A Study Of The Patchawarra Formation, Tirrawarra Field, Southern Cooper Basin, South Australia
Degree of Master of Science, 1988
University of Adelaide
In the Permian Patchawarra Formation of the Tirrawarra Field sedimentary patterns exposed in core and interpreted from geophysical logs are recognised by the writer and previous workers to have modern analogues. Thus, for example, point bar sequences and lag gravels are identified. It is known that the modern deposition rates of such structures are quite rapid. However less than 1200 feet of Patchawarra Formation sub-crop in the Tirrawarra Field and this was deposited over some 10 million years. Therefore considerable reworking of material must have occurred, particularly in view of the low gradient of the land surface at the time. Reworking of modern sediments is difficult to recognise in the field and so is not often considered in the interpretation of ancient sedimentary section.
It is envisaged that meandering streams entering from the north, crossed and re-crossed the Tirrawarra structure, depositing, eroding, transporting and then redepositing the sediments. Stacked point bar deposits occur on the high spots of the Tirrawarra anticline where structural growth (a slower rate of subsidence ) has taken place. Sand percentages are found, in most cases, to be influenced by structure but elsewhere may reflect shore line patterns and preferential stream direction. In the latter two cases there are potential exploration targets outside the field area. High porosities occur in some low sand percentage areas, and this too is of economic interest. Future work on the field might involve porosity investigation in the lower parts of the Patchawarra Formation.