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Depositional Environment And Diagenetic Controls On Porosity And Permeability Of Early Cambrian Basal Siliciclastics In The Georgina Basin.

Eyre, Bradley

Honours Degree, 1989

University of Adelaide


Pacific Oil and Gas Pty. Ltd. have recently defined Early Cambrian basal siliciclastics as potential reservoir rocks within their leases during ongoing petroleum exploration in the Southern Georgina Basin.

The siliciclastics were deposited in a series of alluvial fan-deltas, as sediment was shed off local areas of relief. Potential reservoir rock properties vary significantly among the rocks studied. Permeabilities are consistently low with a few local exceptions, while porosities are quite variable, reflected by the type of porosity developed. Porosities and permeabilities are controlled primarily by the degree of framework and cement dissolution that has occurred. Although this can produce excellent moldic, intergranular, and intercrystalline porosity it produces very little interconnection and hence low permeabilities.

Samples have predominantly open framework textures, with quartz cementation and minor grain rotation resulting in tighter packing being the major primary porosity reducing processes. The degree of framework and cement dissolution that occurs is a manifestation of the diagenetic stages to which a sample has been subjected, within a particular diagenetic sequence. The path which a sample follows within a diagenetic sequence, is dependant upon its original porosity and permeability and hence its original texture, composition and depositional environment.

Locally, exceptionally high porosities and permeabilities occur as the result of particular diagenetic stages. These include (1) early migration of hydrocarbons, (2) high microporosity from the partial dissolution of framework components and (3) enhancement of porosity and permeability by a second dissolution phase. Rocks that have been subjected to the above diagenetic stages are the most favourable potential reservoirs.

Outcrop samples have been subjected to various telodiagenetic alterations, with climate the most important controlling factor. With careful recognition of these, an assessment of potential reservoir characteristics from outcrop samples could be carried out.

Australian School of Petroleum



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