Processes and products of footwall degradation, northern Brent Field, Northern North Sea

Abstract

Analysis of seismic, electrical log and core data from the northern part of the Brent Field enables the syn-tectonic fault scarp degradation complex which is mounted on the footwall scarp of the main block bounding fault to be characterized. The seismic geometry of this package of structurally degraded and sedimentologically reworked sediments describes a hierarchy of thickness variations that occur not only in a N–S orientation in response to changes in the underlying footwall geology, but also on a smaller, sub-km scale, where the complex is characterized by amalgamated cuspate-shaped forms. Within the complex, primary sedimentological integrity is generally preserved and displaced pre-rift formations remain stratigraphically the right-way-up. These observations support a model of translational–rotational sliding as the dominant mechanism for mass wastage from the scarp. Integrating these descriptions with biostratigraphical information from hanging wall Well 211/29–8, a nine stage model for the development and evolution of fault scarp degradation in the northern part of the Brent block is proposed. This describes the initiation of normal faulting and degradation in the Late Bajocian, followed by systematic failure of the scarp until the cessation of significant tectonic activity in the Volgian. The scarp continued to be denuded, probably by surface sediment reworking (e.g. slumps and flows), until complete burial was achieved in the Campanian. The mechanism of failure and the topography of the scarp are a function of the environment (submarine) and different relative strengths of the footwall rheologies. Although this study concludes that slide blocks have a potential to add significant volumes to the field hydrocarbon reserves, it also serves to illustrate the problems that are associated not only with accurate reservoir characterization but also in constraining the degree of compartmentalization within the complex.