Buffalo Oil Field ? Geophysical Success Under Big Bank

Abstract

The Buffalo Oil Field is located in Production Licences WA-19-L and WA-21-L in the Bonaparte Basin, approximately 300 km off the coast of northern Western Australia. The discovery well, Buffalo-1, was drilled in September 1996 and encountered a 45 metre oil column within sandstone reservoirs of the Callovian Elang Formation. The Top Elang boundary is typically very difficult to interpret on seismic data. Modelling has shown this boundary to have a Class II AVO response, i.e. where apolarity reversal of the seismic event occurs across the CMP gather, resulting in attenuation of the event during stacking. In addition to this fundamental geophysical problem, the Buffalo field underlies a seafloor carbonate bank (``Big Bank'), which rises from the seafloor, 300 m below sealevel, up to a depth of 27 m below sea level. Interpretation of the underlying geology on conventionally processed seismic data is made very difficult by severe ray-path distortion from the steep sides of this bank and the large velocity contrast with the surrounding water. At top reservoir level the very poor seismic data quality is a consequence of poor signal penetration, poor reflectivity, faulting, multiples and severe imaging problems, which also give the reservoir section the appearance of being highly faulted. A 3D seismic survey was acquired in 1996 over the field, and a vast improvement in data quality and reliability was obtained through advanced processing techniques incorporating Wave Equation Datumming (WED). The remaining depth-structure uncertainty was quantified by a geological-model-based interpretation and mapping process to provide a range of realistic possible structural outcomes. A key assumption - that velocity changes across the field would be a relatively minor contributor to the depth uncertainty - was confirmed by development drilling results, which demonstrated the success of the WED method in removing most of the undesirable effectsof the carbonate bank. This paper focuses on the WED processing and interpretation process which reduced the uncertainty of oil-in-place calculations for Buffalo to an acceptable level and allowed development of the field to proceed.