The Source And Reservoir Potential Of The Late Jurassic Polda Formation, Polda Basin, South Australia.
Jong, John Tiang Shen
Honours Degree, 1991
University of Adelaide
The intracratonic Polda Basin is an elongate, east-west trending, polyhistory basin that extends across Eyre Peninsula and continues offshore to the centre of the Great Australian Bight. The basin had its origin in the Middle Proterozoic and is thought to have been evolved from the same tectonic regime that gave rise to the Amadeus, Officer and Ngalia Basins of central Australia.
Petroleum exploration in the basin has been mainly confined to the offshore central Polda Basin. Three wells were drilled in this part of the basin and results were uniformly disappointing. No shows were recorded, even though targets were identified from the highly structured Permo-Carboniferous Coolardie Formation.
Information on the source and reservoir potential of the overlying Late Jurassic Polda Formation is limited since the relatively shallow depth of burial of this formation precludes it as a potential source rock. Encouraging results were obtained in 1984, when McKirdy assessed the hydrocarbon potential of this formation and found that its source beds had a mean TOC of 2.56%, with fair to very good generative potential (Sl+S2=3-22 kgHC/tonne). The present investigation is a more detailed study of the Polda Formation aimed at documenting basin-wide variations in its source potential and reservoir characteristics.
Potential source rocks (coals and carbonaceous claystones) were selected from the Polda Formation in twelve wells/drillholes for TOC analysis, Rock-Eval pyrolysis and organic petrology. The results show that although thermally immature, the source rocks of the Polda Formation have high TOC values (10-55%) and excellent generative potential (Sl+S2=24-126 kgHC/tonne). Additionally, kerogen facies variation is apparent with samples from Tuckey #1 containing gas-prone Type III kerogen, and samples from other parts of the basin containing oil and gas-prone Type II/III kerogen. This can be interpreted as a result of localised facies variation (vertical or lateral) which is a feature of fluvio-lacustrine depositional environments. Organic petrological investigation revealed a high percentage of liptinite in coal/DOM (8-25%; mean=l4%, by vol). Some samples contain high concentrations of thermally labile liptinite macerals (notably fluorinite, resinite and suberinite) which impart the capability of early generation of so-called immature oil and/or condensate.
The results of the present study also show that coals can source hydrocarbons and the Jurassic Polda Formation has similar source characteristics to the Jurassic Walloon Coal Measures of the Eromanga, Surat and Clarence-Moreton Basins.
Log analysis of the Polda Formation in Mercury #1 and Columbia #1 indicates sandstones with high porosity values (mean=10.9% in Mercury #1 and 13.2% in Columbia #1), with some intervals having porosity values of 25-30%. Therefore, the sandstones of the Polda Formation have excellent reservoir potential.
The unexplored western Polda Basin may have an increased Mesozoic section and burial depth, with Polda Formation anticipated to be buried in excess of 2000 m. Significant potential exists in this area where source rocks of the Polda Formation may be present at sufficient depth to reach maturity. Additionally, the possibility of locally enhanced geothermal/maturity gradients adjacent to salt swells should not be discounted. We now know that appropriate reservoir rocks, valid plays, and possible seals do exist in the Polda Basin. With the acquisition of high quality seismic data and water depths ranging from 50-150 m, the western Polda Basin represents an area of high hydrocarbon potential deserving of further exploration