Biostratigraphic Constraints on the Shuaiba Formation, Abu Dhabi, United Arab Emirates

Shuaiba Formation The Shuaiba Formation is the uppermost unit of the Thamama Group, and is the most prolific oil reservoir in Field A. The Thamama Group is part of a second order supersequence that spans the Valanginian to Aptian stages (Sharland et al., 2001). The Shuaiba is overlain by the Nahr Umr Formation which forms a regional seal. Depositionally the Shuaiba reservoir spans platform, slope and basin environments. Consequently, lithostratigraphic nomenclature varies regionally. This emphasizes the need for a regional, sequence-stratigraphic framework based on chronostratigraphic data (Yose, et al., 2006). Yose et al. (2006), dealt with facies variation in the Shuaiba combined with high-resolution volume visualization of stratigraphic features. Seismic data also provided third and fourth order sequence boundaries (Gombos, 2003), but they were not well constrained in time. It was felt that more precise age constraints were needed to provide a rigourous time-stratigraphic framework for the seismic sequences of the Shuaiba Formation. To this end, a search was undertaken for biostratigraphic fossils which could provide this. A general Aptian age has long been established for the Shuaiba in Abu Dhabi (Alsharhan 1985; Owen and Nasr, 1958). This was mainly based on the presence of Aptian age fossils such as rudists, larger benthic foraminifera (e.g., Orbitolina and Choffatella), and palynomorphs. Ammonites and planktonic foraminifera are not present in the core material. Larger benthic foraminifera and palynomorphs are often excellent paleo-environmental indicators, but their limited paleogeographic distribution and broad time ranges, make them less than optimal zonal fossils. A finer-scaled chronostratigraphic subdivision of the Shuaiba, providing more precise ages for its upper and lower boundaries, is required to place the stratigraphic sequences in their proper time frame. Calcareous nannofossils can provide more precise age determinations. Their ranges are well constratined regionally and when combined with other non-evolutionary events such as anoxic events, they can provide even finer resolution. We use the time scale and cycle chart of Hardenbol et al. (1998) as our sequence and age reference (Figure 1). Biostratigraphy and Sequence Stratigraphy A new and exciting discovery of calcareous nannofossils in lowstand and transgressive systems tracts of the Bu Hasa clinoforms permitted dating of Shuaiba with more precision than had previously been possible. SEM photomicrographs taken for reservoir quality studies by Rudolph (1984, unpublished internal company report) reveal well preserved calcareous nannofossils in the Shuaiba Fromation of Well-3 (Figure 3). This discovery made it possible to more precisely age-date the Shuaiba Formation in the LST and TST of the Bu Hasa clinoforms. Calcareous nannofossils are photosynthetic, planktonic marine algae. They prefer open, well circulated marine environments and therefore are most commonly found in shales or deeper water sediments. The unique setting of Field A on the edge of a deep basin (Bab Basin) with well developed clinoforms is prone to the presence of calcareous nannofossils. Four samples were selected from the same interval of the core in which the calcareous nannofossils were identified in SEM photomicrographs. These were analyzed for nannofossils. The results were encouraging as many age-diagnostic calcareous nannofossils were present. Knowing that calcareous nannofossils were present in some samples enabled us to predict that they were also present in similar sediments, i.e., shale-prone condensed sections. An expanded study was proposed and samples from cores in Well-1, Well-4, Well-5 and Well-6 of Field A were collected. These were analyzed and incorporated into this study.