Sedimentological Characterisation and Sequence Stratigraphy of the Lowermost Lekhwair Deposits of the Southern Tethys Early Cretaceous, Onshore Abu Dhabi
Authors: Jeong J. , Kang J. , Bonin A. 
Published: November 2016

While the Kharaib and Shuaiba carbonate hydrocarbon reservoirs of UAE benefit from numerous investigations enabling the establishment of a relatively accurate biostratigraphical framework and the characterisation of the sedimentological, depositional and sequence stratigraphical context, the lowermost Lekhwair deposits remain poorly known and are rarely the topic of publications. These Valanginian/Hauterivian(?) (Alsharan and Nairn, 1986; Granier, 2000) carbonates are studied in the onshore Abu Dhabi area where they host hydrocarbon accumulations within clean shallow inner ramp deposits. These clean carbonates alternate with non-reservoir units comprising clay/organic matter-prone carbonates thought to have been deposited in distal settings (e.g. Hassan and Wada, 1981) and commonly picked as maximum flooding surfaces (e.g. Sharland, 2001; Strohmenger et al., 2006). However, until now it has not been possible to confirm or refute these interpretations due to the lack of petrographical samples within these non-reservoir rocks, which are usually discarded due to their negligible reservoir potential.

This work consists of the establishment of the sedimentological framework of the lowermost Lekhwair Formation of an onshore Abu Dhabi area based on the detailed sedimentological descriptions from four cored wells and the analyses of petrographical samples. A facies analysis has been carried out which has resulted in the reconstruction of the depositional environments. The integration of key surfaces characterised in core, marked sedimentological changes and core-calibrated openhole log data have helped in the interpretation of a sequence stratigraphic framework across both the reservoir and non-reservoir units.

In summary, an upward change from a dominantly intertidal to clay/organic-prone inner ramp deposition in the lowermost Lekhwair to clean Lithocodium/Bacinella–rich, open and probably marginal inner ramp to proximal mid-ramp deposits reflects a large-scale transgressive phase. The large-scale maximum flooding surface is picked at the base of a c.30–40ft thick massive Lithocodium/Bacinella-bearing interval and is overlain by the upward regression towards more restricted and Lithocodium/Bacinella-free inner ramp deposits. A well-developed karstified surface interpreted as a sequence boundary (SB) caps the large-scale regressive phase. It is overlain by clay/organic-rich inner ramp deposits interpreted to represent the base of the overlying large-scale deepening-upward trend.

At a smaller scale, the lower Lekhwair interval consists of alternations between proximal and clay-rich inner ramp, intertidal deposits and clean inner/proximal mid-ramp carbonates. These alternations are thought to be controlled by intermediate-scale sea level fluctuations with the proximal and clay-rich inner ramp deposits being preserved at the base of transgressive trends and the cleanest carbonates being formed during deep conditions (i.e. flooding intervals).

This evaluation has provided a better understanding of the depositional evolution, helping in the prediction of the facies distribution at the reservoir scale and the establishment of the reservoir architecture.