Impact of Depositional Environment, Sequence Stratigraphy, and Structure on Developing Zubair Reservoirs in North Kuwait

Abstract

Abstract The Lower Cretaceous (Barremian) Zubair Formation in North Kuwait represents a major clastic pulse above the Ratawi Formation. Depositional environments and the sequence stratigraphic framework play a key part in the reservoir development and production strategy with distinct depositional barriers giving rise to multiple fluid contacts. Reservoir structure and fault pattern control fluid redistribution. The Zubair Formation was deposited within a (weakly) tidally influenced deltaic system with episodes of marine influence. The sedimentary sequence consists of highly mature clastic deposits with variable and heterogeneously distributed argillaceous matter, containing negligible amounts of expandable clay minerals. The dominant sandstones range from very fine to medium-grained and are weakly to moderately overprinted by authigenic mineral precipitates. Reservoir quality is mainly controlled by the primary depositional detrital clay content, with additional control by grain size and minor quartz cementation within the cleanest deposits. A sequence stratigraphic framework adopting field-wide correctable surfaces forms the basis for the division of the Zubair layers. Lower Zubair deposition (Z10 gross reservoir unit) occurred within a tidally influenced deltaic system locally with a stronger marine influence and diminished clastic influx at the very base. Above a widespread mud-prone marine barrier, the heterogeneous middle Zubair interval (Z20–30) comprises a mixture of sand and mud-prone delta-top-or-front deposits and tidally influenced channel-fills. The main reservoir unit of the upper Zubair (Z40) comprises at least four episodes of incision and fills by sand-prone, tidally influenced channel deposits. The overlying upper Zubair (Z50–60) is largely mud-prone with only minor channel development, including channel-fill sandbodies incised into more marine-influenced deposits in the uppermost part of the Zubair. Reservoir development to a large extent depends on genetic aspects of the Zubair reservoirs. The tidally influenced upper Zubair channel-fills represent the best reservoir facies in the Raudhatain field and have been the main targets of initial development. The amalgamation of individual channels forms a number of complex, heterogeneous, and variably interconnected reservoirs. There is good aquifer support for the upper Zubair sand in such a depositional setting. The middle Zubair channel sandbodies show lesser support from the aquifer and represent a second priority for development. Shoreface and mouthbar sandstones potentially form more aerially extensive intervals of poorer quality reservoir that are locally interconnected with the channels. Such thin but laterally extensive sands are the target of current and future development of the reservoir with maximum reservoir contact wells. Complex structural aspects, filling, and up-structure oil migration have resulted in a leaking trap in the Zubair reservoir in the Sabiriyah field. Only stratigraphic traps and extensive sealing by deltaic and marine mudrocks have trapped oil in the Lower Zubair sand (Z10). Other prolific oil reservoirs in the Raudhatain field are water wet with residual oil saturation in the Sabiriyah field. The mechanism for the formation of tar plugs in the Raudhatain field has illustrated the importance of leaking faults. The Raudhatain field has been produced for the last six decades. The initial phase of depletion continued until 2000. Subsequently, peripheral water injection began into different zones of the reservoir. The injection plan is based on the reservoir geometry and sandbody continuity, pressure depletion, and the production plan. Well design and type have evolved over time with higher well diameters drilled after effective control of the lost circulation zone in the overlying Shuaiba limestones. The current development plan includes drilling horizontal wells for effective depletion of the reservoir. Production in the Sabiriyah field started in 2008, mainly from thin shoreface, mouthbar, and channel sandbodies at the Zubair base in the southern part of the field.