PROSPECTIVITY OF AREA AC93-3 ON AUSTRALIA'S NORTHWEST SHELF

Ian H. Lavering
Bureau of Resources Sciences
Canberra

During the last 10 years, the Australian government has regularly released acreage on the continental shelf for exploration. The intensity of exploration has increased dramatically during this time, but many parts of Australia still remain relatively under-explored.

A little over 1,100 exploratory and development wells have been drilled on the Australian continental shelf the past 30 years a small number by world standards and especially considering the Australia's offshore sedimentary basins cover about 12 million sq km.

There remain good prospects for the discovery of oil and gas reserves in Australia, with the largest discoveries expected to be made on the continental shelf. Many of the areas recently released for bidding are under-explored and have good potential.

This article examines one such area. Area AC93 3 lies largely in the Browse basin in the Timor Sea between Australia and Indonesia.

REGIONAL SETTING

Significant opportunities for evaluation of Mesozoic plays are available in Area AC93 3, recently released in the second round of Australian offshore exploration bidding for 1993.

The Browse basin contains more than 6,000 m of Mesozoic section, and four major but as yet undeveloped gas fields have been discovered in Triassic and jurassic reservoirs 3,000 4,500 m deep.1 Water depths of 50 600 m are present over much of the basin.

Oil has been recovered from Cretaceous reservoirs.1 Fewer than 30 wells have been drilled in an area occupying more than 100,000 sq km and that contains the largest single gas field in Australia, Scott Reef, with reserves of several trillion cubic feet (Fig. 1).

Area AC93 3 overlies part of the northern sector of the Browse basin where Permo-Triassic fluvio deltaic and shallow marine clastic sequences were block faulted during the Late Triassic into a series of northeast southwest trending tectonic elements. Early to Mid jurassic clastics deposited in fluvial and deltaic settings (Plover formation) onlap and engulf the older block faulted sequences and are oil bearing in Skua field of the Vulcan sub basin, north of the Browse basin (Fig. 2).

A Late Jurassic sequence with lower and upper boundaries defined by Callovian and Valanginian unconformities contains most of the likely oil generating source rocks in the Browse basin and Vulcan sub basin.

The early part of this sequence fringes major graben margins and contains prograding fan delta reservoir units.2 These reservoir units pass laterally into restricted marine, low energy mud-rock facies that are potential source sequences.

The latter part of the Late Jurassic sequence was associated with volcanic active and resulted in deposition of submarine fan systems around the margins of exposed structural "highs."2 Post Valanginian regional sag and some significant local subsidence resulted in transgression across the Browse basin and deposition of thick Early Cretaceous claystone sequences. Sufficient subsidence of the western part of the basin had taken place by the Cenomanian for full oceanic circulation to develop and the formation of a shelf/slope system.2

Differential rates of subsidence and fluctuating sea levels during the Tertiary led to erosion at some locations and extensive deposition in others. The present morphology of the continental shelf and slope was established since the Miocene. Despite the effects of post-Miocene subsidence and sea level rise, outer shelf reef complexes have developed near the Late Miocene shelf edge.

Area AC93 3 alone comprises a significant part of the northern Browse basin, 120 graticular (5 by 5 min) blocks or approximately 10,076 sq km, some 300 km northwest of the coast of Western Australia. Water depths range from less than 200 m in the east to more than 500 m in the west.

Area AC93 3 is bounded to the east by down to the-basin faults delineating the Prudhoe terrace, and to the west by northern extensions of the Scott reef and Buffon "highs." The latter comprise faulted and uplifted Mesozoic and possibly Paleozoic sediments, overlain by relatively thick Cretaceous and Tertiary clastic and carbonate sequences (Figs. 3, 4).

To the north of Area AC93 3 is the drape anticline feature tested in part by the 1B Mt. Ashmore well drilled in 1980 which is notable for the presence of Triassic and older sediments in the lower part of the well. These Triassic sediments are similar to those evident in wells drilled on the Ashmore platform. The Triassic sequence in 1B Mt. Ashmore is overlain by a Lower Middle and Upper Jurassic sequence that thickens off structure, and thick Cretaceous and Tertiary sediments that are truncated, in part, by two unconformities.1

North of the Mt. Ashmore feature the Browse basin is bounded by faults along its northern margin with the Ashmore platform (Fig. 5). The Ashmore platform comprises large uplifted blocks of Triassic sediments and some jurassic volcanic sediments, and possibly lava flows (1 Ashmore Reef), overlain by Cretaceous and Tertiary clastic and carbonate sequences.

The 1 Gryphaea and 1 Asterias wells were drilled on features located on the margin of the Ashmore platform and an extension of the Vulcan sub basin, near the eastern limits of Area AC93 3. Considerable downfaulting of Permian and Triassic sequences is evident where the Vulcan sub basin and Browse basin join.2

Early to Mid jurassic sediments present near the feature drilled by 1 Asterias do not notably thicken in the down faulted adjacent Heywood graben. An expansion in area of Late Jurassic sedimentation (Callovian to Kimmeridgian) took place prior to a period of major faulting that formed many of the major tectonic elements in the Vulcan sub basin.

Along the eastern margin of Area AC93 3, two deep northeast-trending grabens are evident in the Vulcan sub basin. The Late jurassic to Early Cretaceous Upper Vulcan formation is widespread in the Vulcan sub-basin near Area AC93 3.

The widespread nature of this unit is a feature favorable to the petroleum potential of this part of the sequence as it has a proven potential for developing fault scarp related plays such as submarine fans and fan delta systems, such a those identified in the 1 Montara and 1 Bilyara discovery wells to the northeast of Area AC93 3.

The post jurassic sequence comprises fine clastic and calcilutite of Early Cretaceous age and Late Cretaceous and Tertiary carbonate that envelope all parts of Area AC93 3, as well as the Vulcan sub basin and the Ashmore platform to the north of AC93 3.

Thick sequences of the Late Cretaceous and Tertiary carbonates are features of the Browse basin (including Area AC93 3) and have developed as a result of major subsidence since the Early Cretaceous. In the central Vulcan sub basin and Ashmore platform to the north, however, these sequences are considerably thinner but their significance as a guide to the subsidence history of the region is notable. The thickness of such sequences, since the Early Cretaceous, serves as a guide to the longevity of potential migration paths and continuity between potential generative source areas and traps.

One exception to the typically significant thickness o Late Cretaceous is evident across the Mt. Ashmore feature, north of Area AC93 3. Here the Jurassic sequence onlaps and thins across the crest of the feature. During the Early Tertiary the structure was a local "high" and much of the Cretaceous sequence was eroded.1

Since the Miocene, however, the Mt. Ashmore feature has subsided more than surrounding areas and now represents a regional "low" in the bathymetry of the northern Browse basin. Such reversals in subsidence rates and the contrast with sedimentation patterns within the basin provide an opportunity to pursue new play concepts as well as following up successful plays identified in the Vulcan sub basin.

PROSPECTIVITY

Area AC93 3 was covered with parts of several reconnaissance surveys in the 1970s but the 1976 Tessa Troubador, 1977 Haycock-Laminaria, and other infill surveys undertaken in 1979 and 1980 provide the most valuable information on potential leads.

The 1 Bassett well was drilled in 1978 in the southern part of Area AC93 3 in 364 m of water. The well tested an Upper Cretaceous objective section in a faulted anticline and reached a total depth of 2,706 m, where it terminated in Albian sediments. Formations penetrated appear to be normally pressured, and while drilling at 2,185 m a trace of oil scum was recorded on the blender mud. Campanian sandstones have an average net thickness of 83 m and porosity of 18%; no other potential reservoirs were intersected in the Cretaceous target interval.3

The 1 Gryphaea well was drilled in 1987 on a major tilted fault block in the northeast of Area AC93 3 on the southeast margin of the Ashmore platform where it adjoins the northern Browse basin. The primary objective of the well was Late Triassic sandstones and Aptian calcarenites were the secondary objective. Gas readings during drilling were low to moderate over most of the sequence penetrated; Maastrichtian silty sandstone generated traces of butane in the drill mud. Fluorescence was evident in sidewall core samples from Creatceous and Triassic sandstones. Porosity readings in the Cretaceous were 10 30%, in the Triassic less than 10%. No formation tests were undertaken. Results of the well indicate that at this location reservoir quality in the Triassic is limited and the prospect lacks connection with mature source sequences. A prospect with mature jurassic source and adequate reservoir in the Browse basin or Vulcan sub basin could have an improved chance of success.3

The 1 Asterias well drilled in 1987 tested Cretaceous and Late Jurassic sediments in a horst block that underlies the eastern part of AC933, in the southern Vulcan sub basin. Porosity readings in the Cretaceous were 12 and the Late Jurassic 15%. A peak drill mud gas reading of 17% was recorded with a pressure kick in the Tithonian sequence. Post drill interpretation suggests the well may have intersected a saddle between two closures and may have some additional potential for similar prospects.3

The 1B Mt. Ashmore well was drilled in 1980 about 25 km north of Area AC93 3 on a large closed, drape anticline. An eroded and condensed Jurassic sequence has a porosity of 17% while porosity in the Triassic is 10%. Fluorescence is evident in the Lower Cretaceous and Jurassic, but the feature has been interpreted to be too young to trap hydrocarbons. Significant potential could be inferred for the more extensive and potentially mature Jurassic sequence downflank of the crest or within the central part of Area AC93 3.3

The three wells drilled in the southern part of the Vulcan sub basin and the eastern Browse basin margin, 1 Discorbis, 1 Kalyptea, and 1 Productus, highlight the potential of jurassic and Cretaceous source sequences in the Vulcan sub basin. The apparent willingness of permittees in ACP 3 to undertake additional exploration for these plays suggests some potential for similar plays in the eastern part of Area AC93 3.3

Existing seismic data quality over Area AC93 3 is moderate at best with much of it acquired and processed in the period from 1976 79. At least three potential leads are evident in the central and western parts of Area AC93 3, with potential for additional leads in the southern part of the Vulcan sub basin in the east. Main potential source sequences are the jurassic and possibly the Cretaceous. The largest lead identified to date is a partly faulted closure evident on 1976 79 seismic data. The greatest risk with such a lead is the quality of jurassic and Cretaceous reservoirs at depths of 4,500-5,500 m at the possible crestal location of the feature. However, as much of Area AC93 3 lies in 200 400 m of water, this together with a Cainozoic section up to 3,000 m thick, suggests that the present depth of burial of the Creatceous jurassic sequence in Area, AC93 3 would not rule out the preservation of good quality reservoir sands.

Cretaceous and Jurassic closures have the most evident potential in Area AC933, either within the Browse basin proper or the southern extension of the Vulcan subbasin in the eastern part of Area AC93 3. "Hour glass" features typical of the Jabiru terrace in the northern part of the Vulcan sub basin are known to be present and provide considerable scope for additional plays as well as plays currently known in other arts of the Bonaparte basin.2

Additional potential exists to identify Browse basin gas plays typical of accumulations previously discovered in the basin at Scott reef and other major regional high trends.1

CONCLUSIONS

Significant opportunities exist for the delineation and definition of drillable prospects in the "frontier" acreage within Areas AC93 3, in the northern part of the Browse basin and southern Vulcan sub basin on Australia's Northwest shelf.

Considerable scope exists for the presence of a mature source rock sequence, particularly the Late Jurassic, within Area AC93 3. The quality of jurassic and Cretaceous reservoir rocks is good given their present burial depths of 2,500 4,500 m (subsea), although a significant part of these depths is due to coverage by 300 400 m of water. The thickness of Cretaceous and Tertiary carbonates shows a general continuity in subsidence pattern and could be used to identify likely migration pathways from potential jurassic source areas.

Considerable scope exists for the development of new exploration concepts and the application of concepts that are successful in other parts of the Vulcan sub basin.

ACKNOWLEDGMENTS

Alan Williams and Russ Temple of the Bureau of Resource Sciences and Geoff Furnell and Brian Kay of the Petroleum Division, Department of Primary Industries and Energy, reviewed the article prior to publication. Brian Pashley of the cartographic services unit of the Australian Geological Survey Organisation supervised drafting of the figures.

REFERENCES

Willis, I., Results of exploration, Browse basin, North West Shelf, Western Australia, in Purcell, P.G., and R.R. (eds.), The North West Shelf, Australia: Proceedings of Petroleum Exploration Society Australia Symposium, Perth, 1988, pp. 259 272.
Patillo, J., and Nicholls, P.J., A tectonostratigraphic framework for the Vulcan graben, Timor Sea region, APEA journal, Vol. 30, No. 1, 1990, pp. 27 31.
Cadman, S.J., Connolly, Passmore, V.L., Maung, T.U., West, B.G., Blevin, J.E., Miyazaki, S., Vuckovic, V., Stephenson, A.E., Resiak, E., Staunton, J., Jung, P., and Ransley T., Browse basin petroleum prospectivity study, Bureau of Mineral Resources, Geology and Geophysics, Australia, Record 1991/83.