The Neoproterozoic Gillen Formation, Amadeus Basin, central Australia: an intra-salt petroleum system and viable exploration target?
Phillip S. Plummer
+ Author Affiliations
- Author Affiliations
Visiting Research Fellow, Department of Earth Sciences, University of Adelaide, Adelaide, South Australia, 5005, Australia. Email: phil.plummer@adelaide.edu.au
The APPEA Journal 61(1) 236-252 https://doi.org/10.1071/AJ20040
Submitted: 16 November 2020 Accepted: 21 January 2021 Published: 2 July 2021
Abstract
The Gillen Formation, the basal component of the lower Neoproterozoic Bitter Springs Group, occurs throughout the Amadeus Basin of central Australia and comprises a basal sequence of black shales and stromatolitic dolostones overlain by an evaporitic sequence of a thick halite sandwiched between two anhydrite/dolomite units. These Gillen lithologies include potential source, reservoir and seal units for petroleum, suggesting an analogy could be drawn with the prolific evaporitic Ara Group Petroleum System of South Oman. Although well data penetrating the Gillen Formation are very limited, organic-rich shales have been penetrated in both the north and south of the basin, while both well and seismic data indicate the ubiquitous presence of sealing evaporites across the basin. The Ara Group Petroleum System spans the Neoproterozoic–Cambrian boundary and is blessed with carbonate reservoirs composed of metazoan ‘reefs’ comprising Cloudina and Namacalathus, which provide an open framework porosity approaching 50%. By contrast, the Gillen Formation was deposited during the Tonian Period of the Neoproterozoic, some 300–350 million years older than the Ara Group, and has reservoir facies limited to stromatolitic carbonates with fenestral to vuggy porosities that are typically less than 15%. Nevertheless, seismic data reveal numerous intra-evaporite opportunities within the Gillen Formation that, seismically, are virtually indistinguishable from those being exploited in South Oman, suggesting that a Gillen Intra-Salt Petroleum System is a viable exploration target throughout a majority of the Amadeus Basin.
Keywords: Amadeus Basin, Ara Group Petroleum System, carbonate stringers, evaporites, halotectonics, exploration target, Gillen Formation, intra-salt petroleum system, Neoproterozoic, salt seal, shaley source rocks, South Oman analogy, stromatolitic reservoirs.
Phillip S. Plummer is a Chartered Geologist and Exploration Geoscientist with over three and a half decades of experience in the international upstream oil and gas industry. After graduating with a PhD from the University of Adelaide, he joined Shell International in 1980 for whom he interpreted seismic data in exploration ventures in Holland, Oman, Tanzania and Gabon. The industry downturn of the mid–late 1980s saw him consulting in Adelaide, Sydney, Melbourne and New Zealand before becoming Head of Resource Management for the Seychelles National Oil Company where, throughout the 1990s, he promoted the petroleum potential of the Seychelles offshore to the international oil industry. Returning to Australia, he joined Santos Ltd in 2000, initially reviewing the petroleum potential of the Perth Basin before exploring for stratigraphic gas in the Cooper/Eromanga Basin. Moving into oil exploration, he studied the geochemistry of oils from the Cooper/Eromanga Basin, finding that their gas chromatography trace profile shapes indicate the organic maturity of the parent source rock rather than its depositional environment, a finding that provided a new slant to the exploration effort. His final move with Santos was to evaluate the petroleum potential of the Neoproterozoic succession in the Amadeus Basin of central Australia and develop frontier exploration opportunities. For the past three years, he has held the position of Visiting Research Fellow in the Department of Earth Sciences at the University of Adelaide, where he continues his research on Neoproterozoic stratigraphy, both in the Amadeus Basin and in the Flinders Ranges of South Australia. |
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