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Hydrocarbon Genesis and Organic Facies in Cambrian Carbonates of the Eastern Officer Basin, South Australia

By
David M. McKirdy
David M. McKirdy
South Australian Department of Mines and Energy, Eastwood, S.A. 5063, Australia1
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Agu J. Kantsler
Agu J. Kantsler
Wollongong University, Wollongong, N.S. W. 2500, Australia2
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John K. Emmett
John K. Emmett
Australian Mineral Development Laboratories, Frewville, S.A. 5063, Australia3
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Alan K. Aldridge
Alan K. Aldridge
Masspec Analytical, Stroud, Gloucestershire GL5 3JA, England
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Published:
January 01, 1984

Abstract

In mid-1979, a stratigraphic well drilled by the South Australian Department of Mines and Energy (SADME), Byilkaoora-1, encountered shows of aromatic-naphthenic and naphthenic oils while coring Cambrian carbonates in the northeastern Officer basin, South Australia. The oils occupy vugs and partly healed fractures in an alkaline playa-lake sequence of algal-plate dolomite mudstone, dolomite mudstone with Magadi-type chert, and organic-rich dolomitic argillaceous mudstone containing calcite pseudomorphs of sodium carbonate-bicarbonate minerals, notably trona and shortite. Oil-source-rock correlations confirm that the oils originated within those facies drilled. The Byilkaoora oils are thus the first reported examples of nonmarine Cambrian petroleum. They are unusually rich in the C15-C25 and C30 acyclic isoprenoid alkanes, except where severely biodegraded. Sterane and hopane distributions indicate that the oils were expelled from marginally mature source rocks. On the southeastern flank of the Officer basin, micritic carbonates deposited in a marine-sabkha environment are marginally mature to mature and have good oil-source potential, although minor staining is the only evidence of oil generation and migration in these rocks.

The primary organic facies of the Officer basin carbonates determines whether they are oil or gas prone. Kerogen in the oil-prone carbonates is either Type I (deposited in an alkaline playa–lacustrine environment) or Type II (deposited in a marine-sabkha or lacustrine environment). Marine lagoonal carbonates contain gas-prone Type III kerogen derived from algal (including cyanobacterial) mucilage. Petrographically, the major components of the oil-prone kerogens are lamellar alginite and bituminite. Likely precursors include the lipids of cyanobacteria (blue-green algae) and various heterotrophic bacteria. The sesterterpanes and squalane present in high concentrations in the Byilkaoora oils and their source rocks may be biological markers of halophilic and/or methanogenic archaebacteria.

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Contents

AAPG Studies in Geology

Petroleum Geochemistry and Source Rock Potential of Carbonate Rocks

James G. Palacas
James G. Palacas
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American Association of Petroleum Geologists
Volume
18
ISBN electronic:
9781629811642
Publication date:
January 01, 1984

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