2006 lease sale adds investment opportunities
Dr. Marita Bradshaw, Geoscience Australia
null
The pace of exploration and development activity has accelerated in deepwater Australia, especially offshore Western Australia. And there are significant new opportunities for investment in deepwater exploration in the upcoming 2006 release of offshore acreage, scheduled for May. The drivers of this upswing in activity include higher oil prices and increased demand for LNG in the Asia-Pacific, as well as exploration success.
Exploration
Significant recent deepwater discoveries are the super-giant Jansz gas field (2000, in 1,321 m of water), the Enfield (1999, 544 m), Laverda (2000, 840 m), Stybarrow (2003, 825 m) oil accumulations, and the Pluto gas field (2005, 976 m).
Woodside’s Enfield development is under construction with first oil expected in 2006, and Pluto LNG has been granted major project facilitation status by the federal government with first export cargo scheduled for late 2010.
This environment has translated into lively bidding for recent deepwater acreage in the 2004 and 2005 rounds, but given the size of offshore Australia, large prospective areas remain to be offered in the 2006 release.
The area of Australia’s marine jurisdiction around the mainland and its island territories, at over 12 million sq km, is larger than the size of onshore Australia, and much of it is underlain by sedimentary basins.
However, less than 70 exploration wells have been drilled in waters deeper than 400 m and only 23 in depths below 1,000 m. Deepwater drilling has been concentrated along the North West Shelf in the Carnarvon, Bonaparte, and Browse basins, with only a handful of deepwater wells drilled elsewhere. In southeastern Australia, five exploration wells have been drilled in waters deeper than 400 m in the Gippsland basin, one in the Otway basin, only two in the vast expanse of the Great Australian Bight.
Map of Carnarvon basin, offshore Western Australia, showing oil and gas fields, currently held acreage, and the proposed release areas for 2006.
Deepwater exploration began in Australia in the late 1970s and early 80s with the Exmouth Plateau drilling campaign in the outer Carnarvon basin. Eight deepwater (up to 1,360 m deep) wells were drilled, which resulted in a giant gas discovery in an Early Cretaceous basin floor fan with the Scarborough-1 well (1979, 923 m water depth). At the time, the available technology and the undeveloped LNG market made the remote, deepwater gas accumulation uneconomic to develop.
There was a more than a 10-year gap until deepwater exploration returned to the North West Shelf. During this time, LNG production for export was established from giant gas fields located in shallower waters along the Rankin Trend.
The renewed deepwater exploration of the mid-90s initially focused on the established Triassic play along the eastern margin of the Exmouth Plateau, outboard of the Rankin Trend. Chrysaor (1994, at 806 m water depth) and Dionysus (1996, 1092 m) were drilled to test seismic anomalies north of the giant Gorgon field and proved to be giant gas fields with significant quantities of associated condensate.
In 1999 to 2001 multi-trillion-cubic-feet gas discoveries were also made at Greyon, Orthrus, Maenad, and Urania. All were drilled in waters greater than 1,000 m and have strong amplitude vs. offset (AVO) signatures, and together contain an estimated 7.4 tcf of recoverable reserves.
Most of the discovery wells tested Triassic horsts where good quality reservoirs were intersected in the Late Triassic, and to a lesser extent the Early Jurassic section. Overlying seals in the Late Triassic and Early Jurassic were preserved on some fault blocks, and where this section has been removed due to Late Jurassic erosion, traps can occur beneath the regional Cretaceous seal.
In contrast, the super-giant Jansz/Io gas accumulation introduces a new play type to the Carnarvon basin, being stratigraphically trapped in an Oxfordian shallow marine sandstone.
Projects
Market conditions have changed markedly since the 1970s, with major gas contracts secured to supply LNG to China, in addition to the established trade with Japan. An indication of the viability of the deepwater plays on the Exmouth plateau is the recent uptake of acreage by BHP Billiton, Woodside, Apache, and jointly by Chevron and Shell.
The Scarborough gas is under active consideration in BHP Billiton’s Pilbara LNG project, just one of several major developments proposed for giant deepwater gas fields on the North West Shelf.
Another in the Carnarvon basin is Chevron’s Greater Gorgon project. Approximately 40 tcf of gas and 240 MMbbl of condensate are present at Gorgon and the more westerly Jansz/Io deepwater fields.
The Gorgon field development plan involves the installation of a subsea gathering system and a 70-km subsea pipeline connected to a gas processing facility on Barrow Island. This development is also planned to include the Jansz/Io gas field, which has an in-place gas resource estimated at 20 tcf and is the largest gas field found to-date in Australia.
Lease sale
In the 2006 acreage release large deepwater areas are available in the Carnarvon basin located between the Scarborough gas field and the Greater Gorgon complex.
There are also major gas and condensate resources (26 tcf, 540 MMbbl) in the Browse basin in fields including Scott Reef and the Ichthys complex. A possible third hub for gas processing may be established in the basin, midway between the already producing Carnarvon and Bonaparte basins.
In the western area of the Browse basin, Woodside is embarking on an appraisal program to assess the feasibility of developing the Browse project that includes the deepwater Brecknock and Brecknock South fields. The fields are remote from shore and Woodside is considering concepts including the production and export of LNG, as well as sales gas into domestic markets and gas supply to an onshore gas-to-liquids (GTL) facility.
In the 2006 acreage release, large deepwater areas will be available in the Browse basin located inside of the Brecknock and Scott Reef fields as well.
Map of Otway, Bass and Gippsland basins, offshore Victoria and Tasmania, showing oil and gas fields, currently held acreage, and the proposed release areas for 2006.
Deepwater acreage is also part of the 2006 release in the Gippsland basin, a petroleum province discovered under the waters of the Bass Strait in southeastern Australia in the 1960s. The billion-barrel Gippsland basin oil fields have underpinned Australia’s oil supply for the past 30 years. The oil play was extended into deepwater with the Blackback discovery in 1989, drilled in 418 m of water.
Seismic data
Geoscience Australia supplies pre-competitive information to assist explorers in their assessment of open acreage and other investment opportunities in Australia. Public access to E&P data (e.g., digital seismic tapes and core and cuttings samples from wells) results from legislation that initially subsidized exploration in return for lodgment and public availability of the data. When the subsidy was removed, the requirement that data be lodged with government and made available for future exploration continued. This historical legacy was further enhanced with the 2001 Federal Government’s Spatial Information and Data Access Policy, so that company data are publicly available at the cost of transfer, after a relatively brief confidentiality period.
The 2003 federal budget provided funds to preserve Geoscience Australia’s seismic data archive by transcribing it onto high density stable media. The funding has also enabled Geoscience Australia to re-process selected seismic and collect new industry standard data in frontier basins.
The initial focus has been offshore southwestern Australia, where seismic data was collected in the deepwater Mentelle basin and the Bremer sub-section of the western Bight basin. This was the first seismic data to be acquired from this part of the Great Australian Bight since 1974.
The seismic data has been integrated with dredge information from a geological sampling survey to enable explorers to assess the prospectivity of this un-drilled frontier basin as part of the 2005 acreage release.
In 2006, Geoscience Australia’s new data acquisition program will include a marine sampling survey for the deepwater central North West Shelf to investigate sites of possible hydrocarbon seepage.•
For more information about offshore acreage releases, access to petroleum data, or Geoscience Australia’s program in frontier basins, please contact Marita Bradshaw ([email protected]).
