Burrowed matrix powering dual porosity systems – A case study from the Maastrichtian chalk of the Gullfaks Field, Norwegian North Sea

Chalk reservoirs are commonly modelled as dual-porosity systems, in which a very porous but low permeable matrix is intersected by highly permeable vugs and fractures from which oil and gas can be produced. The Gullfaks Field in the Norwegian North Sea contains such a reservoir, which is producing from Maastrichtian chalk in addition to the conventional Triassic and Jurassic siliciclastic reservoirs. However, in comparison to the prolific chalk fields in the southern North Sea (e.g. Ekofisk, Valhall), chalk reservoirs in the northern part (e.g. Oseberg and Gullfaks fields) experience challenged production due to reservoir presence and quality related to depositional facies and structural conditions. Analyses of cores from three wells in the Maastrichtian Shetland Group of the Gullfaks Field reveal that this interval is completely bioturbated during several stages, e.g. mottling with diffuse bioturbated texture in an early softground stage that became subsequently overprinted by more discrete burrows with active and passive fill and different rock properties during the stiffground and firmground stages of the ooze. A rich and moderately diverse trace-fossil assemblage consists of abundant Zoophycos, common Chondrites, Taenidium, Thalassinoides and Virgaichnus, and rare Nereites, Planolites, Spirophyton and Teichichnus. Ichnological features allow the differentiation of five recurrent ichnofabrics (Thalassinoides, Zoophycos, Chondrites, Nereites and Zoophycos-Taenidium ichnofabrics) with variable influence on porosity and permeability. The Thalassinoides ichnofabric in chalk has the highest impact on improving reservoir quality, whereas Zoophycos and partly Chondrites ichnofabrics, in marly chalk and chalky marlstone respectively, contribute to creating potential reservoir zones if burrow density is high enough. Thin-section analysis of the different ichnofabrics illustrates the negative or positive effect of burrows on porosity distribution, whereas micro-CT imaging reveals an intriguing system of partly open micro-burrows (e.g. Virgaichnus) within the matrix, which serves as source for porosity. This burrow porosity provides a connection in the matrix that hosts open vugs and fractures, thus improving oil production.