Characterisation Of The Cretaceous And Early Tertiary Sandstone Reservoirs Of The Duntroon Basin, Offshore South Australia.
Anderson, Bruce William
Honours Degree, 1989
University of Adelaide
Restricted exploration of the Duntroon Basin so far has defined sedimentation in a rift system developing prior to the breakup of the Australian and Antarctic continents, at about 96+4 Ma (Veevers, 1988), followed by a post-rift passive margin regime developing open marine conditions in the Early Tertiary if not earlier. Drilling to date has intersected Cretaceous and Early Tertiary clastic sediments followed by Tertiary open marine carbonates.
Early Tertiary sandstones exist in all three wells drilled so far in the Basin, however only two, Platypus #1 and Duntroon #1, show suitably porous Cretaceous sands and so only these were studied.
Potential reservoirs were characterized in terms of petrography, diagenesis and sedimentology combined appropriately to define reservoir quality and the main factor controlling it, porosity. This has been carried out using cuttings and core samples, measured porosity and permeability data from core in Duntroon #1, and wireline log data, gamma ray and sonic in particular.
Sonic travel time derived porosity (PHIT) was calculated as it is least effected by the rugose conditions of both wells examined. Parameters for PHIT calculation in rift phase sediments, and the post-rift Platypus Formation and Wombat Sandstone Member of similar petrography, were derived using comparisons of PHIT calculations and core derived porosity and the petrographic and wireline log character of sediments.
PHIT calculation parameters for the overlying formations were derived from petrographic and wireline log data only, as the sediments have a very different character.
Producible porosities and sand thicknesses were encountered throughout the sandy intervals in both wells with the exception of the base of Platypus #1 which becomes marginal. Porosities are primarily with some possible minor secondary development and permeabilities appear high.
The main factors affecting porosity are compaction, which is broadly steadily increasing with depth, cementation, which is variable, and patchy in the post-rift sediments but more constant in the lower sections, and shaliness which is depositionally controlled.