MICROFACIES AND DIAGENETIC CONTROLS OF POROSITY IN CRETACEOUS TERTIARY CHALKS, ELDFISK FIELD, NORWEGIAN NORTH-SEA

The pattern of porosity preservation in North Sea chalk hydrocarbon reservoirs is a function of the composition of the chalk, overpressuring history, and the timing of hydrocarbon migration. Porosity in chalks with low acid insoluble residue concentrations was reduced in Eldfisk field from soft chalk values of about 45% to values in the 25-30% range mainly by consolidation (mechanical compaction). Further reductions in porosity occurred primarily by stylolitization and cementation. The present-day effective stress-vs.-depth gradient in Eldfisk field is too small to account for the overall decrease in porosity with depth. The porosity-vs.-depth profile of Eldfisk field must have largely developed during earlier diagenesis under a different effective stress-vs.-depth regime. Porosity layering was caused by differential consolidation in response to microfacies correlated variations in the insoluble residue (mainly authigenic quartz) concentration of the chalk, rather than differences in mode of deposition. Chalk intervals consisting of microfacies with low biogenic silica (radiolarian) concentrations, such as the Tor Formation, have low authigenic quartz concentrations (< 5%) and thus tend to be zones of comparatively high porosity and matrix permeability. Pelagic chalks with high insoluble residue concentrations (> 10%) can retain porosities of 40% or more if they have a structurally high position.