Reservoir Heterogeneities, in Fractured Fluvial Reservoirs of the Buchan Oilfield (Central North Sea)

Abstract

The Buchan Oilfield in the central North Sea is a structurally complex, pervasively fractured Upper Devonian-Carboniferous reservoir comprising vertically stacked, sandstone-dominated, fining-upward sequences deposited predominantly by braided streams. Hierarchical analysis of reservoir quality at the microscale (thin sections), mesoscale (lithofacies and facies sequences) and megascale (zones composed of more than one mesoscale sequence) levels shows that the reservoir can be divided into six megascale units based on their sedimentological properties, poroperm values and electric log response. The microscale and mesoscale properties of these units, particularly the presence of fractures and variations in the correlation coefficient between the logarithm of permeability and porosity, provide a means of defining effective and non-effective reservoir zones, which correspond with, or occur within the units. The most effective zone, between 2738 and 2788 m, consists predominantly of extensively fractured subarkoses which differ from other sandstones in the reservoir in that they contain more preserved primary intergranular porosity and secondary fracture porosity, with porosity values up to 30. 2%, and permeabilities up to 1475 mD. This zone extends across most of the field where it defines, more precisely than has previously been possible, the best quality and most productive part of the reservoir section.