--> Abstract: Western Australia - Evolution and Inter-Action; #90063 (2007)

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Western Australia - Evolution and Inter-Action

 

Purcell, Peter G.1 (1) P&R Geological Consultants Pty Ltd, Scarborough, Australia

 

The outcrops of the Devonian reefs in Australia's Canning Basin have been one of main exploration objectives in the basin over the past 50 years. That exploration has been primarily based on seismic reflection data, and interpreters have been greatly influenced by the image of the reef in outcrop. That image of the ‘reef', both as geological model in outcrop and as seismic anomaly in the subsurface, has evolved over time, and each has influenced the other.

 

In the 1950s and 1960s, the reef complex was seen as an extensive single-cycle complex, and early seismic interpretations were based on a similar subsurface model. Drilling on the Lennard Shelf adjacent to the outcrop immediately showed a relatively more complex depositional and tectonic history. Extensive mapping by industry and government geologists, notably Dr Phil Playford, in the 1970s redefined the outcrop in terms of separate Frasnian (Pillara) and Fammenian (Nullara) reef systems.

 

Geophysicists working with reprocessed and new seismic data in the late 1970s and early 1980s focussed mainly on the atoll images of the Pillara reef cycle, and applied this to the entire Devonian sequence. Blina Oilfield was discovered in 1980 but is very small, and far below the potential ascribed at that time to the ‘reef play'. Many local moundforms and other seismic anomalies were interpreted as ‘reefs' at that time. However, drilling was to show that many anomalies were not reef-related at all: some were the product of Permian fluvio-glacial erosion; others, of salt-evacuation origin; others, basement knobs. Regional control was poor and many features were not of Devonian age.

 

A closer integration of outcrop and seismic models in the mid-late 1980s led to very successful seismic mapping of Frasnian reefs. Drilling showed a ubiquitous porosity problem – but the geophysicists were not to blame for that!

 

AAPG Search and Discover Article #90063©2007 AAPG Annual Convention, Long Beach, California