MAPPING DIAGENETIC FLUID-FLOW WITHIN A RESERVOIR - K-AR DATING IN THE ALWYN AREA (UK NORTH-SEA)

The Alwyn area demonstrates that variation in the timing of diagenesis on the field scale depends on the structure and the proximity to the sources of diagenetic fluids. Variation in reservoir quality within the structure is the result of facies control on fluid migration. Illite in Alwyn South formed in the Middle Palaeocene to Middle Oligocene between 60 and 38 Ma. At this time the Brent Group was at depths of between 2400 and 3000 m and reservoir temperatures ranged from 110 to 120-degrees-C. Extensive K-Ar dating shows that illitization, pre-oil emplacement, proceeded from the deepest reservoir panels along and up-structure. Fluid migration may have occurred along the deep seated Alwyn South-Ninian Fault System. Where the reservoir is lithologically homogeneous illite precipitation proceeded systematically base-upwards at rates of 5.5 m Ma-1. The duration of illite formation at any one level was between 2 and 8 Ma. In contrast, in heterolithic reservoirs fluid migration occurred initially along high permeability fairways and consequently illite ages do not vary systematically with depth.