Purcell, Peter G.1 (1) P&R
Geological Consultants Pty Ltd,
The outcrops of the Devonian reefs in
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