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AAPG GEO 2010 Middle East
Geoscience Conference & Exhibition
Innovative Geoscience Solutions – Meeting Hydrocarbon Demand in Changing Times
March 7-10, 2010 – Manama, Bahrain

Sedimentation and Diagenesis for Improved Reservoir Characterization - Upper Cretaceous Carbonate, Onshore Oil Field, Abu Dhabi (United Arab Emirates)

Magdy A. Hozayen1; Mohamed T. Shuaib1

(1) RDD, ADCO, Abu Dhabi, United Arab Emirates.

A big carbonate oilfield, located onshore Abu Dhabi, has been producing from the Upper Cretaceous (Maastrichtian) carbonate reservoir since 1983. Detailed sedimentological, diagenetic, seismically interpreted high-resolution sequence stratigraphy, flow units and reservoir rock type studies have been carried out, integrating approximately 7,000 feet of core material, approximately 3,500 thin sections, and well-log data of 46 wells. Core description along with semi-quantitative petrographic examination of thin sections allowed establishing a new reservoir characterization model.

The sedimentological study suggested sixteen lithofacies types (LF1 to LF16) representing a wide variety of depositional environments, ranging from upper ramp, rudist-bioclastic shoals to open marine mid to outer ramp mud-dominated settings. The 3D seismic interpretation was integrated with the results of the sedimentological study and seventeen high resolution fourth-order sequences were defined which were finally utilized the basis for the reservoir model layers. The diagenesis study showed a variety of diagenetic events and processes. Leaching, calcite cementation and dolomitization formed a strong diagenetic overprint, and have produced a number of flow units masking the primary petrophysical properties of the lithofacies. Karsts are developed at certain intervals within the reservoir. Karst intervals commonly contain solution channels, solution enhanced fractures, collapse features, and breccias. Challenges encountered during the reservoir rock type characterization process necessitated the use of various RRT techniques and procedures to achieve a robust reservoir rock type (RRT) scheme.

The integration of rock texture, facies, diagenesis, the newly developed high-resolution sequence stratigraphic layering scheme together with the defined flow units suggested that the reservoir comprises nine reservoir rock types; RRT1 being the best to RRT9 the poorest. A 3D static model was built and the nine rock types were distributed within the seventeen fourth-order sequence stratigraphic layers. The new 3D static model will be used as input for reservoir flow modeling.