BIDS SOUGHT FOR TRACTS OFF AUSTRALIA

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

Eleven areas are up for bids: two off Northern Territory, one within the territory of Ashmore and Cartier Islands, three off western Australia, three off South Australia, and two off Tasmania.

Companies wishing to participate in the bidding are required to submit permit applications and supporting data in duplicate along with $3,000 (Australian) by 4 p.m. May 6, 1994, to the proper state or territorial agency.

NORTHERN TERRITORY

Tracts NT93-3 and NT93-4 cover 56 and 87 blocks, respectively, in the central Bonaparte basin about 300 km northwest of Darwin in water generally less than 100 M.

The tracts overlie the northeast-southwest trending Malita graben, a depositional trough that originated during the middle Jurassic. They also cover the flanks of the graben, the Sahul platform to the north, and the northwest trending Petrel subbasin to the south.

The Malita graben contains a thick section in the late to recent Mesozoic and probably is underlain by a Paleozoic section possibly as thick as 8,000 m.

The Sahul platform on the northern flank of the graben is an area of elevated Mesozoic and Paleozoic sediment that has northeast-southwest trending Mesozoic faulting parallel to and toward the graben.

The Petrel subbasin is a Devono-Carboniferous rift that has Paleozoic and Mesozoic sediments estimated to exceed 10,000 in. The subbasin is separated from the Malita graben by a northeast trending faulted hinge line, downfaulted to the northwest.

Exploration in the area started in the mid-1960s. About 420 line km of seismic surveys have been conducted over the two areas, where three wells have been drilled on currently available tracts.

Main potential reservoirs include the Jurassic Plover formation, Flamingo group sandstones, and Cretaceous Bathurst Island group sandstones. The Plover covers a wide area in the region but is expected at depths below 5,000 m in the graben and may be too deep for a primary reservoir due to diagenetic alteration. Early migration of hydrocarbons into Plover sands, however, may have inhibited the diagenetic processes.

The Flamingo turbidite sandstones, such as Sandpiper, shed from the Sahul platform to the north and Darwin platform to the east, providing potential targets on the tracts.

Cretaceous sands in the Bathurst Island group are shallow marine to marginal marine and provide potential as reservoir targets as do sandstones in the overlying Paleocene section. These sands should be well developed, and a thicker section should be found within the Malita graben.

The main seal in the area is a massive claystone and siltstone section at the base of the Bathurst Island group. Bathurst Island thickness increases toward basin depocenters, like the Malita graben, so the risk of seal breaching should be reduced in these areas.

There are three potential source rocks across the tracts.

The Plover has intervals with fair to good oil source potential over a large area. Total organic carbon values indicate the best source beds are in the Petrel subbasin to the south. Flamingo sediments show good to excellent source potential, mainly in the Petrel subbasin and Malita graben. The Bathurst Island group also has good source potential.

ASHMORE/CARTIER

Tract AC93-3 includes 120 blocks covering about 10,076 sq km about 300 km off Western Australia along the northern end of the Northwest Shelf in the Timor Sea. Water depths range from less than 200 m in the east to more than 500 m in the west.

The tract crosses the northern part of the Browse basin. It is bounded on the east by faults that delineate the Prudhoe Terrace and bounded on the west by northern extensions of the Scott reef and Buffoon highs, which are faulted and uplifted Mesozoic and possibly Paleozoic sediments overlain by fairly thick Cretaceous and Tertiary clastic and carbonate sequences.

Several reconnaissance geophysical surveys conducted in the 1970s cover the area, and infill surveys were conducted in the late 1970s and 1980.

A well drilled in 1978 in the southern part of the area cut an upper Cretaceous objective section on a faulted anticline, reaching total depth at 2,706 m in Albian sediments. A trace of oil scum was noted in blender mud while drilling at 2,185 in. Campanian sandstones averaged 83 m thick with 18% porosity.

A well drilled in 1987 in the northeast part of AC93-3, on the southeast margin of the Ashmore platform, found fluorescence in sidewall cores of Cretaceous and Triassic sandstones. Cretaceous porosities ranged from 10% to 30%, Triassic less than 10%.

The western and central parts of the area have at least three potential leads, with additional potential in the southern part of the Vulcan subbasin to the east. Jurassic and possibly Cretaceous sediments are main potential source sequences.

A partly faulted closure evident in 1976-79 seismic data represents the largest potential play. However, there is a risk of poor quality Jurassic and Cretaceous reservoirs 4.5-5.5 km deep at the possible crest of the structure.

WESTERN AUSTRALIA

Tracts W93-6 through W93-8 lie on the Northwest Shelf about 175-230 km north-northeast of Dampier and cover 48, 38, and 28 blocks, respectively. Water depths range from 100 m in the southeast to more than 1,200 m in the northwest.

The areas overlie the central and western parts of the Beagle subbasin, the northernmost Mesozoic depocenter of the Carnarvon basin. The subbasin is bounded on the east by the Canning basin and on the southwest by shallow basement areas of the Lambert shelf and De Grey nose.

While the northern boundary of the Beagle subbasin is poorly defined, it is bounded on the southwest by the Dampier subbasin and on the west by the Rankin platform, both proven hydrocarbon areas.

It is estimated that the main depocenter of the subbasin has more than 10,000 m of Mesozoic to Cenozoic sediments, underlain by an unknown thickness of Paleozoic sediments.

Structurally, the areas are dominated by a series of broad north-northeast trending uplifted fault blocks separated by structural lows.

The areas are covered by a broad regional grid of variable quality seismic data acquired between 1965 and 1978, and there is some post 1978 seismic coverage over all the areas.

Area W93-6, in the deeper water region of the outer Beagle subbasin, is considered the most underexplored in the subbasin. It overlies the northern Sable and Swift blocks, parallel, north-northeast trending uplifted fault blocks separated by narrow, faulted structural lows.

Main exploration targets are reservoirs subcropping the late middle Jurassic main unconformity on the uplifted fault blocks. The blocks dip to the north and are draped and onlapped by a late Jurassic to Aptian marine claystone seal. Early to middle Jurassic potential source rocks may be present in adjacent structural lows. One well has been drilled on the tract.

Area W93-7 is dominated by southern extensions of the Sable and Swift blocks.

Parallel bedded intervals of early to middle Jurassic fluvial sandstones in the rotated fault blocks are the main exploration targets. Potential seals are intraformational claystones in the fault blocks or marine claystones that drape the underlying faulted structures.

Plays include anticlines downdip and marginal to the main fault blocks and potential marginal fan systems. Middle to early Jurassic source rocks may be present in structural lows, however migration systems from faulted lows to adjacent structures are expected to be complex. There are no wells on the tract.

Three wells have been drilled in area W93-8, which is in the central part of the subbasin overlying the Ronsard block, a north-northeast trending intrabasin fault block. The southern part of the area overlies the Cossigny trough, a narrow and elongate late Mesozoic depocenter between the Ronsard block and the northern margin of De Grey nose.

Main reservoir targets are middle Jurassic fluvial sandstones and late Jurassic to early Cretaceous marine sandstones. Potential seals are Neocomian to Aptian marine shale. Main source rocks are a thick section of early to middle Jurassic claystone known locally as Picard shale. Potential traps include rotated Triassic to Jurassic fault blocks related to the Ronsard block and submarine fans systems along the margins of the block and Cossigny trough.

SOUTH AUSTRALIA

Areas S93-1 with 392 blocks and S93-2 with 380 blocks are in the Ceduna subbasin of the Bight basin in water 100-1,500 m deep. Both areas extend from the continental shelf over the Ceduna terrace to the lower continental slope.

The Bight basin is part of the Mesozoic southern Australian rift system that includes several extensional and transtensional subbasins. The Ceduna subbasin, the main depocenter, holds at least 10,000 m of Jurassic to Neogene sediment.

First seismic data in the area were recorded in the late 1960s, and more recent coverage includes data from 1989.

Lower Cretaceous reservoirs are variable, ranging to as much as 30 m thick with porosity as high as 25%. Upper Cretaceous reservoirs are as much as 50 m thick with porosity as high as 30%.

Adequate source rocks should be present in the deeper parts of the subbasin, and the main seal is upper Cretaceous Wigunda.

OTWAY BASIN

Area S93-3 lies in the South Australia sector of Otway basin and includes 80 blocks in water generally less than 200 m deep.

No hydrocarbons have been recovered from the part of Otway basin off South Australia, but there have been commercial gas discoveries off Victoria in the basin, east of S93-3.

The area has more than 3,000 m of early Cretaceous and 5,000 m of late Cretaceous clastic sediments with many potential source rocks and reservoirs. Early Cretaceous and older rocks have not been fully evaluated in the area.

Area S93-3 is almost completely within the Voluta trough, which has as much as 5,000 m of late Cretaceous Sherbrook sediments overlain by a Tertiary section that ranges from about 300 m in the northwest to an estimated 2,500 m in the southeast.

Since 1963 about 4,900 line km of seismic data have been acquired in area S93-3.

A well drilled on the tract by Esso in 1968 found residual bitumen. A second well was drilled in 1984. Gas shows were noted while drilling the Waarre sandstone, but the zone could not be logged or properly tested due to engineering problems.

A third well, drilled in 1989, cut good quality reservoir rocks in late Cretaceous, but no significant shows were noted.

Nine wildcats have been drilled adjacent to the area onshore, with several recorded hydrocarbon shows in early and late Cretaceous rocks.

Five prospects identified on S93-3 are large enough to hold commercial volumes of hydrocarbons. Four are in the Voluta trough, and one is on the Chama terrace.

TASMANIA

The T93-1 area covers 109 blocks in the northern part of the Bass basin. T93-2 includes 114 blocks at the southern end of Bass basin and includes a large part of Durroon subbasin. Water on both tracts is less than 80 m deep.

Bass basin is a late Mesozoic-Cenozoic intracratonic basin that trends northwest-southeast between Victoria and northern Tasmania. The basin consists of as much as 16 km of nonmarine and marine sediments and late Jurassic to recent volcanics. Durroon subbasin has similar structural and stratigraphic features to Bass basin, however rifting in Durroon occurred in late Cretaceous, while Bass basin rifting occurred in early Cretaceous.

Main targets under both tracts are upper Eocene Eastern View interbedded sandstone, siltstone, mudstone, coal, and volcanics. The group contains suitable source rocks. Several noncommercial discoveries such as Pelican gas/condensate field and Yolla oil and gas/condensate field have been drilled in the Tasmanian part of the basin.