Eromanga basin

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Map of both the Cooper and Eromanga Basin

This page is currently being authored by a student at the University of Oklahoma. This page will be complete by Dec 13, 2020.

Introduction

The Eromanga Basin covers an area of approximately 1,000,000 sq. km and extends over South Australia, Northern Territory, Queensland, and New South Wales.[1] This is seen on the location map on the right. It is comprised of a broad down warp with two main depocenter’s: the Poolowanna Trough and the Cooper region. [2] The Cooper-Eromanga basin is Australia’s most prolific oil and gas basin and continues to yield new discoveries.[1]

History

Eromanga Basin Borders

Petroleum exploration commenced in the 1950s when licenses covering the Cooper and Eromanga basins were first acquired by Santos, who went against conventional wisdom that commercial accumulations of oil would not be found in Mesozoic formations within the Great Artesian Basin.

Initial exploration involved surface mapping, stratigraphic drilling, aerial surveys, gravity, and aeromagnetic surveys and seismic. The first petroleum well was drilled in 1959.

The first commercial hydrocarbon to flow from the Eromanga Basin was gas produced from Namur 1 in 1976. Oil was discovered in 1977 with an uneconomic flow from Poolowanna 1 in the Poolowanna Trough. The first economic oil flow was recorded from Strzelecki 3 (Cooper region) in the following year and this stimulated a major oil exploration program.[3]

Since 1959 over 2,000 wells have penetrated the Eromanga Basin sequence and over 100,000 km of seismic has been acquired.  A new phase of exploration for oil in the Eromanga Basin commenced in 2002 with 27 new licenses. Most new entrant explorers are currently targeting Eromanga Basin oil plays. [2]

Stratigraphy Map

Geologic Risks and Uncertainties

Earthquakes and Faults

The Cooper–Eromanga Basin contains a regionally extensive Cretaceous polygonal fault system. The fault system contains vast numbers of small faults with no preferred direction of either strike or dip, meaning the pattern of faulting on one horizon will not persist to overlying or underlying horizons.[4]

Bore Water Geochemistry

A surprisingly large number of water bores in the Eromanga Basin and adjacent Surat Basin were drilled to basement. These bores, combined with bores reaching only shallower aquifers, provide the opportunity for investigation of not only the stratigraphy, but also the type, source, and quantity of hydrocarbons in artesian waters. The bores draw water from the Wyandra and Hooray Sandstones in the Eromanga Basin.

Water samples are collected directly from the flumes of flowing bores, from pressure plugs in the bores, or from pumped sub-artesian bores. Several samples of free-flowing gas have also been collected.[5]

Petroleum Elements

Source Rock

Vertical migration of oil from Permian (Cooper Basin) source rocks has been widely accepted as the principal source of most Eromanga-reservoir oil (in the Cooper region). Both Cooper and Eromanga mature source rocks have contributed to oil accumulations in the region, however each oil accumulation needs to be considered with respect to the extent of mixing from Permian and Mesozoic sources. The Poolowanna and Birkhead formations contain organic-rich shales that are oil-prone and in places at peak maturity for oil generation.[2]

Migration

Many reservoirs have been charged from local source rocks with only short migration paths.  Others, as evidenced by recent commercial oil discoveries at the margins of the underlying Cooper Basin, have required extensive migration of oil from deeper source regions within the open aquifer systems of the Eromanga Basin.  Extensive lateral migration of both Permian and Eromanga sourced oil becomes possible where it has entered permeable carrier beds within the open aquifer system of the Eromanga Basin.[6]

Seals and Traps

Seals consist of intraformational diagenetic sandstones, siltstones, and shales of the Poolowanna, Birkhead, and Murta formations in the Cooper region. In the Poolowanna Trough, they consist of intraformational siltstone and shale.  Additionally, potential seals include the Cadna-owie Formation and Bulldog Shale in the Wallumbilla Formation).[2]

Structural Features

In the Cooper region the structural framework of the Eromanga Basin is largely inherited from mild but widespread compression, regional tilt, and erosion in the Late Triassic which produced the Nappamerri unconformity surface– N seismic horizon. Regional down warping commenced in the Early Jurassic.[2]

Reservoir

Principal reservoirs in the Cooper region are the braided fluvial Hutton and Namur sandstones with 25% porosity and permeability of about 2500 mD. Oil is also reserved in meandering fluvial Poolowanna and Birkhead formations.[2] Natural gas reservoirs are found in sandstones in multiple formations including the Patchawarra Formation, Epsilon Formation, and Toolachee Formation.[1]

Future Potential

The New South Wales portion of the Eromanga Basin remains under-explored. Large distances from recognized sources and the perceived lack of mature petroleum source rocks has deterred exploration efforts.[7] Excellent porosities and permeabilities in the Hooray Sandstone have indicated the existence of reservoir rocks in the Mesozoic section. Geochemical analysis of the artesian waters attests to the presence and movement of hydrocarbons in regionally extensive aquifers in the Eromanga Basin.

Recognition of the troughs underlying the Eromanga Basin provides new possibilities for the presence of thickened sections containing mature source rocks adequate for local hydrocarbon generation.[7]

Production Facilities

Eromanga is famous for being the town furthest from the ocean in all of Australia.  There is not much in the area, as it is where one and a half million barrels of oil are produced each year.  Therefore, production sites are one of the more exciting parts of the town and there are abundant amounts of pump jacks and well sites.[8]

References

  1. 1.0 1.1 1.2 Cooper / Eromanga Basin. Beach Energy. https://www.beachenergy.com.au/cooper-eromanga-basins/
  2. 2.0 2.1 2.2 2.3 2.4 2.5 corporateName=MER; contact=8226 3821; address=Level 4, 11 W. S. (2015, December 1). Metadata.custom. Department for Energy and Mining. https://energymining.sa.gov.au/petroleum/prospectivity/eromanga_basin
  3. Sharperedge. (2019, November 15). The Eromanga Basin - PESA - Petroleum Exploration Society of Australia. PESA. https://pesa.com.au/eromanga_basin_symposium_summary_papers_p20-42-pdf/
  4. Geoteric. Polygonal faults: baffles or conduits for hydrocarbon migration? https://www.geoteric.com/cooper-eromangacase-study
  5. Hall, Lisa & Palu, Tehani & Murray, Andrew & Edwards, Dianne & Hill, Anthony & Troup, Alison. (2019). Hydrocarbon prospectivity of the Cooper Basin, Australia. AAPG Bulletin. 103. 31-63. 10.1306/05111817249.
  6. Eromanga Basin. Eromanga Basin - NSW Resources and Geoscience. https://resourcesandgeoscience.nsw.gov.au/miners-and-explorers/geoscience-information/nsw-geology-overview/sedimentary-basins/eromanga-basin
  7. 7.0 7.1 Geological Survey. Eromanga Basin - Geological Overview | NSW Trade & Investment. https://web.archive.org/web/20120414001809/http:/www.dpi.nsw.gov.au/minerals/geological/overview/regional/sedimentary-basins/eromanga
  8. Welcome to Queensland, Australia. https://www.queensland.com/us/en/places-to-see/destination-information/p-56b25e47d5f1565045da0e69-eromanga

External Links

“Aquifers.” Aquifers - SEG Wiki, Aquifers - SEG Wiki.

“Dictionary:Aeromagnetic.” Dictionary:Aeromagnetic - SEG Wiki, Dictionary:Aeromagnetic - SEG Wiki.

“Dictionary:Basin.” Dictionary:Basin - SEG Wiki, Dictionary:Basin - SEG Wiki.