AAPG Middle East Region Geoscience Technology Workshop:
3rd Edition Carbonate Reservoirs of the Middle East

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Advances in Characterization and Modeling of a Lower Cretaceous Heterogeneous Carbonate Reservoir Onshore Abu Dhabi, UAE

Abstract

A Lower Cretaceous carbonate reservoir onshore Abu Dhabi is characterized by high heterogeneities, whose nature and distribution is controlled by the interplay of depositional facies and diagenetic overprints within the sequence stratigraphic reservoir architecture. These heterogeneities are believed to have a great impact on the reservoir fluid flow behaviour, the understanding and prediction of these heterogeneities are crucial to mitigate the uncertainties associated with 3D reservoir modeling. In this work a comprehensive reservoir characterization and modeling study have been conducted to capture the nature of the geological heterogeneities controlling the reservoir properties. The impact of diagenesis on the rock types is driven by the characteristics of the depositional facies, and is captured in a refined geological facies scheme. Through core and petrographic observations and analysis, four classes of geological facies are defined: 1) rudist-bearing rudstones and floatstones, 2) Bacinella/Lithocodium-coral floatstones, rudstones and breccia, 3) Orbitolinid- and skeletal-rich facies and 4) fine-grained, matrix-supported facies. The link between geological facies and depositional settings and how they control the reservoir quality is established. The nature, areal distribution and vertical stacking pattern of the geological facies has been used to refine the sequence stratigraphic architecture and geological concept of the reservoir. The geological facies have been integrated during the petrophysical synthesis and an excellent correlation with the petrophysical groups have been reached, allowing the generation of seven Static Rock types, these are divided in two classes, based on their stratigraphic occurrence and petrophysical evidences. The best Static Rock Types are represented by the rudist-rich floatstones/rudstones and the Bacinella/Lithocodium-coral facies, both are characterized by a multiple porosity system because of the diagenetic imprint. Based on the revisited geological concept and sequence stratigraphic architecture, the Static Rock Types are distributed within the reservoir, and permeability modelled accordingly. A fit-for-purpose 3D static model has been developed. Significant sequence stratigraphic surfaces tied to interpreted seismic horizons are the main input to model the reservoir structural framework. Static Rock Types have been used to condition the reservoir properties model, the latter have been distributed by using a novel approach to ensure accurate lateral and stratigraphic distribution and, therefore, to improve the quality of the prediction of the reservoir properties.