LOWER CAMBRIAN RELIEF SANDSTONE, EASTERN OFFICER BASIN, SOUTH AUSTRALIA: AN EXAMPLE OF ECONDARY POROSITY DEVELOPMENT

Abstract

Interest in the Relief Sandstone as a potential economic oil-bearing sandstone is supported by excellent reservoir quality (up to 26.6 per cent porosity and 4839 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 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 permeability 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. 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.