Shu'aiba Fm

Deposited during a highstand (carbonate ramp). Type section is Zubair-3 in Southern Iraq (see van Bellen et al., 1959/2005). The reference section in Oman is well Lekhwair-258. Columns: Syria, Oman Subsurface, Kuwait, Qatar, Column: Bahrain, Saudi Arabia Subsurface, Saudi Arabia Outcrop, Iraq Central, United Arab Emirates; Uppermost formation in Thamama Gr in Kuwait and of the Kahmah Gr in Oman subsurface.

Synonym: Shuaiba Fm, Shu'aiba Platorm Fm, upper part includes the Bab Member (Qatar, United Arab Emirates) or coeval Bab Basin Clinoforms (Oman).

The depositional patterns in the Shu’aiba Fm are highly complex, and include open-marine areas, shallow-marine (rudist-rich) shoals and lagoonal (semi-restricted) environments. In Iraq, it is 62m of oolitic limestone, sometimes sandy, and fine-grained. Grades into white, chalky limestone and has shale streaks at the top. In Oman, in general, there is a clear trend towards more open-marine conditions from the ocean facing northeast towards the Lekhwair area in northwest Oman, where deeper water carbonates link up with the more muddy intra-shelf carbonates of the Bab Basin near Oman’s border with the UAE. In Oman, it is divided into a Lower Member of "clean" limestones, and an Upper Member of interbedded calcareous clays and limestones. The Oman Upper Member corresponds to the Bab Member (Bab Fm of Granier, 2000) of the UAE. In Oman, the Shu’aiba Fm varies in thickness from 130 m in the northwest to 0 m in Central Oman. The regional thinning trend to the south is related to uplift and truncation below the base Nahr Umr Fm unconformity as well as a subtle onlap onto the Al Huqf High.

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

Deposited during a high stand (carbonate ramp). In Oman region, the Shu’aiba Formation basically represents the establishment of a carbonate platform at the margin of the shelf following the flooding of the exposed Kharaib platform. This carbonate complex prograded towards the shelf interior and deposition ended with regional exposure. Facies analysis shows that during the initial transgression low-angle depositional profiles predominated and a microbial/algal carbonate factory produced the sediments. During the late transgression algal mounds developed a differentiated topography. This topography was filled during the subsequent highstand by progradation from rudist carbonate factories that first established in shallows, shedding large amounts of sediment, laterally prograding into the deeper intra- shelf areas. The resultant clinoforms are steep and show strong lateral variations in the amount and direction of progradation. The carbonate platform became subaerially exposed during the following fall in sea level and the remaining depositional lows on the shelf, the ‘Bab’ intra-platform basin, were filled by deposits consisting of alternating offlapping wedges of fine-grained clastics and carbonates. Parallel and laterally continuous clinoform belts infilling the Bab Basin suggest a strong component of alongshore transport in the distribution of the clastics. These clastics provide additional stratigraphic sealing components for hydrocarbon traps in northwest Oman.

"The carbon isotope stratigraphy of the Shu’aiba Fm has been documented by Vahrenkamp (1996), van Buchem et al. (2002) and Immenhauser et al. (1999, 2000, 2004). The carbon isotope data for the lower part of the Shu’aiba consistently show a negative excursion followed by increasing trend towards high d¹³C values. This is very similar to the Early Aptian global carbon isotope profile (see Jones and Jenkyns, 2001). However, correlation of isotope trends in shallow-water carbonates, which are likely to have significant and numerous hiatus, variations in sedimentation rates and a strong diagenetic overprint, with the continuous global record from deep-sea sections are questionable. For the upper part of the Lower Shu’aiba and the Upper Shu’aiba, the isotope trends are highly variable both vertically and between closely spaced wells and do not clearly tie to the published curves. … In the Shu’aiba Formation, a regionally correlatable interval characterized by a high uranium content (Gamma trend) associated with condensed sedimentation and high organic matter content may be recognized. This has been correlated to the global, earliest Aptian event OAE1a (also know as the ‘Selli’ level, Weissert et al., 1998), which is associated with the beginning of the positive d¹³C excursion and the ‘nannoconid crisis’ (Gombon et al., 2008)."