BPC manager Mike Middleton will discuss the characteristics of this part of the offshore central Perth Basin.
Good oil and gas shows have been found in the Vlaming Sub-basin and most exploration has been directed towards various structural features associated with the Breakup Unconformity feature.
The Vlaming is characterised by a thick development of early Cretaceous-aged sediments, which is thin where it occurs elsewhere in the Perth Basin.
“Although few wells have penetrated deep than the Upper Jurassic sediments in the Vlaming Sub-basin, it is believed that the depositional and structural history prior to the Cretaceous was similar to that of the remainder of the Perth Basin,” Middleton said.
During the latter stages of the deposition of the Yarragadee Formation (Tithonian), a large half graben began to develop with a major listric-normal fault on the western margin of the sub-basin and a hinge-axis in the vicinity of the Badaminna Fault system, which is nearly coincident with the present day coastline.
The Parmelia Group sandstones and shales were deposited in this half graben during the Berriasian stage, with thickness in the western part of the depocentre being in excess of 3 seconds two-way travel time (TWT) or roughly 5000m.
At the beginning of the Valanginian Age, an almost convulsive period of tectonism occurred.
This tectonism formed an unconformity surface (commonly termed the Breakup Unconformity), which is characterised by systems of north-south trending extensional graben, and an apparent left-lateral strike-slip “transfer” fault trending northwest to southeast across the middle of the sub-basin.
This tectonism developed associated topographic relief of up to 800m in places.
“It is conjectured that this tectonism was accompanied by uplift, as the initial deposition (Gage Sandstone) after the extension and development of topographic relief was only in the base of the dominantly north-south trending valleys cut into the unconformity surface,” Middleton said.
This basal sandstone was gradually covered by the South Perth Shale, during the Valanginian to Barremian stages.
At about this time, flows of Bunbury Basalt were also erupted along east-west valleys, which may also be associated with the unconformity surface.
The age of the Bunbury Basalt is dated radiometrically between 123 and 130 million years (Ma), and the Breakup Unconformity (Berriasian-Valanginian boundary) is variously dated as 135Ma or 140Ma, depending on which timescale is used.
After the filling of the valleys, the Vlaming Sub-basin subsided in a sag-like fashion throughout the remaining Cretaceous and Tertiary periods.
Middleton graduated from the University of Sydney with a BSc (Hons) in 1973 and PhD in 1979.
He has worked for Amoco, CSIRO, ECL, the Geological Survey of Western Australia, the Nordic Energy Research Program, Curtin University and also as a private consultant. He currently works for BPC and manages the company’s Perth Basin permits.
PESA’s luncheon meeting will start at 12pm and end at 2pm.
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For non-members, the fee of $75 applies.
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