Application of Fault Plane Attribute Mapping in the Gambier Embayment of the Onshore Otway Basin, South Australia

Boult, Peter J.¹, Sandy Menpes², Gordon Wakelin-king² (1) PIRSA, Adelaide, Australia (2) Essential Petroleum, Victoria, Australia

The rift-sag-rift Otway Basin is one of the best known and most actively explored of a series of Mesozoic rift basins that span the southern coastline of Australia. Its structural evolution is analogous to basins such as the Tucano–Reconcauo Basin of Brazil and the Gabon and Cabinda Basins of the west coast of Africa.

3D interpretation of 2D seismic and display of fault plane attributes in the Gambier Embayment reduces exploration risk in an area with significant, but as yet undeveloped, petroleum potential. Wet gas seeps, asphaltite strandings and oil shows are evidence of a working petroleum system with marine, type II source rocks. Non-commercial oil, along with commercial CO2 is produced from the base of the Late Cretaceous in Caroline-1 and “oil sands” were described at the base of the Tertiary in SAOW-2 in 1926. Juxtaposition of marine shales against fluvial/marginal marine sands in fault bounded, 3-way dip closures and horsts are the main trapping mechanisms and there is good potential for shale gouge seal development.

Karst topography significantly degrades the seismic signal in many places, but fault plane modelling and displacement mapping reduces interpretation risk by providing confidence in fault correlation and a robust understanding of cross-fault geometries. Three sets of faults are distinguishable, being early NW–SE basement related faults, later NNW–SSE cover faults, which show displacement highs at the K/T boundary, and neotectonic ENE–WSW thrust faults.

Interpretation demonstrates shale beds of at least 220m and 240m thick are needed to provide robust juxtaposition seals for the Harris Flat lead at the top Flaxman and Waarre formations respectively. Nearby well intersections indicate that shale beds will have sufficient thickness.