Depositional Environment and Controls on Porosity Development and Distribution, in the Late Albian Mauddud Formation, Sabiriyah and Raudhatain Fields, Kuwait, as Input to a Geocellular Model

Abstract

Abstract Detailed description of core (19,500 ft) from 57 wells in the Cretaceous (Late Albian) Mauddud Formation, Sabiriyah and Raudhatain Fields, Northern Kuwait, formed the basis for an updated depositional model. Numerous reservoir units occur within carbonate-dominated, mixed carbonate-clastic, and siliciclastic-dominated intervals. Core description and interpretation software (WellCAD/CoreCAD) was used to record observations and interpretations in digital format which was integrated into a master database for geocellular modeling, static, and dynamic reservoir modeling. The predictive depositional model demonstrates lithofacies distribution, primary depositional and secondary diagenetic controls on porosity occurrence, distribution of high permeability ‘thief zones common in both fields, and distribution of flow baffles and barriers within the vertical sequence. The Lower Mauddud comprises carbonate-dominated shallowing-upward cycles deposited in a carbonate ramp environment and localized clastic wedge deposits and associated tidal channels. The Upper Mauddud evolved to carbonate platform geometry with bioclastic shoals, rudist banks and patch reefs, and restricted back-reef and lagoonal highstand deposits. Late highstand deposition culminated in widespread subaerial exposure near the top of Upper Mauddud. Most porosity development and distribution is controlled by depositional fabric and texture, bioturbation, and dissolution during subaerial exposure at cycle tops. Widespread exposure surfaces and related brecciation and dissolution associated with late highstand deposition are major contributors to thief zone development. Several marine flooding events influence widespread and correlative vertical flow baffles. ‘Bank and tidal channel’ topography is dominant in the upper Lower Mauddud, where shoals and patch reefs developed between tidal channels. Tidal channels were filled in part with poor to non-reservoir clastics, creating baffle zones. Bioclastic shoals comprise Upper Mauddud reservoir units. Middle-late highstand reservoirs are composed of bank crest rudist buildups, patch-reefs and shoals. Late highstand deposits record increasingly restricted depositional setting, increase in frequency and duration of subaerial exposure, and development of karst brecciation with associated dissolution.