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Time-lapse dedicated 3D seismic surveys were acquired across the Pyrenees oil and gas field, Exmouth Sub-basin to map production-induced changes in the reservoir. Rock-physics 4D modelling showed that changes in pore pressure and fluid saturation would produce a time-lapse seismic response of sufficient magnitude, in both amplitude and velocity, to overcome time-lapse noise. The dominant observed effect is associated with gas coming out of solution. The reservoir simulation model forecasted that reservoir depletion would cause gas breakout that would impact the elastic properties of the reservoir. The effect of gas breakout can be clearly observed on the 4D seismic data as a change in both amplitude and velocity. The analysis of the seismic datasets was proven to be enhanced significantly by using inversion methodologies. These included a band-limited extended-elastic impedance (EEI) approach, as well as simultaneous 4D elastic inversion. These datasets, combined with rock physics modelling, enabled quantitative interpretation of the change in 4D seismic response which was a key tool for assisting with the infill well placement and field development strategy.

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The Blackback structure is a large, top-Latrobe, eroded and faulted anticline within the Gippsland Basin of south east Australia. Despite its large areal extent, it is a commercially marginal field due to reserves uncertainty, deep water and the practical difficulties of development. Proximity to existing infrastructure helps to make this marginal field commercial. Five wells have been drilled on the Blackback feature, and have intersected reservoirs with greatly varied geological and geophysical properties. The resulting uncertainty over the ultimate recovery remains, despite the five wells drilled to date. This paper describes the geological and development risk mitigation undertaken by a non-operating Joint Venturer to reduce the technical and commercial uncertainty pertaining to a second phase of development. This includes the development of a predictive depositional model targeted at the upside potential and maximum use of existing infrastructure. P. 209