Full field reservoir geological model integrating high resolution sequence stratigraphy: Nahr Umr sandstone, Block 5, offshore Qatar

Oil bearing ultra thin, shallow marine sandstones, with a thickness of only 3-5 ft are present in the Upper Aptian - Middle Albian lower part of the Nahr Umr Formation in Block 5, offshore Qatar. The sands have been produced for 16 years from horizontal wells and Nahr Umr has now reached a mature development stage where identification of further infill opportunities requires very detailed reservoir characterization studies and extensive simulation work. This paper describes how integration of core studies and detailed palynological analysis has improved the understanding of both the reservoir architecture and the reservoir quality distribution. This approach allowed construction of a deterministic 3D static model, which was used for dynamic simulation work. The high resolution sequence stratigraphic model is based on the detailed description of 28 cored vertical wells, a core gamma-ray controlled log calibration, and a closely spaced palynological sampling. The lower part of the Nahr Umr Formation consists of a large variation of sedimentary facies, including sandstones, claystones, oolitic ironstones and carbonate mudstones and wackestones. In addition, locally, carbonate and siderite cement are present as well as varying amounts of authigenic glauconite. These facies are organized in six different types of small scale (decimeter to metre scale) cycles, which form a stacking pattern controlled by the 3rd order depositional sequences. The obtained insight in the systematic and predictive spatial distribution of sedimentary facies forms the framework for the reservoir model, with the best reservoir units occurring in the sandstone dominated cycles, whilst low permeable silts, iron-rich clays, iron ooids and carbonates form flow baffles between the main pay zones. Facies maps for each cycle were refined with input from horizontal well data, thus further improving the input for the full field static reservoir model. Calibration of the modelled flow properties against dynamic flow data obtained from numerous Mini-DST's provided a valuable quality check of the model properties. Copyright 2011, Society of Petroleum Engineers.