CONTROLS ON SEASPRAY GROUP SONIC VELOCITIES IN THE GIPPSLAND BASIN—A MULTI-DISCIPLINARY APPROACH TO THE CANYON SEISMIC VELOCITY PROBLEM

Abstract

Seaspray Group carbonate sediments of Oligocene to Recent age overlie the main hydrocarbon producing Upper Cretaceous to Eocene Latrobe Group in the offshore Gippsland Basin. Their sonic complexity creates major difficulties for hydrocarbon exploration. Carbonate facies are divisible into three subgroups based on seismic character, sonic logs, velocity profiles, carbonate content, petrologic character and age. The oldest unit is the Angler Subgroup that consists of carbonate pelagic marls (CaC03 <50%). The middle carbonate-rich Albacore Subgroup (CaCO3 50 to 70%) comprises bioclastic marls preserving multiple channel cut and fill features. The youngest Hapuku Subgroup includes high carbonate marls (CaC03 >70%) with interbedded clastic-rich units.

Zones of high velocity (>3,000m/s) are restricted to the deeply buried parts of the Albacore Subgroup, at TWT's greater than 0.8 seconds. The characteristics of this high velocity facies are:

a composition of fine grained bioclast-rich packstones and wackestones with less than 10% silt sized quartz;

the carbonate content exceeds 60%;

the intervals are prone to cementation and are stylolitised;

they are diachronous (i.e. cut across seismic boundaries);

velocities progressively increase with depth;

highest velocities occur where the unit is thickest towards the centre of the basin;

velocity increases laterally with steepness of angle on downlap surfaces due to coarser grain sizes and inferred greater initial porosity; and

velocities increase with stratigraphic age in the Albacore Subgroup.

Regardless of burial depth the Angler and Hapuku Subgroups contain no significantly high velocity zones.

An empirical relationship derived from this data set provides a basis for re-interpreting average velocity to the top of the Latrobe Group in areas underlying high velocity canyon-fill sediments.