Early Cambrian Relief Sandstone Officer Basin, South Australia: Subdivision, Diagenesis And Porosity-Permeability Distribution.
Gaughan, Christopher J
Honours Degree, 1989
University of Adelaide
The Early Cambrian Relief Sandstone has been drilled and cored at seven locations in the eastern Officer Basin, Central Australia. Core, well log, petrographic and XRD analysis have been used to subdivide the Relief Sandstone into stratigraphic units, individual depositional environments, and to determine controlling factors on known reservoir distribution and quality.
Interest in the Relief Sandstone as a potential economic oil-bearing sandstone is supported by excellent reservoir quality (up to 26.6% porosity and 4,839 md permeability). Potential source rocks are found above, below and interfingering with the Relief Sandstone. There are several occurrences of live oil bleeding from vugs and fractures in a stratigraphically higher carbonate. Traces of oil in the Relief sands, and the presence of live oil in relatively close proximity, suggests that the Relief Sandstone could host an economic oil accumulation.
The majority of the Relief Sandstone was deposited in aeolian or braided fluvial environments, with some alluvial fan sedimentation in the east, and tidal to shallow marine deposition in the west. Distribution of reservoir-quality sands is bimodal. In the east, porosity and permeability for the most part is very poor to average. In the west, porosity and pemeability is generally good to excellent. The bulk of the economic porosity is secondary, a result of dissolution of cement and matrix, with minor porosity from leaching of grains. The lower reservoir quality in the east is due to diagenesis associated with compaction and authigenic illite. Grain packing with suturing and silica overgrowths have reduced primary porosity to non-economic levels. Permeability has been reduced by the above processes and by the blocking of pore throats with authigenic illite. In the west, the porosity and permeability are high and generally due to dissolution of clay cement and primary matrix. In some cases where the clay has undergone less dissolution, it remains as grain rims and still blocks pore throats. This significantly reduces permeability although the porosity may remain high.
Future exploration in the Relief Sandstone should aim for deeper sands within, and flanking, the Wintinna Trough, in the area between the presently defined eastern and western regions.