New Insights on the Lower Lekhwair Formation offshore Abu Dhabi: Understanding the Key Role of Lithocodium/bacinella Floatstones on Large Scale Reservoir Quality Variations

Abstract

The Lower Cretaceous Lekhwair Formation is one of the most prolific oil reservoirs in onshore and offshore UAE, yet the available literature on this interval remains limited. Based on a recent study carried out in collaboration with ADNOC Offshore, the present paper provides new insights into the comprehension of the interplay between primary depositional and secondary diagenetic controls on the reservoir performance, which is of crucial importance for the refinement of the static and dynamic models. In offshore Abu Dhabi, the Lower Lekhwair Formation is characterised by an alternation of relatively thick argillaceous (dense zones) and clean limestones (reservoir zones). Reservoir zones consist of basal, low to moderate energy inner ramp deposits, grading upward into thick inner and mid-ramp sediments. Lithocodium/Bacinella is the volumetrically dominant skeletal allochem and can form m-thick, stacked floatstone units. Such Lithocodium/Bacinella-rich floatstones are interpreted to originate from a mid-ramp depositional setting as a result of an increase in the accommodation space. By contrast, the contribution of Lithocodium/Bacinella floatstones is significantly reduced in inner ramp settings where these tend to form cm- to dm-scale, laterally discontinuous interbeds. The combination of sedimentological findings with diagenetic data provided an enhanced understanding of the origin and variations of the reservoir quality across the Lower Lekhwair Formation. In more detail, the best reservoir quality occurs within poorly cemented, Lithocodium/Bacinella-rich floatstones with grain-supported matrices, which favoured the preservation of a macropore-dominated pore system allowing an effective fluid flow. By contrast, the mud-supported textures with only rare and localised occurrence of mm- to cm-scale Lithocodium/Bacinella clumps, present the poorest reservoir quality due to the isolated nature of the macropores and the relatively tight micrite matrix surrounding them. At the large scale, the Lower Lekhwair shows an upward increase in reservoir quality, consistently with the upward increase in abundance and thickness of the Lithocodium/Bacinella-rich floatstones. The integration of depositional features with diagenetic overprint in the Lower Lekhwair Formation shows the fundamental role played by Lithocodium/Bacinella-rich floatstones with grain-supported matrices on the reservoir quality distribution. The impact of the Lithocodium/Bacinella floatstone matrices on the reservoir performance was never investigated before and hence represents an element of innovation and a powerful tool to predict the distribution of the areas hosting the best reservoir properties.