Petroleum generation and migration modelling for the Capel and Faust basins, eastern offshore Australia - GNS Science Report 2011-22

The Capel and Faust basins, located on the Lord Howe Rise in water depths between 1,300 m and 2,500 m, were the focus of a series of marine surveys by Geoscience Australia in 2006 and 2007. Their interpretation of high-fold 2D seismic reflection, gravity and magnetic, multi-beam bathymetry, sonobuoy refraction, heat flow and geological sample data suggested the basins have petroleum potential. Analysis of petroleum generation and migration, based on structural maps, lithological and other data supplied by Geoscience Australia, is the focus of this study. Basin models predict that most of the deeper depocentres in the Capel and Faust basins, mapped as containing Jurassic-aged pre-rift and Cretaceous-aged syn-rift source rocks, have the potential to expel oil and gas, and charge nearby syn-rift and post-rift reservoir formations from Cretaceous time to the present day. Multi-1D thermal and petroleum generation models predict: - Pre-rift (215 - 165 Ma) and Syn-rift 1 (130 - 100 Ma) megasequences within the deeper depocentres are within the oil or gas generation window; - Based on the expected presence of petroleum-generative coaly source rocks, total oil and gas expulsion from the major depocentres exceeds 5 MMbbl/km2 and 25 Bcf/km2 respectively from the Pre-rift source rocks, and 20 MMbbl/km2, and 300 Bcf/km2 respectively from the Syn-rift 1 source rocks. In terms of timing, 80% of total hydrocarbon expulsion is predicted by the end of the Eocene, with maximum expulsion taking place between the Late Cretaceous and the Late Eocene (c. 68-36 Ma); - A significant increase in paleo-water depth in late Cenozoic time has supressed further heating related to post-Eocene burial. However, modelling predicts post-Eocene expulsion of oil and gas may have been partly enhanced by post-rift magmatism. In this study total expelled oil and gas volumes are 'migrated across' mapped horizons to assess charge of and fill-spill relationships between structural traps. This map-based charge modelling assumes certain reservoir properties with no migration losses and predicts that: - Accumulations within potential reservoir facies, such as deltaic, shoreline and turbidite sandstones of the lower Post-rift unit (70 - 68 Ma) are dominantly gas with volumes generally about 5 to 9 Tcf at burial depths of 400 - 700 m; - Accumulations within similar sandstones of the upper Syn-rift 2 unit are mixed oil and gas (about 2 to 3 billion bbl oil and 10 Tcf gas) at burial depths of 400 - 800 m; - Similar accumulations are present in the lower Syn-rift 2 and Syn-rift 1 fluvial sands; - Most of the mapped structural traps are buried to relatively shallow depths and seal effectiveness for containment must therefore be a significant risk. Deeper structures and stratigraphic plays may further contribute to the petroleum potential in the basins. The model presented here illustrates the potential for petroleum charge of structural traps in the Capel and Faust basins and highlights the risks associated with source rock distribution and type, reservoir distribution and quality, and seal effectiveness. Volumetric and charge assessments could be further refined using higher density seismic data and appropriate rock property data for reservoir and seal rocks in combination with 3D modelling.

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