ABSTRACT This chapter demonstrates a workflow to characterize the impact of heterogeneity in paralic deposits from full field to core scale that can be used for meeting the challenges of managing reservoir performance complexity in a supergiant field. The Upper Burgan member of the Cretaceous Burgan Formation was deposited in a tide-dominated coastline, which laterally passes into transgressive shoreface to offshore deposits. Variations in depositional subenvironments in time and space generated heterogeneity ranging in scale from millimeters to several meters (tens of feet) in the reservoir, which impact the permeability architecture of the reservoir. Construction of a sequence stratigraphic framework allowed understanding of the reservoir geometry and the extent of large-scale shale barriers. On the basis of variability in reservoir connectivity, the entire field was separated into three broad areas. A test case was run in an area of moderate reservoir connectivity to capture interwell heterogeneities. In the basal reservoir unit, a conceptual depositional model was developed through integration of regional geology, core-based sedimentology, and log motifs. The three-dimensional (3-D) depofacies volumes generated with the help of this model ultimately constrained the facies model. The defined object-based modeling (DOBM) method provided flexibility in modeling the mixture of facies that was used as a constraint to the petrophysical property models. Finally, micromodels for each reservoir facies were developed representing millimeter-scale heterogeneities. The facies classified from core interpretation and borehole images were incorporated to model heterogeneity at a high resolution. Streamline simulations were run to compute permeability anisotropy ratios in each facies. The results provided insight into the nature of heterogeneity within a framework of reservoir connectivity. They also provided high-quality input to larger simulation models and greatly influenced reservoir management. The methodology is applicable to other areas of varying connectivity.

ABSTRACT During the Mesozoic Era, episodes of siliciclastic input onto the dominantly carbonate Arabian shelf form important elements of petroleum plays, forming proven and potential reservoirs, source rocks, and seals. This chapter examines the temporal and spatial extent of these siliciclastic episodes. It then compares them against known tectonic, climatic, and eustatic events affecting the Arabian plate that may have been acting independently or coincidently to control siliciclastic input by means of hinterland uplift, influence on denudation and run off, incision, and creation of sediment pathways and accommodation space. Particularly important phases of siliciclastic input occur in (1) the Early Triassic (Olenekian Sudair shale) coincident with major eustatic lowering, an episode of humid climate and rifting on the northern part of the Arabian plate; (2) Late Triassic (late Norian initial Minjur Sandstone) coincident with East Mediterranean rifting, a humid episode and a major eustatic sea-level fall; (3) Middle Jurassic (early Bajocian initial Dhruma Sandstone) coincident with localized uplift and a humid climate and immediately postdating a eustatic sea-level fall in the Aalenian; (4) Early Cretaceous (late Valanginian–Barremian Zubair sandstone) postdating a Valanginian eustatic lowering and coincident with humid climate and uplift in northern and western Arabia; (5) Mid-Cretaceous (latest Aptian–middle Albian Burgan Sandstone) coincident with Arabian shield uplift, humid climate, and a eustatic low. Other episodes of siliciclastic input also occur, although they tend to be more localized. Important seals are formed during the progradation of siliciclastic systems “poisoning” carbonate shelves or during transgression when distal pro-delta siliciclastic systems retreat back across the shelf, capping up-systems tract fluvial or shelfal sandstones, or when they are located above major unconformities, capping carbonate reservoirs. Siliciclastic reservoirs include the well-known and prolific fluvial and paralic sandstones that contribute, for example, to the Burgan field in Kuwait and to the Zubair and Nahr Umr reservoirs of the northern Gulf. Lowstand sands (both lowstand deltas and slope and basin gravity flow deposits) form viable, but underexplored, reservoir targets. Source rocks may be deposited in front of prograding delta systems linked to high nutrient supply and water stratification caused by freshwater overhang, leading to anoxia and preservation of organic matter. A well-known example is the Kazhdumi Formation of the Iranian Zagros. A better understanding of the fundamental controls on siliciclastic input onto the Arabian plate will enable better predictions of these key petroleum play elements and a better understanding of the subsurface risk associated with their occurrence.

ABSTRACT The Jal Az-Zor Formation sediments in north Kuwait were deposited on the southwestern margin of the Zagros foreland basin. They are organized into 30–60 ft (9–18 m)–thick repeated fining upward high-frequency sequences. The sequences generally consist of conglomerate and pebbly sandstone, coarse- to fine-grained sandstone, muddy sandstone, siltstone, and mudstone lithofacies, ideally stacked in ascending order. In upper part of the sequences, burrowing and fossiliferous horizons are developed, which reflect marine influence during their deposition. Each sequence is interpreted to have been developed by infilling of an incised valley during base-level rise. Such valleys had been cut down into the underlying strata during base-level drop. The formation was sourced from west–southwest from the Arabian Massif. From the biostratigraphic analysis, late Burdigalian–early Langhian age is assigned to these sediments in north Kuwait. The uppermost mudstone interval (Cap Shale) contains Miogypsina remains considered early Langhian in age. Based on this, we suggest possible correlation of this zone to the Arabian Plate MFS Ng30. Considering the age limits for the studied section, there is another possibility of MFS Ng20 within the basal mudstone horizon.

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 correlatable 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.