Miocene carbonate reef complexes in the Browse Basin and the implication for drilling operations

Abstract

As Shell embarked on a new phase of exploration in permit WA–371–P a previously unrecognised drilling hazard came into focus. An exploration well intersected a high permeability zone in the Miocene Oliver Formation, which resulted in substantial losses of drilling fluids. An integrated study using newly reprocessed 3D seismic data, logs and cuttings has since shown that the well penetrated the margin of a regionally extensive carbonate reef complex. Seismic facies observed from the reprocessed data shows a remarkable array of reef morphologies, which include barrier reefs, lagoons, patch reefs and atolls. This 80 km long barrier reef system occurs directly above and parallel to the local structural hinge-lines, suggesting that a subtle inversion event may have produced a broad submerged plateau that was colonised by reef building organisms in the Miocene. These features have subsequently been drowned and buried by more than 600 m of sediment. Loss of drilling fluids whilst drilling through this section indicates that zones of high permeability are scattered throughout the reef.