AAPG Middle East Region, Second EAGE/AAPG Hydrocarbon Seals of the Middle East Workshop

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Characterization of Primary Seals in Tuba Reservoir in Sabiriyah Field, North Kuwait


The Tuba Formation is part of the Middle Cretaceous Ahmadi Formation of Onshore Kuwait. It is a well-developed limestone reservoir with an Ahmadi Shale layer residing on the top as a regional seal. The Tuba Limestone has been divided into three units as Upper, Middle and Lower Tuba based on the sedimentological characteristics and the reservoir rock properties. The Formation was deposited in marine environments during the Late Albian-Early Cenomanian time. Its thickness increases up to 400 ft towards the northeast. The Lower Tuba unit consists of several transgressive-regressive cycles. It comprises the most variation in lithofacies compared to the other Tuba units. Argillaceous mudstone and wackstones dominate at the lower section and gradually merge into Wara Shale Formation. Towards the upper section, it has much cleaner lithofacies, consisting of pack-grainstones and grainstones with well-developed porosity and permeability for hydrocarbon bearing. The Middle Tuba unit is deposited in very low energy depositional environments. It is represented mostly by argillaceous-rich lithofacies with poor reservoir quality. Normally, this unit is treated as a barrier between the Upper Tuba and Lower Tuba. The Upper Tuba unit of most wells drilled in the northern area hold the best reservoir quality. It also has variable argillaceous content along with the dominated wackstones and packstones. The interpretation of seals for Tuba Formation is reviewed in this article in terms of the integrated study of sedimentology, petrography, wells correlation, log data, RFT/Sampling, and cores (RCA and SCAL). The static and dynamic reservoir models built recently are also included to improve and assist analysis. The seals are identified in three categories: (1) Primary shale seals bounded to the top and bottom of Tuba Formation through entire field with extreme low permeability. (2) Shale seals which are mudstone dominated with highest argillaceous contents, recognizable in the middle Tuba unit. (3) Localized diagenetic seals which happens by subsequent diagenetic effects specifically dissolution of matrix and calcite cementation resulting in reducing porosity and permeability. Some sealed faults have been interpreted as part of the structure seals which stop the hydrocarbon migration laterally.