Unconventional Middle Cambrian petroleum systems in the Georgina Basin

Geoscience Australia has begun a systematic evaluation of the shale gas/oil (unconventional) resource potential of Australia's onshore sedimentary basins. According to the Australian Gas Resource Assessment 2012 [1] Australia's unconventional gas resource endowment is likely to be greater than its estimated total conventional gas resources with some basins likely to have significant unconventional oil potential. An assessment of Australia's unconventional resource potential will use methodology developed by the United States Geological Survey based on statistically derived estimates of hydrocarbon recovery from actual production data, or basin analogues in data-poor areas. The Georgina Basin, containing Proterozoic-Paleozoic age sediments and covering an area of ~325,000 sq. km in south-central Australia, is the first basin to be assessed and since there is no petroleum production history, suitable analogues will be sought. The assessment also relies heavily on the updated stratigraphy, tectonic history, petrography, geochemistry and petroleum systems modelling, with a discussion emphasis on the latter two datasets.

The Georgina Basin is host to basin-wide oil staining and contains proven petroleum systems with relative short migration distances from source to trap, which likely represent multiple hybrid unconventional systems and breached conventional reservoirs. For example, the result of localised migration is exemplified in the composition of residual free hydrocarbons from organic-rich mudstones in which light and heavy hydrocarbons were recorded in samples 3 m apart. The most prolific oil-prone effective sources occur in the Middle Cambrian Thorntonia Limestone (early to middle Ordian) and overlying Arthur Creek Formation (latest Ordian to late Boomerangian). These source rocks were diachronously deposited from west to east under marine anoxic bottom waters, which periodically extended into the photic zone, and represent the local expression of a prolonged Middle-Late Cambrian oceanic anoxic event that lead to deposition of organic-rich 'black shales' on a global scale. The oil stains are varyingly altered by biodegradation and are geochemically characterised by a strong isotopic depletion in 13C, high abundance of monomethylalkanes, C15-C23 odd carbon number predominance for n-alkylcyclohexanes, C27 slightly dominant over C29 desmethylsteranes and high content of tricyclic terpanes. Source richness and maturity estimates are derived from Rock Eval, saturated and aromatic hydrocarbons, FAMM and hydrogen isotopic relationships between n-alkanes and isoprenoids. For example, the 'hot shale' unit comprising predominately dolostone at the base of the Arthur Creek Formation, currently the focus of drilling activity for unconventional hydrocarbons, has TOC and HI up to 15.5 % and 500 mg hydrocarbons/g TOC, respectively. Maturity levels range from the early oil to early dry gas windows. This unit appears to have all the geochemical pre-requesites for a significant unconventional hydrocarbon play. Geohistory modelling using formation-specific compositional kinetics indicates petroleum generation and expulsion begins in the latest Cambrian-Early Ordovician due to relatively rapid burial of the Arthur Creek Formation. Deposition ends with the start of the Alice Springs Orogeny and following uplift and erosion during the Devonian, hydrocarbon generation ceases. An unconventional petroleum resource assessment of the Georgina Basin will be undertaken in February 2013 and available for benchmarking and refinement against any future shale gas and shale oil production. 

[1] Geoscience Australia and Bureau of Resource and Energy Economics, 2012, Australian Gas Resource Assessment 2012, Canberra, 56 p. https://www.ga.gov.au/products/servlet/controller?event=GEOCAT_DETAILS&amp;catno=74032

Tags: Earth Sciences, GA Publication, Published_External, hydrocarbons, organic geochemistry, petroleum potential, resource assessment
Organization: Australian Ocean Data Network