GEOLOGICAL CONTROLS ON SONIC VELOCITY IN THE CENOZOIC CARBONATES OF THE NORTHERN CARNARVON BASIN, NORTH WEST SHELF, WESTERN AUSTRALIA

Abstract

The Cenozoic carbonates of the Bounty-Talisman region can be divided into five major facies. From oldest to youngest, these are: Paleocene to Eocene basinal facies, Oligocene to Miocene slope-canyon facies, Oligocene to Miocene shelf facies, Oligocene to Miocene near-shore facies, and Pliocene-Quaternary shelf facies. This represents a shallowing-upwards cycle up to the late Miocene, followed by a significant transgression and a return to more open marine conditions in the Pliocene- Quaternary. The dominant geological processes controlling sonic velocity in the Cenozoic carbonates are physical compaction, burial calcite cementation, dolomitisation, and anhydrite/gypsum cementation. In the more open marine facies of the Cenozoic carbonates, compaction and burial calcite cementation have been the dominant geological processes that have controlled sonic velocity. Large-scale carbonate content variations associated with submarine canyon-fill sediments have also produced lateral sonic velocity variations. Dolomitisation and anhydrite cementation have produced localised high velocity zones within the near-shore facies of the carbonates.