhydrites and argillaceous calcareous mudstones, all variably developed. In various settings any number of the salt units may be absent but particularly on the Eastern Flank of the South Oman Salt Basin, salt may not be present at all (e.g. Thamoud-6, Figure 16.24).

The Birba Formation comprises carbonates and evaporites of the Ara Group (A0 to A3 cycles). Carbonate units vary in thickness from 50–200 m; intervening evaporites are more difficult to estimate due to variable degrees of post-depositional halokinesis, but anhydrite deposits are on the order of 10–20 m thick and halite deposits are many tens to hundreds of metres thick. Interbedded volcanic ash beds, as part of a very distinctive basal carbonate-shale unit are characteristic of the Basal Birba Carbonate sequence (A0C). These volcanics aid correlation and have provided U-Pb based age data.

Subsurface recognition: At the well site, the Birba Formation is difficult to distinguish from similar evaporite/carbonate-dominated sequences. The evaporite and/or carbonate sediments are very similar to those of under- and overlying formations. The similarity of the Al Noor and Birba salt sequences, for example, is such that the presence of the Athel and/or the U Formation is sometimes required to separate the two (Kapellos et al., 1992), but generally the Al Noor Formation can be distinguished based on its clastic content.

In North Oman, the top of the Birba Formation is usually picked at the first occurrence of dolomites below confirmed Haima clastics. In the absence of any real data, it is the observation that the Angudan unconformity erodes significantly into the Ara that suggests this interpretation.

Similarly in Central Oman, the top of the Birba Formation is usually picked at the first occurrence of dolomites or salt.

There is a positive drill break when entering salt and negative drill breaks going into anhydrites and dolomites.

Post drilling, positive identification of this Formation is only achieved upon completion of the well when all data can be reviewed. Logs and offset well correlations are necessary to both recognise and accurately delimit the Formation.

Salts show low Density, wide negative Density-Neutron separation and very high Resistivity.

Dolomites have ‘Bag of Nails’ (i.e. erratic dips) Dipmeter character contrasting with the uniform character (regular dips) of the underlying Buah Formation. Often this Dipmeter change has been used as the critical element in determining the lower boundary of the Formation.

The Birba salt sequence comprises generally clean salt with no intercalations of shale and siltstone (unlike the Al Noor salt sequence where shale/siltstone intercalations can occur). Initial results indicate that Birba salts exhibit primary evaporation trends with respect to bromine concentrations in contrast to the general recycled readings and trends in the Al Noor salt (Schramm, 2009).

Carbon isotope trends can help distinguish between the A0C to A3C units, and the A4C. Carbonates of the Birba Formation show slightly positive δ13C values of 1 to 3.5‰ with a subsequent decrease to as low as –5‰ for carbonates in the U Formation.

Boundaries: Generally the Birba Formation is interpreted to unconformably overlie the Buah Formation (see boundary discussions of Ara and Nafun groups).

The Birba Formation is variably overlain by the Al Noor, U, Athel or Dhahaban formations or the Haima Supergroup or Haushi Group. In North Oman the Birba Formation is generally overlain by clastics of the Haima Supergroup, but it should be noted that this is based on a limited number of well penetrations.

Distribution: The Birba Formation has a wide regional distribution. Most well penetrations are in South Oman; penetrations in North Oman are rare (e.g. Miqrat-1, Figure 16.22).

Deposition: The depositional environment of this Formation ranges from hypersaline in a restricted basin to marine carbonate shelf.

Subdivision: The Birba Formation has no lithostratigraphic subdivisions, but instead a sequence stratigraphic subdivision is commonly used, comprising the A0 to A3 evaporite-carbonate sequences discussed below. The subdivisions crucially depend on the presence of evaporites between the carbonates. In the absence of these, at the edges of the salt basins, the carbonates stack-up and are very difficult to distinguish from each other and from the underlying Buah Formation. In such cases the stacked Birba carbonates have been informally referred to as the ‘basal Ara’.

Sequence Stratigraphy: The Birba Formation comprises three well-developed evaporite-carbonate sequences (A1 to A3) and a lowermost, A0, carbonate and shale section representing the initial transgression following the deposition of the Nafun Group. Each of the three successive sequences, A1 to A3, comprises a lowstand evaporite succession, progressively developed in increasingly restricted environments, followed by a basal anhydrite level corresponding to the initial transgression of the basin and a final highstand carbonate section. Well-preserved bromine depositional trends in the evaporites indicate a confined basin setting, with isolated sub-basins following different evaporation trends (Schramm, 2009; Warren, 2009).

Ara Sequence A0

At the base, the A0 carbonates are interbedded with cm-to-dm-scale green to grey, locally red argillaceous shales and clay to silt-grade volcanic ash beds. This lower unit is present in most of the deeper well penetrations in the South Oman Salt Basin and forms a regionally correlatable marker for base salt. Locally the ash content can be significant, dominating sections over several metres (e.g. Asala-1, which produced the U-Pb date of 546.7 Ma, see Bowring et al., 2007).

In the Birba area, the A0 carbonates develop into the incipient Birba platform rim. The base of the A0 is characterised on logs by high Gamma readings related to transgressive argillaceous shales and/or interbedded ash beds. Towards the top the Gamma readings decrease to low values around 10–20 API units, the overall character, however, is quite serrate.

Ara Sequence A1

The Birba shelf edge develops as prominent physical barrier between the shelf and the deeper basin because of differential subsidence. The shelf edge comprises shallow-water reefal carbonates (stromatolites and thrombolites).

Behind the rim, restricted shallow and deeper-water carbonates (sapropelic laminites) develop during periods of high accommodation (transgressive system tract), resulting in a thick, sheet-like, laterally continuous facies distribution. Most of the accommodation space remains unfilled during periods of reduced accommodation (highstand system tract), since only thin thrombolite or grainstone units cap the sapropelic laminites.

Ara Sequence A2

This sequence is characterised by carbonate platform growth during transgressive to highstand accommodation conditions.

In the Harweel Field area the A2C platform is developed as an extensive, 100 m-thick, shallow-water carbonate platform over an area of more than 150 sq km (Grotzinger, 2001). Significantly thicker (several hundreds of metres) off-platform carbonates with very poor reservoir properties have been encountered in what are interpreted to be intra-platform lows. Marked differential subsidence has to be invoked to explain such thick carbonates.

In the greater Birba area, the A2C is thinner and comprises non-reservoir anhydritic carbonates and anhydrites. The Birba shelf edge is comprised of stromatolitic reefal carbonates. Further north, no A2C carbonates have yet been identified.

A2C carbonates are characterised by a clean Gamma signature and a relative constant thickness of the gross carbonate.

Ara Sequence A3

This sequence is dominated by thrombolite reef complexes in South Oman.

The sequence represents a phase of significant carbonate growth. A3 platforms are characteristically more differentiated with respect to shelf margin and slope-to-basin facies (Grotzinger, 2001; Grotzinger, 2003).

A3C carbonates are characterised by a clean Gamma signature and a variable thickness (100–160 m) of the gross carbonate. This carbonate sequence is always separated from the underlying carbonates by a package of salt (unlike preceding Birba carbonates). This indicates that the basin margins must have stepped back significantly and that A3 carbonates no longer followed old tectonic trends.

The A3C carbonates have also yielded volcanic ash U-Pb dates of 542.3 Ma and 542.9 Ma (see Bowring et al., 2007).

Age: Latest Ediacaran, ca. 547–542 Ma. The sediments of this Formation have been radiometrically (U-Pb) dated ranging from ca. 542.3 Ma in the A3C carbonate to 546.7 Ma in the A0C carbonate (Bowring et al., 2007).

Biostratigraphy: The Birba Formation yields Cloudina and probable Namacalathus (Amthor et al., 2003), which are found in both thrombolite and grain/packstone facies within the A2C to A3C carbonates. Cloudina is also recorded in A1C carbonates.

*
Members are described individually.

Figures & Tables

Figure 16.1:

Location map: Ara Group.

Figure 16.1:

Location map: Ara Group.

Figure 16.2:

Composite electrical logs, lithology and lithological description of the Ara Group in well Shamah-1, South Oman. See Figure 16.1 for location.

Figure 16.2:

Composite electrical logs, lithology and lithological description of the Ara Group in well Shamah-1, South Oman. See Figure 16.1 for location.

Figure 16.3:

Composite electrical logs, lithology and lithological description of the Dhahaban Formation, Ara Group, in well Dhahaban South-1, South Oman (Mohammed et al., 1997). See Figure 16.1 for location.

Figure 16.3:

Composite electrical logs, lithology and lithological description of the Dhahaban Formation, Ara Group, in well Dhahaban South-1, South Oman (Mohammed et al., 1997). See Figure 16.1 for location.

Figure 16.4:

Composite electrical logs, lithology and lithological description of the Dhahaban Formation, Ara Group, in well Al Noor-2, South Oman (Mohammed et al., 1997). See Figure 16.1 for location.

Figure 16.4:

Composite electrical logs, lithology and lithological description of the Dhahaban Formation, Ara Group, in well Al Noor-2, South Oman (Mohammed et al., 1997). See Figure 16.1 for location.

Figure 16.5:

Ditch cuttings from the Ara Group: (a) Dolomite from the Dhahaban Formation in Dhahaban South-1; (b) Salt from the Al Noor Formation in Dasimi-1H2; and (c) Salt from the Al Noor Formation in Al Noor-1H2 (scale grid is 1 x 1 mm) (Mohammed et al., 1997).

Figure 16.5:

Ditch cuttings from the Ara Group: (a) Dolomite from the Dhahaban Formation in Dhahaban South-1; (b) Salt from the Al Noor Formation in Dasimi-1H2; and (c) Salt from the Al Noor Formation in Al Noor-1H2 (scale grid is 1 x 1 mm) (Mohammed et al., 1997).

Figure 16.6:

Composite electrical logs, lithology and lithological description of the Al Noor Formation, Ara Group, in well Al Noor-1, South Oman. See Figure 16.1 for location.

Figure 16.6:

Composite electrical logs, lithology and lithological description of the Al Noor Formation, Ara Group, in well Al Noor-1, South Oman. See Figure 16.1 for location.

Figure 16.7:

Composite electrical logs, lithology and lithological description of the Al Noor Formation, Ara Group, in well Suwaihat-5H2, Central Oman (Mohammed et al., 1997). See Figure 16.1 for location.

Figure 16.7:

Composite electrical logs, lithology and lithological description of the Al Noor Formation, Ara Group, in well Suwaihat-5H2, Central Oman (Mohammed et al., 1997). See Figure 16.1 for location.

Figure 16.8:

Composite electrical logs, lithology and lithological description of the Al Noor Formation, Ara Group, in well Birba-1, South Oman (Mohammed et al., 1997). See Figure 16.1 for location.

Figure 16.8:

Composite electrical logs, lithology and lithological description of the Al Noor Formation, Ara Group, in well Birba-1, South Oman (Mohammed et al., 1997). See Figure 16.1 for location.

Figure 16.9:

Ditch cuttings from the Ara Group: (a) Anhydrite from the Al Noor Formation in Al Shomou-1; and (b and c) Organic shale from the Thuleilat Member in Athel-1 (scale grid is 1 x 1 mm) (Mohammed et al., 1997).

Figure 16.9:

Ditch cuttings from the Ara Group: (a) Anhydrite from the Al Noor Formation in Al Shomou-1; and (b and c) Organic shale from the Thuleilat Member in Athel-1 (scale grid is 1 x 1 mm) (Mohammed et al., 1997).

Figure 16.10:

Composite electrical logs, lithology and lithological description of the Athel Formation, Ara Group, in well Athel-1, South Oman (Mohammed et al., 1997). See Figure 16.1 for location.

Figure 16.10:

Composite electrical logs, lithology and lithological description of the Athel Formation, Ara Group, in well Athel-1, South Oman (Mohammed et al., 1997). See Figure 16.1 for location.

Figure 16.11:

(facing page): Composite electrical logs, lithology and lithological description of the Thuleilat Member, Athel Formation, Ara Group, in well Thuleilat-2, South Oman (Mohammed et al., 1997). See Figure 16.1 for location.

Figure 16.11:

(facing page): Composite electrical logs, lithology and lithological description of the Thuleilat Member, Athel Formation, Ara Group, in well Thuleilat-2, South Oman (Mohammed et al., 1997). See Figure 16.1 for location.

Figure 16.12:

Composite electrical logs, lithology and lithological description of the Thuleilat Member, Athel Formation, Ara Group, in well Al Noor-4, South Oman (Mohammed et al., 1997). See Figure 16.1 for location.

Figure 16.12:

Composite electrical logs, lithology and lithological description of the Thuleilat Member, Athel Formation, Ara Group, in well Al Noor-4, South Oman (Mohammed et al., 1997). See Figure 16.1 for location.

Figure 16.13:

Composite electrical logs, lithology and lithological description of the Al Shomou Member, Athel Formation, Ara Group, in well Marmul NW-7, South Oman (Mohammed et al., 1997). See Figure 16.1 for location.

Figure 16.13:

Composite electrical logs, lithology and lithological description of the Al Shomou Member, Athel Formation, Ara Group, in well Marmul NW-7, South Oman (Mohammed et al., 1997). See Figure 16.1 for location.

Figure 16.14:

(facing page): Composite electrical logs, lithology and lithological description of the Al Shomou Member, Athel Formation, Ara Group, in well Athel-1, South Oman (Mohammed et al., 1997). See Figure 16.1 for location.

Figure 16.14:

(facing page): Composite electrical logs, lithology and lithological description of the Al Shomou Member, Athel Formation, Ara Group, in well Athel-1, South Oman (Mohammed et al., 1997). See Figure 16.1 for location.

Figure 16.15:

Composite electrical logs, lithology and lithological description of the Thamoud Member, Athel Formation, Ara Group, in well Thamoud-6, South Oman (Mohammed et al., 1997). See Figure 16.1 for location.

Figure 16.15:

Composite electrical logs, lithology and lithological description of the Thamoud Member, Athel Formation, Ara Group, in well Thamoud-6, South Oman (Mohammed et al., 1997). See Figure 16.1 for location.

Figure 16.16:

Ditch cuttings from the Ara Group: (a) Silicilyte from the Athel Formation in Al Shomou-1; (b) Silicilyte from the Athel Formation in Athel-1; and (c) Organic shale from the U Formation in Athel-1 (scale grid is 1 x 1 mm) (Mohammed et al., 1997).

Figure 16.16:

Ditch cuttings from the Ara Group: (a) Silicilyte from the Athel Formation in Al Shomou-1; (b) Silicilyte from the Athel Formation in Athel-1; and (c) Organic shale from the U Formation in Athel-1 (scale grid is 1 x 1 mm) (Mohammed et al., 1997).

Figure 16.17:

Composite electrical logs, lithology and lithological description of the U Formation, Ara Group, in well Al Noor-1, South Oman (Mohammed et al., 1997). See Figure 16.1 for location.

Figure 16.17:

Composite electrical logs, lithology and lithological description of the U Formation, Ara Group, in well Al Noor-1, South Oman (Mohammed et al., 1997). See Figure 16.1 for location.

Figure 16.18:

Composite electrical logs, lithology and lithological description of the U Formation, Ara Group, in well Thuleilat-2, South Oman (Mohammed et al., 1997). See Figure 16.1 for location.

Figure 16.18:

Composite electrical logs, lithology and lithological description of the U Formation, Ara Group, in well Thuleilat-2, South Oman (Mohammed et al., 1997). See Figure 16.1 for location.

Figure 16.19:

Core photograph of (a) base A4C stringer carbonate, (b) high Gamma ‘tuffaceous mudstone’ and (c) A4 anhydrite, in well Birba-4.

Figure 16.19:

Core photograph of (a) base A4C stringer carbonate, (b) high Gamma ‘tuffaceous mudstone’ and (c) A4 anhydrite, in well Birba-4.

Figure 16.20:

Ditch cuttings from the Ara Group: (a) Dolomite from the U Formation in Birba-1; and (b) Dolomite from the Birba Formation in Areeq-1 (scale grid is 1 x 1 mm) (Mohammed et al., 1997).

Figure 16.20:

Ditch cuttings from the Ara Group: (a) Dolomite from the U Formation in Birba-1; and (b) Dolomite from the Birba Formation in Areeq-1 (scale grid is 1 x 1 mm) (Mohammed et al., 1997).

Figure 16.21:

Composite electrical logs, lithology and lithological description of the Birba Formation, Ara Group, in well Shamah-1, South Oman (Mohammed et al., 1997). See Figure 16.1 for location.

Figure 16.21:

Composite electrical logs, lithology and lithological description of the Birba Formation, Ara Group, in well Shamah-1, South Oman (Mohammed et al., 1997). See Figure 16.1 for location.

Figure 16.22:

Composite electrical logs, lithology and lithological description of the Birba Formation, Ara Group, in well Miqrat-1H1, North Oman (Mohammed et al., 1997). See Figure 16.1 for location.

Figure 16.22:

Composite electrical logs, lithology and lithological description of the Birba Formation, Ara Group, in well Miqrat-1H1, North Oman (Mohammed et al., 1997). See Figure 16.1 for location.

Figure 16.23:

Composite electrical logs, lithology and lithological description of the Birba Formation, Ara Group, in well Budour NE-2, South Oman (Mohammed et al., 1997). See Figure 16.1 for location.

Figure 16.23:

Composite electrical logs, lithology and lithological description of the Birba Formation, Ara Group, in well Budour NE-2, South Oman (Mohammed et al., 1997). See Figure 16.1 for location.

Figure 16.24:

Composite electrical logs, lithology and lithological description of the Birba Formation, Ara Group, in well Thamoud-6, South Oman (Mohammed et al., 1997). See Figure 16.1 for location.

Figure 16.24:

Composite electrical logs, lithology and lithological description of the Birba Formation, Ara Group, in well Thamoud-6, South Oman (Mohammed et al., 1997). See Figure 16.1 for location.

GROUPFORMATIONMEMBERSEQUENCE‘SUB-UNIT’/SEQUENCE CODING
AraDhahaban A6A6 Carbonate/Shale (A6C)
A6 Salt + Anhydrite (A6E)
Al Noor A6 Intrasalt Clastics (A6CL)
A5A5 Carbonate (A5C)
A5 Salt + Anhydrite (A5E)
AthelThuleilatA4*A4 Athel Shale (A4AS)
Al ShomouA4 Athel Silicilyte (A4Sil)
ThamoudA4 Athel Carbonate (A4AC)
U A4 U-Carbonate (A4UC)
A4 U-Shale (A4US)
A4 Carbonate (A4C)
A4 Salt + Anhydrite (A4E)
Birba A3A3 Carbonate (A3C)
A3 Salt + Anhydrite (A3E)
A2A2 Carbonate (A2C)
A2 Salt + Anhydrite (A2E)
A1A1 Carbonate (A1C)
A1 Salt + Anhydrite (A1E)
A0A0 Basal Carbonate + Shale (A0C)
GROUPFORMATIONMEMBERSEQUENCE‘SUB-UNIT’/SEQUENCE CODING
AraDhahaban A6A6 Carbonate/Shale (A6C)
A6 Salt + Anhydrite (A6E)
Al Noor A6 Intrasalt Clastics (A6CL)
A5A5 Carbonate (A5C)
A5 Salt + Anhydrite (A5E)
AthelThuleilatA4*A4 Athel Shale (A4AS)
Al ShomouA4 Athel Silicilyte (A4Sil)
ThamoudA4 Athel Carbonate (A4AC)
U A4 U-Carbonate (A4UC)
A4 U-Shale (A4US)
A4 Carbonate (A4C)
A4 Salt + Anhydrite (A4E)
Birba A3A3 Carbonate (A3C)
A3 Salt + Anhydrite (A3E)
A2A2 Carbonate (A2C)
A2 Salt + Anhydrite (A2E)
A1A1 Carbonate (A1C)
A1 Salt + Anhydrite (A1E)
A0A0 Basal Carbonate + Shale (A0C)
*

No relative stratigraphical position implied. Some key uncertainties remain concerning the geographical and temporal relationships of some units within the A4 sequence.

Contents

GeoRef

References

Related

Citing Books via

Close Modal
This Feature Is Available To Subscribers Only

Sign In or Create an Account

Close Modal
Close Modal