Australia opens latest exploration bidding

• Where Acerage is Available Off Australia [36481 bytes]

Australia has opened a bidding round for 38 offshore areas in 10 regions, including acreage near recent discoveries in the Bonaparte and Carnarvon basins.

Included in the offering is prospective acreage in the central part of world class Gippsland basin off Western Australia and lightly explored blocks off Victoria and the island of Tasmania.

Operators working last year on Australia's North West Shelf gauged significant oil and gas discoveries. Corallina prospect in northern Bonaparte basin confirmed expectations for the region raised by discovery in 1994 of Laminaria field. Also in Bonaparte, delineation is under way on Bayu and Undan gas discoveries.

Discoveries in 1995 in Carnarvon basin included Perseus gas field on Rankin platform, where reserves are estimated at several trillion cu ft. Carnarvon's exploration record includes oil strikes at Blencathra, Wonnich, and Agincourt.

Australia in its 1996 offshore tender is seeking work program bids on acreage in proven producing areas, as well as in frontier areas, in water depths ranging from extremely shallow to as deep as 2,000 ft.

The bidding round, which opened June 17 in Darwin, represents Australia's first offshore acreage tender in more than a year. Companies interested in acquiring rights to explore any of the tracts on offer must submit bids by Jan. 30, 1997.

Bonaparte basin tracts

Australia has included 10 areas in the 1996 offshore bidding round spread across the Bonaparte basin.

Areas NT96-1 and NT96-2 cover about 19,000 sq km in the northeastern part of the basin. The areas overlie Calder and eastern Malita grabens, which contain a thick Mesozoic section with associated highs on its flanks.

The acreage offers a range of structural and stratigraphic, pre- and post-Jurassic breakup plays, from broad central graben drape fault blocks and fan sands to basin margin fault zones. Potential reservoirs are Jurassic and Cretaceous fluvial and marine sandstones.

Areas NT96-3 and NT96-4, farther west in Bonaparte basin and adjacent to the Australia-Indonesia Zone of Cooperation, overlie the major depocenter of Malita graben. Play types include fault dependent anticlines and horst blocks, as well as Jurassic and upper Cretaceous stratigraphic plays.

Previous activity in the latter two areas includes only three wells, and seismic coverage is light. Water depths across the two tracts mostly are less than 100 m.

Vulcan subbasin acreage

Areas AC96-1 through AC96-6 lie along the western part of Vulcan subbasin and eastern part of Ashmore platform in an area where bidding was strong in Australia's last acreage release.

Previous activity has proven Vulcan graben to be oil productive, especially from upper Jurassic source rocks. Additional oil sources might include Cartier trough in the north and Browse basin to the south. Triassic through Cretaceous sands provide effective petroleum reservoirs in the region.

Operators have begun producing oil through floating production facilities from Triassic and Jurassic reservoirs immediately east of the acreage on offer.

Play types include fault blocks within the graben, traps associated with salt diapirism, structural and stratigraphic plays at the graben margin, and fault and unconformity traps on the platform.

Browse basin areas

Australia included in its 1996 offshore tender eight Browse basin areas in 40-200 m of water.

Australia's biggest gas accumulation, Scott Reef field, was discovered in 1971 on the basin's western margin. However, until 1 Gwydion wildcat discovered oil last year, touching off a play on the basin's eastern margin, Browse had yielded only tantalizing oil shows.

Now, as well as reinvigorating terrace and central basin plays in the basin's thick Triassic through Cretaceous section, Gwydion proves long range migration for oil generated in the depocenters.

Officials expect exploratory targets in Areas W96-1 to W96-8 to occur at less than 1,200 m in marine lower Cretaceous sands draped over basement highs. Other evidence indicates additional stratigraphic trap potential including pinchouts against such highs. Also in the area, Cretaceous shales have good seal potential, and seismic data quality is good.

Carnarvon basin acreage

Australia included 13 areas in Carnarvon basin in its 1996 offshore tender.

Areas W96-9 to W96-15 lie in 900-2,000 m of water on Exmouth plateau, a lightly explored area where drilling beginning in the early 1980s resulted in discovery of undeveloped giant Scarborough gas field.

Exmouth lies west of oil and gas developments in Barrow and Dampier subbasins.

Exploration targets of the seven Exmouth areas likely occur in sub-breakup unconformity Triassic to mid-Jurassic fault blocks and overlying Cretaceous sands. Some development potential also is indicated for younger Jurassic sands.

Seals occur at several horizons through the area's Jurassic and Cretaceous sections and include a regional Cretaceous seal. Triassic and Jurassic source rocks apparently charge hydrocarbon reservoirs in the area.

Four areas-Areas W96-16 through W96-19-straddle Alpha arch in southwestern Barrow subbasin. Oil and gas targets on the four tracts include Cretaceous deltaic and fan sands in drape and stratigraphic traps, as well as good quality fluvio-deltaic Triassic to Middle Jurassic sands in tilted fault blocks.

Alpha arch focuses hydrocarbon migration from generative, mainly Jurassic, source rocks on either side. Oil, gas, and gas/condensate fields lie north, east, and south of the blocks.

Farther south in Exmouth, the lightly explored Area W96-20 offers a similar play scenario, though in shallower water. Area W96-21 lies on trend with Rankin platform oil and gas fields and west of the Middle Jurassic 1 Nebo oil discovery.

The main plays within these tracts involve structural traps below the Cretaceous regional seal and structural/stratigraphic trapping within the Jurassic section. Nebo developed as a combination of faulting during mid-Jurassic breakup and arching associated with Neocomian wrenching.

Perth basin area

Operators produce oil and gas onshore in Perth basin, although no discoveries have been reported within Area W96-22.

Officials said, however, oil was recovered from a well adjacent to the tract, and about 2,300 km of recent seismic data are available on the area.

Area W96-22 is within Perth basin's Vlaming subbasin where a thick Jurassic sequence is broken by major Neocomian structuring.

The main part of Vlaming subbasin consists of Jurassic to Cretaceous clastic sediments interrupted by Neocomian rifting and an associated major unconformity. A recent review of the basin confirmed or identified play concepts and leads associated with unconformity closures, drapes and pinchouts, and sub-unconformity fault traps, as well as a lower Tertiary channel play.

There is an extensive amplitude anomaly associated with a unit immediately overlying the Neocomian unconformity.

Acreage off Victoria

Officials released two tracts off Victoria in Gippsland basin.

Area V96-G2 is on the northern flank of the basin, on potential migration routes out of the main basin, just north of proven fields and south of a coastal oil field that produced during World War II.

Data on the 750 sq km tract show traps at the base of the regional seal, and there are proven intraformational seals at older horizons, as well. Water in Area V96-G1 mostly is less than 50 m deep.

Geology of Area V96-G2-surrounded by producing fields in the center of the basin-is marked by upper Cretaceous to Tertiary targets in anticlines, fault traps, erosional highs, and channels mostly associated with the Latrobe group. Water across the tract mostly is less than 100 m deep.

Area V96-01, immediately southwest of Melbourne in Otway basin's Torquay subbasin also offers shallow water exploratory targets.

Previous activity on the block includes only three wells, including one drilled off structure and the other two apparently located poorly in relation to hydrocarbon migration fairways.

Potential source rocks on the tract are expected in the lower Cretaceous to Cainozoic Australia-Antarctica break- up section, inlcuding lacustrine shales. Officials expect traps in three main stratigraphic levels, including basement onlap, tilted fault blocks, and hanging wall structural/stratigraphic traps.

Acreage off Tasmania

Area T96-2, farther south in Otway basin and west of King Island, is a lightly explored area with trap potential in tilted fault blocks and drape-over deep structures.

The tract's geologic record includes as much as 8,000 m of Cretaceous terrestrial to marine sediments and Cainozoic clastics and carbonates.

Previous activity includes only two wells on the acreage, the last in 1970. The most recent seismic survey of the area by industry occurred in 1982, but the government has collected data more recently. Australia has mapped three upper Cretaceous leads in the area.

Several companies have explored Area T96-3 off Tasmania's western coast in Sorrell basin's Strahan subbasin. Formed in the breakup of Australia and Antarctica, Strahan consists of upper Cretaceous and Tertiary sediments in a series of half grabens, parallel to the coast and rotated clockwise.

Seismic data cover the 6,000 sq km area in a 1-1.5 km grid. The only well drilled within the tract, 1 Cape Sorrel, encountered oil shows beneath a Tertiary-Cretaceous unconformity but later was shown to have no closure and is regarded as an invalid test.

Area T96-3's best potential lies in rotated fault blocks and overlying draped sandstones. Prospects and leads have been mapped in the area, and thermogenic hydrocarbons have been sampled at the sea floor.

Area T96-1 is offshore north of Tasmania in a series of half grabens in the southern extremity of Bass basin, known collectively as Durroon subbasin.

Durroon shares broad structural and stratigraphic elements with Bass basin but with different timing of major tectonic events and related sedimentation. The subbasin is marked by as much as 7 km of Cretaceous sediment overlain by a thin Tertiary cover.

Area T96-1 covers part of the main depocenters for Otway group sediments, and the primary objectives within Durroon likely will be Otway and Furneaux group sediments.

Seismic data have imaged basin margin alluvial fans, which could provide adequate reservoirs, and there is potential for fault block traps with Cretaceous sandstone reservoirs.

Water on Area T96-1 is less than 90 m. Water depths across the other two Tasmanian tracts are less than 200 m.

Copyright 1996 Oil & Gas Journal. All Rights Reserved.