A Model for the Evolution and Petroleum Potential of the Offshore Polda Trough
Thrupp, Benjamin N.
Honours Degree, 2002
University of Adelaide
The Polda Trough is an east-west trending, graben-like feature located on the eastern side of the Great Australian Bight. It extends into the onshore Gawler Craton. Three offshore wildcat wells have been drilled within the Polda Trough and all were plugged and abandoned, with only one trace hydrocarbon show of 40ppm of methane recorded.
Excellent source rocks and reservoir lithologies occur in the Late Jurassic Polda Formation. Source rock intervals could potentially be present in the Neoproterozoic Kilroo Formation equivalent, but the reservoir potential of this interval is poor.
Thermal modelling shows that the Late Jurassic interval is thermally immature in the Central Polda Trough where borehole temperature data and vitrinite maturity data are available. The Jurassic section in the Western Polda Trough is also thermally immature, based upon modelling using a gradient model derived from the Central Polda Trough well data.
Major and minor element Inductively Coupled Plasma Ion Chromatography (ICP) analysis shows the Neoproterozoic halite in the Mercury-1 well to be very pure, suggesting that it represents a concentric basin central precipitate, which was preserved by later rift formation. Under thin section the largely featureless salt appears to have been recrystallised at least twice since deposition.
The Mesozoic clastics and Kilroo evaporites were probably deposited in a broad synclinal feature that began rifting in the final stages of Neoproterozoic evaporite/carbonate deposition. Bounding fault movement created accommodation space for the Permian veneer and the Jurassic section within the rift structure, probably via sedimentary loading.
The salt in the Polda Trough is a preservational feature of Neoproterozoic rift formation after salt deposition; thus, salt could be more widespread in the GAB area where Neoproterozoic successions are also preserved. Equivalents of the Jurassic Polda Formation source rock could also be present in other regions in the GAB where they could be more mature.