Controls on reservoir quality and fracture distribution in the Old Red Sandstone of the Buchan field, UK central North Sea

The Buchan field, located in blocks 20/05a and 21/01a of the UK Continental Shelf, produced 148 MMstb of oil between 1981 and 2017 before production ceased for facility integrity reasons. A redevelopment is being considered and requires renewed reservoir models to estimate remaining potential. Reliable predictive modelling has historically been hindered by the complex behaviour of the Devono-Carboniferous Upper Old Red Sandstone reservoir, where low-permeability matrix sandstone is variably enhanced by natural fractures and localised high-permeability matrix streaks. To quantify modelling uncertainties, an integrated subsurface reassessment was undertaken combining a structural review, focused seismic reprocessing, field-wide petrography and a fresh interpretation of historical production behaviour. The work shows that matrix reservoir quality is controlled strongly by local variations in diagenetic processes as well as depositional facies, while fracture distribution is linked to structural position, faulting and geomechanical contrasts between reservoir zones. Reprocessed seismic data improved imaging of intra-Devonian faulting and internal geometry, leading to a revised structural framework for the field. Integration of these results with production data has helped explain marked variations in well performance and unique dynamic behaviour of the different stratigraphic zones. The study demonstrates that revisiting subsurface fundamentals in this mature field has materially improved predictions of structure, fracture distribution and matrix quality, providing a more robust basis for reservoir modelling and redevelopment decisions.