Isabel Edmundson

Elucidating key controls on column heights across the Snøhvit gas field, Hammerfest Basin, SW Barents Sea

Izzy Edmundson, Atle Rotevatn, Roy Davies, Kjetil Broberg and Graham Yielding

Faults can act as pathways or barriers to fluid flow and so profoundly influence subsurface fluid migration, accumulation, retention and leakage. Assessment of the retention/seal risk of structural hydrocarbon traps depends upon robust knowledge of the capacity of faults to seal or leak fluids. Faults commonly exhibit spatial variability in sealing properties, and can alternate between sealing and leaking through geological time as the fault undergoes dynamic changes in deformation, diagenesis and applied regional or local stresses. However, typical methods that attempt to predict hydrocarbon column heights based on the mechanical sealing properties of the rock, often neglect the complexity of factors that affect fluid flow at faults and fault intersections. Such estimations predict significantly larger column heights than those measured in 18 underfilled discovery boreholes across the Hammerfest Basin.

To investigate this discrepancy between estimated and discovered column heights, fault-seal analysis was performed on five structures within the Snøhvit gas field. Results indicate that all modelled faults are capable of sealing significantly taller hydrocarbon columns than currently in-place, a result that is corroborated by hydrocarbon shows present below the fluid contact. This suggests that across-fault capillary leakage is unlikely to explain the underfilling. Current work now attempts to assess how the roles of fault intersection and reactivation can cause top-seal breach, leading to enhanced permeability and fluid flow. This may not only help to shed some light on why some traps in the Snøhvit gas field are more underfilled than others, but also lead to better estimations of lateral seal risk and hydrocarbon column heights for future prospect analyses.