Sequence Stratigraphy and Reservoir Architecture of Tuba in Sabriyah Field, North Kuwait

Abstract

Introduction The Sabiriyah Tuba Reservoir is an Upper Cretaceous Carbonate reservoir overlying the Mauddud Formation. The Tuba reservoir is a limestone member of Ahmadi Formation and is sandwiched between underlying Wara shale at the bottom and overlying Ahmadi shale at the top. It has been penetrated by all deep wells in Sabiriyah as well in surrounding Raudhatain and Bahrah structures. Together with other "Minor Reservoirs" they represent a significant component of the North Kuwait Development Plan, and are scheduled for full development. The OIP is uncertain. However recent mapping shows that the Tuba reservoir footprint is similar in area to that of Sabiriyah Mauddud suggesting 1100 mmbbl STOIIP. Although it has been penetrated by more than 260 wells, it has been tested in only four. Tuba is currently producing only from one well @500 b/d from a reservoir layer. Based on well log signature and core analysis, the vertical distribution and relative stacking of lithofacies associations within the Tuba Reservoir has lead to the identification of a clear Lower, Middle and Upper reservoir units/subdivisions of Tuba. This tripartite scheme provides a large-scale sequence stratigraphical framework within which a highresolution zonation is constructed, from which field-wide reservoir architecture is predicted. The Lower Tuba (reservoir zones TL10 to TL60) forms a highstand systems tract of predominantly shallow mid to outer ramp lithofacies associations organized into a series of progradational parasequences. These culminate at the top of the succession with high quality rudist dominated shallow mid-ramp lithofacies. The Middle Tuba (reservoir zones TM10 to TM30) comprises a transgressive systems tract that is made up of the outer ramp to offshore/basinal lithofacies associations. This reservoir interval could be considered as a potential field-wide non-reservoir succession and is likely to act as a key transmissibility barrier between hydrocarbon bearing zones at the top of the Lower Tuba. The Upper Tuba (reservoir zones TU10-TU90) comprises a basal highstand systems tract and an upper transgressive systems tract, both of which are dominated by deep mid to outer ramp, and offshore/basinal lithofacies associations. Thus, suggesting deposition of Tuba in carbonate ramp setting. Finally, these outcomes is considered in terms of sequence stratigraphy, which interprets the depositional stacking exhibited and provides a more predictive framework that will have potential for full-field application, and implications for reservoir architecture, which will assess how the sequence stratigraphy will impact on reservoir architecture and ultimately flow behaviour. Background and Scope Tuba reservoir is an upcoming reservoir of Sabiriyah Field, North Kuwait. Most of the wells drilled on the structure have penetrated Tuba but only four wells have been tested in this reservoir, out of which only one well is on production at present. The activities in Tuba so far have been confined mainly to the northern part of the field with cores, only in seven wells, falling in this part. The study of cores from seven wells was carried out by Badley-Ashton in 1999 that provided significant information about the reservoir. The data acquired from additional wells drilled were studied which when coupled with earlier findings has provided an insight and useful information for Tuba reservoir in Sabiriyah Field.