Modeling reservoir architecture with limited data
After successfully developing Cretaceous reservoirs in several northern
Kuwait fields, an operator discovered hydrocarbons in deeper, naturally
fractured carbonate reservoirs of Jurassic age. High-temperature/high-pressure
wells were drilled in six structures, producing commercial quantities of
gas-condensate and volatile oil.
At the time, however, well penetration was still limited and wireline
logs were unable to capture the full variability of carbonate facies. Also,
existing 3D seismic provided inadequate resolution of internal reservoir
architecture for two reasons. First, seismic data had been acquired to target
shallower reservoirs. Second, inter-bed multiples from overlying salt-anhydrite
layers strongly affected the seismic signature of deeper reservoirs.
Existing geological models had been built on conventional layer-cake
correlations, leaving considerable uncertainty as to the areal distribution of
depositional facies and reservoir properties. To develop these reservoirs going
forward, the operator needed a more reliable 3D geological model with a higher
level of predictability.
The operator's geoscientists and engineers worked with Schlumberger
petrotechnical experts to develop a new high-resolution static model of the
largest and most important Jurassic reservoir based on sequence stratigraphic
Building the 3D geological model
The multidisciplinary workflow began by conducting sedimentological
studies on approximately 12,000 ft of cores. Typically, a third of the
reservoir had been cored per well. In cored intervals, detailed descriptions
successfully identified shallowing upward depositional cycles and distinct
breaks in sedimentation representing sequence boundaries and maximum flooding
surfaces. In uncored intervals, well log responses were tied to core
descriptions to help identify these cycles. Integrating log and core data, the
team interpreted distinct sedimentary facies.
For each key reservoir unit, gross depositional environment (GDE) maps
outlined the areal distribution of facies such as anhydrite, dolomite, multiple
types of limestone, and shale in depositional environments ranging from tidal
flats to shelf and basin. The best reservoir facies were found in highstand
tracts representing progradation of clinoforms toward the basin.
Construction of the 3D geological model took place in three stages.
First, the sequence stratigraphic framework was built using the structural
trend of a single good seismic reflector, faults mapped from 3D seismic, well
tops, and stacked isopach maps of clinoforms. Second, 3D facies were modeled
between wells using the 2D GDE maps as guides for each stratigraphic zone
within each depositional sequence. Lastly, the 3D geological facies model was
used to distribute key reservoir properties such as porosity, permeability, and