AUSTRALIA SEEKING BIDS FOR OFFSHORE ACREAGE

Australia is accepting applications for petroleum exploration permits on portions of its continental shelf.

Thirteen areas are up for bids: three within the territory of Ashmore and Cartier Islands, six off Western Australia, two off South Australia, and two off Victoria.

Companies wishing to participate in the bidding are required to submit permit applications and supporting data in duplicate along with $3,000 (Australian) for each area by 4 p.m. Oct. 5 to the proper state or territorial agency.

Much of the geology in these areas was detailed in a previous offering (OGJ, Dec. 20, 1993, p. 28).

ASHMORE/CARTIER

Leading Australia's first offshore offering this year are three parcels off Western Australia just south of where Australian operators last year drilled a string of significant oil and gas discoveries. Successes included Woodside Petroleum Group Pty Ltd.'s 1 Laminaria, which the government said may be a 300 million bbl oil field (see map, OGJ, Jan. 2, p. 22).

Tracts AC95-1, AC95-2, and AC95-3(53624 bytes) consist of 44, 51, and 7 blocks, respectively, in the northern and central parts of the territory of Ashmore and Cartier Islands in water depths of 80-200 m.

The three unlicensed areas lie in the outer Bonaparte basin.

The oldest significant geological features readily evident in this part of the basin are the seismic reflector horizons, associated with the uppermost Permian sequence, that are generally at depths of 2.5-4 km. Their faulted nature displays a typical horst and graben structural pattern evident on the Londonderry high, adjacent parts of the Vulcan subbasin, and overlying Cartier trough and Sahul syncline.

Most exploratory interest in the three areas is expected to focus on the potential of the Jurassic Plover formation and Flamingo group.

These units are the major potential source and reservoir units in Jabiru and Skua oil fields, respectively. They are the likely source units for several oil discoveries in Zone of Cooperation Area A in the Timor Sea.

Australia and Indonesia jointly administer the Timor Gap Zone of Cooperation (ZOCA).

The first significant oil discovery in the Ashmore/Cartier region was 1 Puffin, in which an ARCO group gauged as much as 4,608 b/d of oil from late Cretaceous Maastrichtian sands of the Bathurst Island group. The first major strike in the region was the Jabiru discovery well in 1983.

Much of the current interest in the Timor Sea region reflects industry successes since ZOCA exploration got under way in 1990-91, notably in 1994 the 5,800 b/d 1 Elang discovery, 13,580 b/d 2 Elang confirmation, and 9,000 b/d 1 Kakatua strike. Especially noteworthy was 1 Laminaria, which flowed 13,400 b/d and cut an oil column 100 m thick in 350 m of water in an area geologically similar to that of the northern portion of AC95-2.

WESTERN AUSTRALIA

Acreage offered off Western Australia covers one parcel in the Timor Sea and five on the Northwest Shelf.

Tract W95-1,(53624 bytes) consisting of 53 blocks, is in less than 100 m of water in the Bonaparte basin of the Timor Sea about 600 km west of Darwin and adjacent to Challis oil field.

The Timor Sea region is structurally complex and consists of a number of Paleozoic and Mesozoic subbasins and platform areas.

Tract W95-1,(53624 bytes) explored as part of offshore exploration permits WA-147-P and WA-201-P, is covered by a broad regional and semidetailed seismic grid consisting of data acquired during 196491. Four wells have been drilled in the area, which is close to the three commercial fields in the Vulcan subbasin: Jabiru, Challis, and Skua.

Data indicate potential source, reservoir, and sealing rocks occur in the area, which is adjacent to the Ashmore/Cartier area.

Tracts W95-2, W95-3, and W95-4,(53624 bytes) are on the Northwest Shelf about 175-230 km north-northeast of Dampier, and consist of 48, 38, and 28 blocks, respectively. Water depths range from 100 m in the southeast to more than 1200 m in the northwest.

The three parcels overlie the central and western portions of the Beagle subbasin, the northernmost Mesozoic depocenter of the offshore Carnarvon basin. All three are covered by a broad regional grid of seismic data acquired in 1965-78 by previous permit holders.

Tract W95-2,(53624 bytes) in deeper water of 4501,500 m, is the least explored part of the Beagle subbasin, stretching from the offshore Canning basin south to the northeast edge of the Rankin platform. It overlies the Sable and Swift blocks, parallel north-northeasterly trending uplifted fault blocks separated by a series of narrow, faulted structural lows.

A single well was drilled there in 1982.

Reservoirs subcropping the late Middle Jurassic Main Unconformity on uplifted fault blocks are the main exploration targets in W95-2.(53624 bytes)

Untested plays in this area include marginal anticlines that formed during late Jurassic to early Neocomian compression and possible late Jurassic fan systems along fault block margins.

No wells were drilled in W95-3,(53624 bytes) although 1 Sable, drilled in 1972, lies immediately outside the area's northern boundary. It is dominated by the southern extensions of the Sable and Swift blocks.

Untested plays in this area are similar to features in W95-2(53624 bytes) and include anticlinal structures downdip and marginal to the main fault blocks and potential postbreakup, marginal fan systems.

Potentially mature middle to early Jurassic source rocks are expected to occur in structural lows. However, migration pathways from the faulted lows into adjacent structures are expected to be complex.

W95-4(53624 bytes) is in the central Beagle subbasin and overlies the Ronsard block, a north-northeasterly trending intrabasin fault block. The southern portion of the area is underlain by the Cossigny trough, a narrow, elongated late Mesozoic depocenter between the southern Ronsard block and the northern margin of the DeGrey nose.

Main reservoir targets are prebreakup middle Jurassic fluvial sandstones and postbreakup late Jurassic to early Cretaceous marine sandstones.

In W95-4,(53624 bytes) one regional survey and three detailed surveys have been conducted since 1985. Operators drilled 1 Ronsard in 1973, 1 Jarman in 1978, and 1 Trafalgar in 1988.

W95-5(53624 bytes) consists of 28 blocks on the Northwest Shelf in water depths from less than 200 in to more than 300 in.

It is immediately west of Goodwyn production license area about 200 km off Karratha. It lies in the Carnarvon basin and includes parts of the Rankin platform, Kangaroo trough, and Brigadier trend.

The area is considered highly prospective, with possible source rocks in Triassic Locker shale and Jurassic euxinic mudstone units and prospective reservoirs in upper Triassic fluviatile Mungaroo formation sands. it holds a Jurassic-lower Cretaceous section with an excellent regional seal provided by the Cretaceous Muderong shale.

Only one well was drilled in the rea, 1 Gandara in 1979.

Tract W95-6,(53624 bytes) consisting of eight locks in less than 200 m of water, lies out 25 km south of Griffin field, 30 m west of Saladin oil field, and 50 km est of Onslow. A gas gathering facility Tubridgi is about 35 km southeast.

W95-6(53624 bytes) lies in the Carnarvon basin ear the boundary of the Kangaroo trough, Alpha arch, Learmonth terrace, nd Barrow subbasin. Source, reservoir, nd sealing rock prospects are the same s in W95-5.(53624 bytes)

Several important discoveries are nearby, including Saladin and Griffin/Chinook oil fields and Macedon gas field. Only one well has been drilled in the area, 1 Somerville in 1987, which found minor shows in Mardie Greensand and Barrow group sands.

SOUTH AUSTRALIA

The two parcels on offer off South Australia are in separate basins.

S95-1(53624 bytes) consists of 392 blocks and covers about 28,700 sq km in the Ceduna subbasin of the Bight basin extending from the western limit of the Polda basin. Water depths are 100-1,500 m extending from the continental shelf over the Ceduna terrace to the lower continental slope.

S95-1(53624 bytes) adjoins the EPP26 permit, awarded in 1991 to BHP, whose 1 Greenly wildcat found significant oil and gas shows that were the first major indication of hydrocarbons in the Duntroon basin.

More seismic surveys and another well are planned for EPP26.

The Ceduna basin is the main depocenter of the Bight basin, which in turn is part of the Mesozoic Australian Southern Rift system. It contains at least 10,000 m of Jurassic or possibly permian to Neogene sediment.

Reservoir characteristics vary, with some sands as thick as 30 m with porosities of 9-25%.

After a dry hole was drilled in the mid-1970s, exploration in the Ceduna subbasin was limited to seismic programs in 1981-82 and 1990.

S95-2(53624 bytes) consists of 80 blocks covering 4,736 sq km and lies in the Otway basin, where very few wells have been drilled since 1975.

However, in the offshore portion of the basin, BHP found two commercial gas fields, Minerva and La Bella, in a geological setting and with plays similar to that in S95-2.(53624 bytes)

Minerva reserves are pegged at 575 bcf.

Three wells have been drilled in the S95-2(53624 bytes) area since 1963, all noncommercial but with hydrocarbon shows. In addition, nine wells have been drilled in the immediate onshore portion of the basin, with several logging hydrocarbon shows of varying significance from early and late Cretaceous rocks.

Five prospects have been identified in S95-2(53624 bytes) with a combined, unrisked reserves potential of 1.5 billion bbl of oil. Gas potential is estimated at a 50% chance of finding more than 200 bcf of sales gas.

VICTORIA

The two areas on offer off Victoria lie in the Otway and Gippsland basins.

V95-01(53624 bytes) consists of 56 blocks off western Victoria in waters from 20 m along the state's 3 mile zone to 200 m along the shelf break and then to more than 1,000 m south along the upper continental slope.

V95-01(53624 bytes) overlies the gently dipping Mussel platform, Portland trough, Voluta high, and Voluta trough.

During the early Cretaceous, nonmarine fluvial and lacustrine rift sediments of the Otway group were deposited in the developing half graben. The Lower Otway group includes the Pretty Hill sandstone, an excellent reservoir sandstone deposited as a series of alluvial fans that coalesced along the edges of the rift.

Structural and stratigraphic plays are common in the area, with main objectives expected to be anticlinal rollover traps at the late Cretaceous Sherbrook group level, where stacked deltaic sands lie within tilted fault blocks.

Only 14 wells have been drilled in the offshore Otway basin, which is believed to be gas prone because of widespread gas strikes in the onshore portion of the basin. However, two onshore wells found small volumes of oil in two stratigraphic units not previously thought to be oil bearing targets, the Tertiary Pebble Point formation and the Cretaceous Heathfield sandstone.

V95-G1(53624 bytes) consists of 33 blocks covering about 2,220 sq km in the southeast portion of the Gippsland basin about 140 km south-southeast of Lakes Entrance and 180 km southeast of Sale. Water depths are mostly less than 200 m but increase to 500 m along the northeast corner of the area, which flanks Bass Canyon.

V95-G1(53624 bytes) lies south and southeast of Gippsland basin producing oil and gas fields that provide about 60% of Australia's domestic petroleum supply.

All of the commercial oil and gas fields in the basin have been found offshore in structural or combined structural and stratigraphic traps mostly at the top of the Latrobe group coarse clastic stratigraphic level, with the biggest fields found early in the basin's exploration.

Several leads have been identified in the area, and reprocessed and infill seismic is likely to identify more.