Chalk reservoirs in the Eldfisk field comprise autochthonous chalks of the Hod Formation, overlain by mixed allochthonous and autochthonous chalks of the Tor and Ekofisk Formations. Core material contains previously undescribed fabrics which are of importance in determining the sedimentological and diagenetic history of North Sea chalk reservoirs.
Resedimentation within the Tor and Ekofisk Formations was dominated by slumping. Evidence of deposition from liquefied flows is scant. Sediment-filled fractures indicate early overpressuring of the sequence in response to burial beneath Tertiary mudrocks.
Within the hydrocarbon zone, the Hod and Ekofisk Formations contain small-scale (about 7.5 cm) rhythmic alternations of oil-stained and unstained layers, related to variation in sedimentation rate and/or early diagenesis. Below the hydrocarbon zone, and in impermeable intervals within the hydrocarbon zone, small-scale alternations are replaced by decimeter-scale interbedding of clay-rich and clay-poor chalks more typical of autochthonous chalk facies and a product of more advanced chemical compaction.
These fabrics suggest that porosity reduction is closely related to lower hydrocarbon saturation, in turn largely a function of clay content, early overpressuring, and structural position within the field.