Monday, 19 May, 2025/ Edition 59

Australia is the smallest of the world’s seven continents, but if it were categorized as an island, it would be the largest of 670,000 islands on Earth. It hosts several sedimentary basins rich in oil and gas resources, and it has been on the world’s petroleum map since the oil and gas discoveries in Gippsland Basin in the 1960s.

Let’s look at some recent findings from Australia’s intracontinental and rift margin basins.

Rasoul Sorkhabi

Editor, Core Elements

Milankovitch Cycles in the Ordovician Sedimentary Record

The Coast of Canning, Australia/ Misha Kincade/ Shutterstock.com

Milankovitch cycles are short-term cycles—some tens of thousands of years—that affect Earth’s climate via Earth’s orbital eccentricity, rotational axial obliquity, and rotational axial precession. The cycles are responsible for glacial and interglacial intervals in the Quaternary Ice Age during the past 2.5 million years.

What’s new: An article in Marine and Petroleum Geology covers how Milankovitch obliquity cycles controlled terrigenous sediment flux in the Middle Ordovician marine sediments in the intracratonic Canning Basin in western Australia.

 Study design:

• The researchers studied stratigraphic data from the Goldwyer Formation within the Tehei 1 well at depths of 1,189–1,595 meters.

• Geochronological constraints on sedimentation came from previously reported biostratigraphic data and new U-Pb zircon ages of 466.07 Ma to 467.12 Ma from volcanic ash beds.

• Well logs provided petrological and petrophysical properties of the sedimentary succession.

• Carbon-13 and carbon-18 stable isotope analyses, as well as ICP-MS elemental analyses, provided geochemical signatures.

• Additional information came from petrographic observation of thin sections and FTIR mineral mapping.

• Researchers predicted sea-level changes in the sedimentary succession using the “dynamic noise after orbital tuning” (DYNOT) model described in this 2018 paper.

Key findings:

• The Goldwyer Formation was deposited in the interval of 9.3 million years in the Darriwilian stage (469.4–458.2 Ma) of the Middle Ordovician. Sedimentation rates are estimated to be 3.5–4.5 centimeters per thousand years.

• The Goldwyer Formation is made up of limestone (marl and wackestone), claystone, dolomite, and siltstone. Detrital grains are coarse silt ranging from 0.03 to 0.05 millimeters.

• Sedimentation in the Goldwyer sediments displayed an obliquity cycle of 1.2 million years.

• Increased rotational obliquity characterized by dry and wet seasons was coeval with increased sedimentation rates and deposition of fine-grained mudstone in the basin.

• Geochemical proxies also indicate chemical weathering combined with enhanced terrigenous sediment flux.

• Obliquity minima were associated with stable winds, sea-level fall, and reduced terrigenous flux.

Why it matters:

• Recognition of Milankovitch cycles in the historic sedimentary record of geologic past is important for stratigraphic reconstruction (third-order depositional sequences), paleoclimate changes, and Earth System Evolution. 

• This study is particularly interesting because of its multidisciplinary methodology.

Go deeper: Read the full paper here.

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Petroleum Systems and Basins in Australia

Basins in Australia/ Geoscience Australia 

Let’s look at some recent studies and papers describing basins across the continent of Australia.

Cooper Basin

An article in Gondwana Research describes the anatomy of the Cooper Basin using 2D seismic images, 1D stratigraphic backstripping, and paleogeographic mapping.

Geology of the basin: The Cooper Basin in eastern Australia is an intracratonic basin developed on the Gondwana supercontinent from the Late Pennsylvanian to Middle Triassic at a paleo-latitude of 50 degrees South. 

• The basin sediments are entirely terrestrial with facies changes from Late Pennsylvanian glacial deposits to Early Triassic warm, dry, red beds.

• The researchers interpret this basin as a failed rift during the Late Paleozoic breakup of eastern Gondwana.

• The basin was superimposed on Cambro-Devonian rocks.

Oil and gas potential:

• Permian shales (Toolachee, Daralingie, Roseneath, Epsilon, Murteree, Patchawarra formations) have great source rock potential. These formations also contain coal beds up to 30 meters thick.

• More than 4,800 wells have been drilled in the basin.

Sydney Basin

A study published in De Gruyter Brill’s Open Geosciences reports new geochemical data on source rocks from the Late Permian to Early Triassic sediments of the Sydney Basin.

Geology of the basin:

• The Sydney Basin lies in southeast Australia.

• Basin rifting occurred during Carboniferous-Early Permian times.

Study design: Researchers collected fifteen core samples from six wells at depths of 493–796 meters for pyrolysis and GC-MS biomarker analyses.

Key findings:

• Samples yielded vitrinite reflectance values of 0.81–3.34 percent and Tmax values of 402–480 degrees Celsius.

• Total organic carbon values ranged from 4.5 to 20 percent, with higher values coming from coaly samples.

• Hydrogen index values ranged from 16 to 816.

Why it matters: Researchers suggest the deeper parts of the Sydney Basin may hold oil or gas reserves in Permian or Triassic sandstones.

Browse Basin

An AAPG Bulletin article describes the Cretaceous sequence stratigraphy and gross depositional environments of the Browse Basin within the North West Shelf of Australia.

About the basin: The Browse Basin is home to the producing Ichthys-Prelude gas field.

Study design: Researchers analyzed 2D seismic images and data from 31 wells.

Key findings:

• First-order sediment space accommodation by thermal and tectonic-eustatic causes were controlled by the plate tectonics of the Australia-India breakup.

• Third-order Cretaceous depositional sequences had three supersequences:

  • Platform (sandstone and mudstone)

  • Slope and basin-floor (siliciclastic claystone and carbonates with total organic carbon values of 1–2 percent)

  • Submarine fan (sandstone bodied interbedded with mudstone, marls, and micrite)

Otway Basin

A study in JGR Solid Earth offers an innovative approach to image deep structures in the Otway Basin.

Geology of the basin: The Otway Basin in southeast Australia records the Cretaceous to Eocene breakup of Australia from Antarctica. The Precambrian-Paleozoic basement is buried under the Cretaceous-Miocene passive margin sediments.

Study design: Researchers used 2D interpreted seismic images, gravity, and magnetic anomaly data, and applied Euler deconvolution and machine learning methods to map deep structures.

Key findings:

• They identified four major faults, two of which segmented the Cretaceous syn-rift Otway Basin.

• These faults indicate inherited heterogeneity within the basement.

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✉️ To get in touch with Rasoul, send an email to editorial@aapg.org.