Nahr Umr Fm
Type Locality and Naming
Clastic deposition dominated the whole shelf during most of the Albian. In Oman the Nahr Umr Fm shales represent this clastic wedge, which provides a world class seal responsible for a large proportion of the retained hydrocarbons in the Oman subsurface. Type section is in Nahr Umr-2 in Iraq (see van Bellen et al., 1959/2005). It is the lower formation in the Wasai Gr (Oman). Column: Oman Subsurface, Oman Outcrop,: Qatar,: Bahrain,: United Arab Emirates,: Central Iraq,: Rutba Uplift (W Iraq). The reference section in Oman is Maqhoul-1 in North Oman (Figure 6.9). Additional reference sections are Baqlah-1 in North Oman and Kulan-1 in Central Oman
Synonym:
Lithology and Thickness
The Nahr Umr Fm in Oman consists of varyingly calcareous shales, marls and some argillaceous limestones. In North Oman the Formation is subdivided into Lower and Upper members on the basis of a thin marker limestone - the Nahr Umr Marker Limestone. This marker limestone, absent from South Oman, is often important as the last downhole reference point for wells targeting the underlying Shu’aiba Fm. Regionally, it can consist of gray, green and, red, silty fine-grained sandstone and coarse siltstone, relatively thin-bedded and divided into Nahr Umr 4 Fm, Nahr Umr 3 Fm, Nahr Umr 2 Fm, Nahr Umr 1 Fm. All units are highly fossiliferous with abundant foraminifera, notably orbitulinids. A widely-traceable, thin, clean ‘Marker’ limestone lies within the lower 30 m of the Formation, but does not extend into South Oman. Towards the south and in the Al Huqf area outcrops, the Nahr Umr thins significantly (e.g. Nimr-1, Figure 6.5) and becomes more silt to sand prone (often with associated chamosite content as burrow fill or ooid grains). To the east of the Eastern Flank/Al Huqf highs and in the extreme south of Oman, thicker limestone/claystone sections indicate a shift to a more proximal, platform edge, setting (e.g. Kulan-1, Figure 6.11).
Relationships and Distribution
Lower contact
Erosion was associated with karstification of the underlying Shu’aiba Fm carbonates. The Formation conformably overlies Upper Shu’aiba (Bab Member clinoforms) infill of the central Bab Basin in northwest Oman. Elsewhere, the base Nahr Umr is an unconformity corresponding to a regional Arabian Platform intra-Late Aptian hiatus. However, the Aptian – Albian boundary corresponds to a global second-order sequence boundary and a major sea-level drop. Widespread erosion on basin edges, associated with that sea-level drop, resulted in a hiatus of variable span, usually intra-Late Aptian, but occasionally down into the Early Aptian. This unconformity expands further southwards and onto structural highs (Eastern Flank) where it progressively truncates older stratigraphic units of the Kahmah Gr, Mesozoic Clastics Gr, Sahtan Gr, Akhdar Gr, Haushi Gr or Mahatta Humaid Gr groups.
It overlies Al-Hassanat IV in Oman Outcrop. In Rutba Uplift (W Iraq), it is Zubair Fm
Upper contact
Regionally, the upper boundary is usually conformable and transitional up into the lower carbonates of the Natih Fm. In Iraq and United Arab Emirates, it is Mauddud Fm. In Qatar and Bahrain, it is Wasia Fm
Regional extent
Oman Subsurface, Oman Outcrop,: Qatar,: Bahrain,: United Arab Emirates,: Central Iraq,: Rutba Uplift (W Iraq). "The Nahr Umr is widespread over Oman, representing a global Cretaceous sea-level rise, and transgresses across the subcrop of many older units to the south and east, thinning in the same directions. The Formation is generally much thinner, with greater sand content in South and Central Oman, with the exception of Kulan-1 and other platform edge wells to the east and south of regional highs. The Nahr Umr Fm is a laterally extensive unit of shallow-marine, calcareous shales, grading into lime mudstones toward the north. The Formation condenses, becomes more sandy and is of shallower depositional aspect to the south. Packer (2004), in contrast to Witt and Gökdag (1994), suggests that most of the southerly wells do contain an Early Albian section and that the base is not necessarily diachronous. However, it is entirely possible that the onlap onto the southern part of the Eastern Flank did not occur until the Middle – Late Albian (e.g. Marmul area, as stated by Witt and Gökdag, 1994, and wells to the south where direct Early Albian evidence is lacking). Thicker, mixed silt/shale-carbonates sections are seen to the east (including offshore) and extreme south of Oman
There is no visible difference between the limestones of the Natih Fm, Nahr Umr and the Shu’aiba Fm. Shales are varicoloured, generally shades of olive brown and greyish green and are indistinguishable from the Natih Fm shales. They can be distinguished from Gharif Fm shales (over which the Nahr Umr Fm sometimes lies, e.g., Nimr-1) by color and being recovered in a slightly more fissile/elongate state than the more massive, amorphous and reddish brown Upper and Middle Gharif Fm shales." The Al Hassanat Fm in the Saih Hatat area of northern Oman is a platform-margin carbonate succession that has been interpreted as correlative with the Nahr Umr Fm (Immenhauser et al., 2000; Immenhauser et al., 2001).
GeoJSON
Fossils
Sub-biozones F575 (Trocholina altispira, Pseudocyclammina hedbergi) – F573/571 (Orbitolina texana/Spineloberis IRK20, Cytherelloidea IRR29). Packer (2004) carried out a study of 33 well sections from throughout Oman, in which he reviewed and updated the existing Sikkema (1991) zonation. He integrated ostracod, benthic foraminiferal and rare planktonic foraminiferal data to essentially split the Nahr Umr into Late – Middle (F575) and Early Albian (F571) (see Group discussion). It should be noted that the study utilized conventional micropaleontological, ‘hand specimen’ sample analysis and the need to further exploit the stratigraphical potential of the orbitulinids (mesobitulinids) via thin section study was not realized. One of the key events used to identify the Early Albian is the last occurrence of the ostracod Spineloberis IRK20, as previously noted by Witt and Gökdag (1994). Sub-biozone F573 of Sikkema (1991) is essentially coeval with the new Sub-biozone F571 and its top was redefined as a downhole influx of Mesorbitulina spp. towards the base of the Late – Middle Albian section. The Sub-biozones F573 and F571 represent different depositional biofacies within the Early Albian and the full range from ‘Mesorbitulina spp. dominated’, through ‘mixed’ to ‘ostracod dominated’ assemblages is evident. Concerns remain about the true range of certain marker species and the use of facies-dependent taxa (ostracods and benthic foraminifers). Further study utilizing nannofossils, dinoflagellate cysts and thin section micropaleontology may prove worthwhile.
Age
Depositional setting
A regional drop in sea level associated with subaerial exposure and influx of fine-grained clastics terminates extensive carbonate deposition (Shu’aiba Fm) on the Arabian Platform in the early Late Aptian. Clastic deposition dominated the whole shelf during most of the Albian. In Oman the Nahr Umr Fm shales represent this clastic wedge, which provides a world class seal responsible for a large proportion of the retained hydrocarbons in the Oman subsurface. Carbonates were only present along the shelf margin.. In sequence stratigraphic terms the basal clays of the Nahr Umr Formation represent the transgressive systems tract of the flooding culminating in the maximum flooding represented by the Marker Limestone Mbr. Immenhauser et al. (1999) used evidence from stable isotopes, brackish fluid inclusions, and petrographic interpretations of root structures to suggest repeated subaerial exposure in the Nahr Umr of northern Oman, but this has been disputed by Davies et al. (2002).
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