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

A big carbonate oilfield, located onshore Abu Dhabi, has been producing from the Upper Cretaceous<br>(Maastrichtian) carbonate reservoir since 1983. Detailed sedimentological, diagenetic, seismically<br>interpreted high-resolution sequence stratigraphy, flow units and reservoir rock type studies have been<br>carried out, integrating approximately 7,000 feet of core material, approximately 3,500 thin sections,<br>and well-log data of 46 wells. Core description along with semi-quantitative petrographic examination<br>of thin sections allowed establishing a new reservoir characterization model.<br>The sedimentological study suggested sixteen lithofacies types (LF1 to LF16) representing a wide<br>variety of depositional environments, ranging from upper ramp, rudist-bioclastic shoals to open marine<br>mid to outer ramp mud-dominated settings. The 3D seismic interpretation was integrated with the<br>results of the sedimentological study and seventeen high resolution fourth-order sequences were<br>defined which were finally utilized the basis for the reservoir model layers. The diagenesis study<br>showed a variety of diagenetic events and processes. Leaching, calcite cementation and dolomitization<br>formed a strong diagenetic overprint, and have produced a number of flow units masking the primary<br>petrophysical properties of the lithofacies. Karsts are developed at certain intervals within the<br>reservoir. Karst intervals commonly contain solution channels, solution enhanced fractures, collapse<br>features, and breccias. Challenges encountered during the reservoir rock type characterization process<br>necessitated the use of various RRT techniques and procedures to achieve a robust reservoir rock type<br>(RRT) scheme.<br>The integration of rock texture, facies, diagenesis, the newly developed high-resolution sequence<br>stratigraphic layering scheme together with the defined flow units suggested that the reservoir<br>comprises nine reservoir rock types; RRT1 being the best to RRT9 the poorest. A 3D static model was<br>built and the nine rock types were distributed within the seventeen fourth-order sequence stratigraphic<br>layers. The new 3D static model will be used as input for reservoir flow modeling.