oa Defining the subtle trap – a Bass Basin case history

The Trefoil prospect is a low relief structural closure in the Bass Basin defined by a vintage 2D seismic grid. For such features, geological uncertainty can be reduced when a number of geophysical tools applied to a seismic data set all independently support a single model of the subsurface. This is particularly relevant for 2D data where the benefit of continuous structural and amplitude coverage of 3D data is not available. Geophysical techniques used to mature this prospect for drilling included pre-stack depth migration (PSDM), horizon-based stacking velocity analysis (HSVA), amplitude versus offset (AVO) analysis/modelling, and frequency attenuation mapping.

The application of these methods improved the understanding of the structure compared with previous work based on the same vintage 2D seismic dataset. These analyses consistently support the geological model that Trefoil is a low relief, four-way dip closed anticline, containing several gas columns that may be filled to spill. It is shown that AVO anomalies at prospective levels closely match the final depth closure, as does a frequency attenuation anomaly. The convergence of this information from different methods has reduced the perceived risk to the point where the prospect is viewed as economically viable and is planned for drilling in 3Q 2004.