ABSTRACT: The Application of Middle Shelf Reefing Concepts to the Re-Interpretation of Canadian and Australian Devonian Subsurface Reefal Facies

JORDAN, CLIFTON F., Mobil Research and Development Corporation, Dallas Research Laboratory, Dallas, TX, and IAN A. RUSSELL, Mobil Exploration and Producing Australia Pty Ltd, Melbourne, Victoria, Australia

The use of Holocene reef systems as lithofacies analogs is an important tool in exploring for oil and gas associated with Devonian reef deposits in western Canada, as well as the basins of Australia's North West Shelf. Devonian reefs in Canada's Swan Hills district were long considered to be either barrier reefs or atolls. This concept meant that reef facies of stromatoporoid boundstone and backreef facies of Amphipora floatstone to mudstone would occur only along outer-shelf facies belts that rimmed offshore atolls on all sides or that occurred in a narrow curvilinear belt along the outer-edge of a continental margin. A re-evaluation of reef trends in the Kaybob South gas-condensate field, based on detailed seismic and core study, indicates that (1) the field actually extends complet ly across a large reef complex, rather than being restricted to its margins; (2) zones of well developed reef facies occur within what had previously been mapped as a mud-rich center of the reef complex; (3) the complex formed as a system of patch reefs trending parallel to each other with distinct inter-reef channels between them; and (4) reef facies were developed along channel margins within the larger reef complex itself. This complex lithofacies pattern was further supported by examples of middle-shelf patch reefs from the modern, adding credibility to the subsurface mapping of a number of highly prospective trends which resulted in some recent discoveries in an area thought to have been explored.

These facies concepts are now being applied to the basins of northwest Australia. The Canning basin, for example, has long been known for its well developed Devonian (Frasnian) stromatoporoid reef systems. The main exploration problem has been the inability to predict porosity trends, with further complications resulting from the existence of algal-rich Devonian (Fammenian) buildups. A reinterpretation of facies orientation combined with seismic and further analogies to Holocene middle-shelf systems may highlight the narrow porous bands predicted by the middle-shelf model.

AAPG Search and Discovery Article #91015©1992 AAPG International Conference, Sydney, N.S.W., Australia, August 2-5, 1992 (2009)