MORE TRACTS OPENED OFF AUSTRALIA

Australia is seeking bids for more offshore exploration permits.

Included are blocks in five areas in the Indian Ocean off Western Australia: W93-1, W93-2, W93-3, and W93-5 in and near the Carnarvon basin and W93-4 in the Bremer basin off the state's southern coast.

Also on offer are four areas in the Bonaparte basin: AC93-1 and AC93-2 off the Ashmore and Cartier islands in the Timor Sea and NT93-1 and NT93-2 in the eastern Timor Sea and western Arafura Sea off Northern Territory.

Blocks in the Carnarvon basin cover about 16,900 sq km, in the Bremer basin more than 7,200 sq km, and in the Bonaparte basin about 51,300 sq km, including almost 41,000 sq km off Northern Territory and about 10,300 sq km in Ashmore/Cartier waters.

All areas off Western Australia except W93-3 include some deepwater tracts. Water depths across W93-1 and W93-5 range from 400 to 1,600 m, across W93-2 100-1,000 m, and across W93-4 200-3,000 n.

Companies wishing to participate in the license round are required to submit a bid packet by 4 p.m. Sept. 24 to the proper state or territorial departments. Each bid is to include permit applications and supporting data in duplicate, along with a $3,000 (Australian) filing fee payable to the Commonwealth of Australia.

Offers for acreage off Western Australia are to be sent to the director of the petroleum division of Western Australia's Department of Minerals and Energy, East Perth and for acreage off Northern Territory and in Ashmore/Cartier territorial waters to the director of energy in the Northern Territory's Department of Mines and Energy, Darwin.

DEEPWATER TRACTS

Areas W93-1 and W93-5 cover 82 and 92 tracts, respectively, on the southern corner of the Exmouth plateau, adjacent to the Barrow subbasin, and include much of the Mesozoic Exmouth subbasin.

The Exmouth plateau is made up of a subsided continental block of thick Paleozoic and Mesozoic sediments with a thin Cainozoic overlay. A thick faulted section of Paleozoic and Triassic rock with thin overlying Jurassic-Cretaceous rock underlies most of W93-1 and W93-5. A thick Triassic to Cretaceous section of the Exmouth subbasin along the southeast edge of the areas appears directly comparable to major hydrocarbon sources of the northern Carnarvon basin.

Source, reservoir, and seal all are proven in the northern Carnarvon basin, and drilling and seismic data confirm their presence throughout W93-1 and W93-5. However, there are no commercial discoveries in the two areas.

Area W93-2 covers 26 tracts on the southern part of the Northwest Shelf about 170 km off Dampier, Western Australia. The area is covered by regional and detailed seismic data acquired in 1966-82. In addition, the Australian Geological Survey Organization in 1992 traversed the area with a regional seismic survey.

A well drilled in 1980 to test closure over a tilted fault block on the Kangaroo syncline's eastern flank cut two thin gas sands in basal upper Jurassic and upper Triassic.

Sediments under W93-2 are expected to be made up of upper Triassic Mungaroo formation and Brigadier overlain by eroded and thin lower to middle Jurassic Dingo claystone, itself overlain by Cretaceous and Cainozoic shale and limestone.

Area W93-4 covers 108 tracts in the main Bremer basin depocenter, mostly on the upper slope and outermost continental shelf off Southwest Australia.

Although unknown offshore, Bremer basin stratigraphy is expected to contain upper Jurassic to Barremian continental sediments overlain by Albian and upper Cretaceous marine sediments with a veneer of Tertiary open marine carbonates.

The hydrocarbon potential of Bremer basin is unknown, but potential source rocks can be inferred by analogy with the Eyre subbasin. Presence of a regional seal and a heat flow regime adequate for generating hydrocarbons is less certain.

Area W93-3 is made up of four tracts in about 70 m of water in the Dampier subbasin southeast of the WA-208-P exploration permit area, about 90 km north of the Burrup Peninsula and 60 km southeast of North Rankin A gas/condensate production platform.

Seismic coverage in the area is relatively dense. At least 10 exploratory wells have been drilled in the vicinity, mostly in 1970-84.

The area's petroleum potential was upgraded after discovery of Wandoo field, which currently is believed to produce Jurassic hydrocarbons migrating from the Lewis trough.

NORTHERN TERRITORY TRACTS

Area NT93-1 contains 245 tracts covering about 20,000 sq km, and NT93-2 holds 261 tracts covering about 21,000 sq km 300 km north of Darwin. Water depths across most of the two areas are 100-200 m but increase to 400 m near the bidding areas' northern boundary.

Together, the two areas extend across the offshore portion of the Bonaparte basin. Exploration interest in the region is focused on the Jurassic Plover formation and Flamingo group and Cretaceous Bathurst Island group. The major regional seal is provided by Bathurst Island marine shale.

NT93-1 and NT93-2 were explored in two phases, first in the 1960s and early 1970s with regional aeromagnetic surveys, regional seismic data, and one well, the 1 Lynedoch in 1973 near NT93-2. In the late 1980s, more detailed seismic data were acquired and two wells drilled in NT93-1, the 1 Evans Shoal in 1988 and 1 Beluga in 1991.

The Lynedoch well, drilled to test a low relief anticline in the Malita graben, found a thin hydrocarbon bearing zone in Lower Cretaceous sediment but was not tested. Logs showed a possible gas saturated Jurassic zone above the well's 3,967 m total depth.

The Evans Shoal well tested a low relief anticline overlying a faulted structure in the Malita graben and found significant Jurassic gas shows above 3,712 m. The Beluga was drilled to 3,100 m adjacent to the southern margin of the Malita graben.

Geology and prospectivity of NT93-1 and NT93-2 were reviewed in publications in 1989 and 1990 by the Northern Territory Geological Survey.

ASHMORE/CARTIER TRACTS

Area AC93-1 covers 16 tracts overlying the southern part of the Vulcan subbasin, a northeast trending horst and graben province, in 100-200 m of water just east of Skua oil field and west of Talbot field.

Exploratory leads can be inferred in AC93-1 similar to adjacent areas, including the productive Swan and Skua graben features to the west.

AC93-1's major potential reservoir sequences are expected in early to middle Jurassic Plover, Montara, and Vulcan. Tilted fault blocks of the type found productive in Skua and Talbot likely will be the best possible leads.

The seismic grid covering AC93-1 is moderate to detailed, and data quality vary. Considerable detailed interpretation is possible of data acquired and processed during the 1980s.

Area AC93-2 includes 108 tracts north of AC93-1 in 100-450 m of water and structural features that form part of the Ashmore platform, Cartier trough, and Sahul syncline.

Wells drilled in or near AC93-2 reported oil shows, including major potential reservoirs in Jurassic and Triassic beds.

Main AC93-2 features include closures on the downthrown sides of major faults and tilted fault blocks on the upthrown sides of such blocks. Possible plays include rollovers adjacent to faults and stratigraphic pinchout and submarine fans.

The area's seismic grid is broadly spaced with little detailed coverage.

Copyright 1993 Oil & Gas Journal. All Rights Reserved.