The influence of variations in illite morphology on the permeability of Middle Jurassic Brent Group sandstones, Cormorant Field, UK North Sea

Abstract

SEM examination of core samples from Brent Group sandstones in the Cormorant Field reveals that critical point drying preserves delicate illite morphologies that are damaged during air drying. This damage may lead to an increase in permeability, as defined by the ‘drying effect’ (air permeability air dried/air permeability critical point dried). The Cormorant reservoir sandstones contain four illite morphologies. Early diagenetic pore-lining illite formed before hydrocarbon emplacement and occurs ubiquitously. Pore-bridging illite, ‘blooming illite’ and illite that replaces kaolinite all formed later and occur preferentially at or below the oil-water contact. Damage to these late illites causes an average drying effect of 3.5 and explains why measured permeabilities predict higher water injection rates than occur in the field. Comparison with data from other reservoirs reveals that the drying effect is influenced by variations in illite particle dimensions as well as illite morphology. TEM shadowing techniques reveal that illite particle sizes describe a trend from small particles in reservoirs with large drying effects to large particles in reservoirs with small drying effects. These size variations probably reflect differing illite growth rates. Permeability in illite-bearing sandstones thus reflects not only the amount of illite present but also the size of the individual particles and their morphology.