ABSTRACT: Halimeda Banks of the Great Barrier Reef, Australia: Modern Analogs of a New Petroleum Play

FLOOD, P. G., Geology and Geophysics Department, University of New England, Armidale, New South Wales, Australia

The calcareous, green macroalga Halimeda is a major carbonate sediment producer within the reefless tract directly behind the shelf-edge barrier reefs of the northern Great Barrier Reef and on the top of drowned reefal shoals within the Swain Reefs Complex at the southern end of the Grat Barrier Reef. In both areas the banks have formed as a result of this in situ accumulation of enormous volumes of diagenetically susceptible aragonite representing the dead segments of the alga trapped by the baffling effect of the living Halimeda veneer. High-resolution seismic records show that although the gross structure of the mounds resemble bioherms, their internal structure is more akin to that of a biostrome. The banks attain thicknesses in excess of 20 m and individual banks may cover areas s large as 18 km x 6 km; containing upwards of 2 x 10{9} cu m sediment. The reservoir potential of such banks is enormous; post-burial diagenetic alteration of the aragonitic skeletal material could produce mouldic porosities in excess of 50%. In addition, the banks in the northern region are situated within the path of the prograding terrigenous clastic wedge which will eventually envelope the carbonate mound. All the ingredients exist for a potential reservoir, namely, reservoir rock, cap and trap. These Halimeda mounds provide modern analogues of Phanerozoic phylloid algae bioherms that occur at the shelf break and that may be important hydrocarbon reservoirs. Elsewhere, modern Halimeda bioherms up to 50 m thick, have been reported from K-Bank 50 km east of the Sunda Shelf margin in t e eastern Java Sea. The inference is that this type of petroleum play warrants careful consideration when examining the Mesozoic and Tertiary basins of southeast Asia.

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