Fault Seal Behavior at Beryl Field, UK North Sea: Observations from 20 Years of Production, Drilling and Injection Data

Steven Buck and Gary Robertson

The Mobil operated Beryl Field is located in UK Block 9/13, on the west flank of the South viking Graben. Hydrocarbons are produced from five Jurassic and Triassic sandstone reservoirs within several large, faulted-bounded structural traps. Within Beryl Field, extensive data, including 150 wells, and a 20 year production history provides an excellent opportunity to study fault-fluid interactions in the subsurface.

Individual fault compartments within the field exhibit a complex charge and seal history with temporal variations over both geologic and production time scales. Sealing faults play an important role in subsurface fluid distribution within the field as demonstrated by different oilwater contacts (up to 1000ft), distinct pressure cells (up to 2000psi differential) and unique geochemicaI and PVT parameters observed between adjacent fault-bounded compartments.

Both membrane (sand-on-sand) and juxtaposition (sand-to-shale) fault seals are interpreted in the field. Based upon cored fault rocks, clay smear deformation is the primary mechanism responsible for the seal potential of the membrane seal fault zones. Broad deformation zones comprised of clay smear and dense cataclastic shear bands typically surround larger faults and locally minor diagenesis is observed.

The history of hydrocarbon production and gas/water injection in the field has highlighted the temporal variability of fault seal behavior within the field. Examples of faults that sealed over geologic time but which broke-down during production are observed. Faults control fluid movement within the reservoir and degrade reservoir productivity adjacent to fault damage zones. Incorporation of fault transmissibilities and seal potentials into reservoir simulation models is necessary to evaluate reservoir and fault behavior under changing production conditions.

An understanding of how faults influence reservoir behavior through production time is critical to the success of the field development plan.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California