Otway basin

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The Otway basin runs from northwest to southeast and is a divergent margin basin. The Otway basin is an onshore and offshore petroleum basin. It is located 500 kilometers from Cape Jaffa in Southern Australia.

Introduction

Figure A

The Otway is a northwest striking basin that extends around 500 kilometers along the offshore and onshore of Southern Australia. Around 80% of Otway is located offshore and the rest is onshore. Overall the basin covers around 155,000 km^2. 45% of the Otway basin is in Victoria, 35% in South Australia, and 20% in the Tasmanian jurisdiction(Regional Geology of the Otway Basin, 2021). The Otway basin is an important gas-producing with discoveries onshore and in shallow waters offshore. The offshore discoveries are around 170-220 km southwest of Melbourne. These discoveries are responsible for delivering gas to the southeast Australian energy market.

Geology

The Otway Basin initially started rifting in the Late Jurassic to Early Cretaceous, with rift progression from west to east. It is a sedimentary basin that formed 145 million years ago. This was when Australia started separating from Antarctica (Otway Basin, 2021). During this process, a series of troughs were formed within the basin and captured sediments over the millions of years.

Figure B

These [1]Early Cretaceous deposit centers include the Penola Trough, Portland Trough, Ardonachie Trough, and the onshore portion of the Shipwreck Trough.

Figure B represents where these important deposit centers are located.

Stratigraphy

The Otway Basin is broken up into seven major basin phrases and their eight component super sequences (Otway Basin, 2021). They are:

  1. Tithonian-Barremian of the Crayfish super sequence.
  2. Aptian-Albian of the Eumeralla super sequence.
  3. Mid-Cretaceous compression and inversion.
  4. Late-Cretaceous rifting of the Shipwreck and the Sherbook super sequence.
  5. Maastrichtian to Middle Eocene of the Wangerrip super sequence.
  6. Local inversion and thermal subsidence of the Nirranda Super sequence, followed by compression of the Heytesbury super sequence.
  7. Plio-Pleistocene deposition of the Whalers Bluff super sequence. 

Main Stratigraphies

The main exploration targets of the basin are the:

  • Waare Sandstone at the base of the Sherbook Group (Shipwreck super sequence)
  • Sandstones of the Pretty Hill Formation of the Crayfish super sequence
    Figure C
  • Katnook Sandstone in the Early Cretaceous section

Figure C represents the cross-section of the super sequences.

Source Rocks

The Casterton Formation represents the richest source rock of in the Otway. It is thought to be the source of the commercial gas accumulations in the Penola Trough region. It is comprised of interbedded shale, siltstones, sandstones, and volcanic lithologies.

The [2]maturity modeling suggests that the northern flank of the Otway Basin represents the most prospective area for shale oil.

Most of the hydrocarbon discoveries have been made in the Otway Group and the overlying Waare Sandstone. This suggests that the major source beds are located within the Early Cretaceous sequence.

The most oil-prone source rocks identified are indeed of Early Cretaceous age and are comprised of carbonaceous shale. At the base of the Crayfish Group is organic-rich [3]lacustine mudstones.

Reservoir

The main producing gas reservoir in the South Australian sector of the onshore Otway Basin is the Pretty Hill Formation, particularly on the Chama Terrace. This is where the Pretty Hill Sandstone is over 1500 m thick and contains individual permeable sand bodies up to 50 m thick.

It has porosities in excess of 25% and permeabilities in excess of 1000 [4]mD. The reservoir permeability does not decrease with increasing depth of burial.

The Late Cretaceous Waarre Sandstone and Flaxman Formation are major onshore and offshore reservoirs. The measured porosities are up to 25% and have a maximum permeability of 3300 mD.

The Timboon Sandstone and Paaratte Formation also have excellent reservoir properties. But they lack effective seals (Energy & Mining, 2021).

Exploration

Since the 1890s, the Otway basin has been actively explored. The first deep exploration well was drilled in 1915. But it was not until 1987 that the first commercial gas discovery was at Katnook, followed by another discovery of the Ladbroke Grove Field in 1989. The lack of early success is attributed to poor quality seismic data. And a poor understanding of stratigraphic relationships in Penola Trough (Energy & Mining, 2021).

Since then, seven gas fields have been discovered in the basin.

Undiscovered

The Otway Basin is a relatively immature exploration area. But it has a high potential for future discoveries. To date, the gas discoveries are modest in size. But some discoveries off the shore of Victoria have portions of larger magnitudes.

Oil discoveries have only recently been made, and there is still a considerable potential for more significant oil discoveries in the future (Energy & Mining, 2022).

Recent Explorations

In 2021, Cooper Energy completed a project to reprocess and merge seven 3D seismic survey offshore of the Otway Basin. The new 3D seismic data improved the identification and definitions of seismic amplitudes. They found seismic amplitude at the top of the Waare Formation, which is a direct indicator of the presence of gas (Wilkinson, 2022).

Green Bill

In December 2021, the Offshore Petroleum and Greenhouse Gas Storage Amendment Bill were introduced to the Senate by a Tasmanian Greens Senator. This bill would amend existing oil and gas legislation to exclude future exploration and extraction in offshore areas, including the Otway Basin. Fossil fuel projects near Victoria’s southwest coast, the Otway Basin area, has a history of causing angst in the lead-up to federal elections (Lamacraft, 2022).

References

Lamacraft, T. (2022, March 2). Bid to ban fossil fuel projects in Otway Basin on ice. Surf Coast Times. https://timesnewsgroup.com.au/surfcoasttimes/news/bid-to-ban-fossil-fuel-projects-in-otway-basin-on-ice/

Otway Basin. (2022, May 10). Energy & Mining. https://wds.squiz.cloud/dem/industry/energy-resources/geology-and-prospectivity/mesozoic-basins/otway-basin

Otway Basin | Geoscience Australia. (2021). Otway Basin. https://www.ga.gov.au/scientific-topics/energy/province-sedimentary-basin-geology/petroleum/offshore-southern-australia/otway#heading-6

Regional Geology of the Otway Basin | Geoscience Australia. (2021). Regional Geology of the Otway Basin. https://www.ga.gov.au/scientific-topics/energy/province-sedimentary-basin-geology/petroleum/acreagerelease/otway

Wilkinson, J. (2022, January 13). Cooper Energy reports first gas into Otway Athena plant. Cooper Energy Reports First Gas into Otway Athena Plant. https://www.listcorp.com/asx/coe/cooper-energy-limited/news/operations-update-2693585.html