The Structural And Stratigraphic Interpretation Of The Strahan Sub-Basin.
Honours Degree, 1991
University of South Australia
The Strahan Sub-Basin structure and fault system formed during the Cretaceous to Eocene by two phases of different extensional direction, within an intracratonic and passive margin depositional environment.
During the Cretaceous, SW-NE extension formed an arcuate proto sub-basin in the north of the Strahan Sub-Basin during the rifting of Australia and Antarctica. Breakup of Australia and Antarctica in the middle Cretaceous resulted in a change to a W-E extension direction, and the collapse of basement. Listric normal faulting in the Upper Cretaceous produced roll-over geometry in alluvial, fluvial and marginal marine deposits. A maximum of approximately one second (-2000 m) of sediments were derived predominantly from elevated basement fault scarps that surrounded the sub-basin to the north, east and south and a pre breakup central high to the west. Approximately 700 m of conglomerates, sandstones with minor siltstones and claystones were deposited at the Cape Sorell #1 location.
The Cretaceous, Palaeocene boundary is marked by a period of significant down-warping along reactivated faults, accompanied by marginal marine inundation. The reactivation of the sub-basin faults is associated with continued W-E extension.
The down-warping also created a graben, cross-cutting the basement high to the north and produced a saddle between the basement high and a minor depression to the south-east of the main sub-basin area. These structural lows connected the more quartz rich sediment supply of the Tasmanian hinterland with the feldspar rich sediments derived from the sub-basin margin. Deposition of approximately 1100 m of predominantly sandstone with minor siltstone and claystone occurred at the Cape Sorell #1 location.
Fault movement continued roll-over geometry in a now marginal marine depositional environment on a passive margin during the Palaeocene.
Early in the Early Eocene a global sea level fall combined with crustal uplift caused deep erosion and channelling of Eocene and Palaeocene deposits, leaving the sub-basin with an irregular and elevated topography. The Early Eocene, after the erosion period, was a time of slow and then fast subsidence with fault movement virtually ceasing at the end of this time. Approximately 1000 m of predominantly sandstone with minor siltstone and claystone were deposited at the Cape Sorell #1 location. Sedimentation rates had not kept up with subsidence during the Early Eocene, resulting in a depositional environment at the close of this period deepening to inner to middle shelf.
By the beginning of the Middle Eocene, the sub-basin had filled with sediments. Sedimentation continued within an inner to middle shelf environment with approximately 360 m of calcareous sandstones with minor siltstones, claystones and carbonates occurred at the Cape Sorell #1 location. The Late Eocene is marked by a period of non-deposition.
By the beginning of the Oligocene the depositional environment had become outer shelf. Towards the end of the Oligocene a transition of sediments from calcareous sandstones to pelagic bryozoan carbonates marks the transition of the sub-basin to fully open marine. Approximately 75 m of calcareous sandstones with minor claystone and limestone were deposited at Cape Sorell #1, followed by 40 m of carbonates during this period. In comparison with previous deposition rates in the sub-basin only a relatively thin packet (approximately 300 m) of Miocene to Recent carbonate sediments is represented at Cape Sorell #1.
Seismic sequence analysis has been applied to the Early Eocene channel in-fill. Two stages of channel in-fill occurred. Fault movement, sediment availability, closeness to fault scarps and movement on fault scarps all influence the form of seismic character found.
The lower channel sediments were deposited during a low stand in a marine environment. Turbidite sequences at the channel origin change to prograding fluvially dominated sediments along the channel trunks which become tide dominated once the channels merge.
The upper channel sediments were deposited in a fluvial environment. Fluvially dominated sequences prograded down the channel and change to tide dominated sequences at the intersection of the two channels,
The general structure of the sub-basin has been present since the Early Eocene leaving source rock maturity as the key point in deciding whether the Strahan Sub-Basin is prospective or not.
A potential field is located within the Sub-Basin, represented by a one and a half to two kilometre wide and 40 to 60 m deep direct hydrocarbon indicator. Hydrocarbon source for this field is possibly from sediments thermally matured by an intrusive. Two faults link the intrusive and the DHI reservoir.