Scott Reef Research Articles

A new genus of small coral-reef goby (Teleostei: Gobiidae) from the Indo-west Pacific, with discussion of its relationships

A new genus and species, Minysicya caudimaculata, is created for a small coral reef gobiine known so far from the Yaeyama Islands, Ryukyu Archipelago, Japan; Scott Reef off northern Western Australia; Lizard Island, Queensland; and the Tuamotu Islands, French Polynesia. Phylogenetic analysis shows that Minysicya n. gen. forms a monophyletic group with five other genera including Bryaninops and Pleurosicya (commensal on various invertebrates and plants), but has fewer of the specialisations exhibited by the other genera.

Island-generated internal waves at Scott Reef, Western Australia

Oceanographic field studies were carried out, in August–October 1993, of the water circulation around Scott Reef, a 39 km wide island rising nearly vertically in 500 m depth on the outer continental shelf slope of Western Australia. Macro-tide prevailed and generated 0.6 m s -1 tidal currents. Internal waves 60 m peak-to-trough occurred around Scott Reef mainly at the semi-diurnal frequencies. A numerical model and the field data suggested that the internal waves were locally generated by the interaction of the tidal currents and the bathymetry. Also these waves were predicted to rotate counterclockwise around the island with maximum amplitude along the island slopes, while radiating energy as free internal waves propagating freely away in the open ocean. They were generated apparently without eddies forming in the lee of the island. The wave frequencies were not restricted to the diurnal and semi-diurnal tidal frequencies which dominated the sea surface fluctuations, indeed higher frequencies were important. The energy at the inertial frequency was negligible. It appears that oceanic islands are internal wave generators and may contribute to the oceanic variability even far away.

Periodicity of spawning aggregations of coral trout Plectropomus leopardus (Pisces:Serranidae) on the northern Great Barrier Reef

I investigated seasonal, lunar and diel patterns in the spawning behaviour of a serranid, the common coral trout Plectropomus leopardus, on the Great Barrier Reef, Australia, using underwater visual census surveys. The study was conducted at Scott and Elford Reefs, 2 mid-shelf reefs off Cairns, for 4 and 3 yr, respectively. At each reef, coral trout aggregated and spawned at the same 'primary' aggregation site in each year. Primary sites are defined as those with the largest aggregations of coral trout. Spawning activity, though not limited to, was concentrated at primary sites. The aggregation site at Scott Reef measured 1700 m² in area, and that at Elford Reef 3200 m². Maximum numbers of coral trout recorded in these sites were 128 fish at Scott Reef and 59 at Elford Reef. Coral trout aggregated and spawned at the primary sites around 3 consecutive new moons each year within the period August-December, 1991 to 1993. In 1990 only 2 aggregations were detected at Scott Reef, both around the new moon. The largest aggregations were recorded at the primary site at Scott Reef: numbers of fish at the aggregation site rose from an average pre-spawning density of 3.9 fish 1000m⁻² to aggregation densities ranging from 37.1 to 75.3 fish 1000m⁻². The median size class of aggregating fish was 41 to 45 cm FL (forklength) at both reefs in all years, with a maximum range of 16 to 80 cm FL at Scott Reef, and 16 to 65 cm FL at Elford Reef. The onset of the spawning season corresponded with a rise in water temperature (>24.00°C) after the austral winter. Spawning aggregations occurred for an average of 5 d, however aggregations were not found at the sites throughout the day. The aggregations appeared to disperse in the morning and re-establish after 13:00 h. Coral trout spawned in pairs predominantly on flooding tides and when current flow at the spawning sites was minimal. Ninety-four spawning rushes were observed which only occurred during a 33 (±4 SE) min period spanning sunset. The spatial and temporal predictability of P. leopardus spawning aggregations makes them vulnerable to overfishing, but also amenable to specialised management through seasonal and/or spatial closures.

Results of Exploration in Browse Basin, Northwest Shelf, Western Australia: ABSTRACT

The Browse basin lies entirely offshore beneath the remote northwestern continental shelf of Australia. It occupies an area of approximately 39,000 mi2 (100,000 km2), most of which lies in water depths exceeding 650 ft (200 m). The basin originated during the Paleozoic as a broad, intracratonic downwarp, which was considerably modified by tectonism association with continental break up during the Jurassic. After passing through restricted rift-basin and rim-basin phases during Early-Middle Jurassic and Late Jurassic-Early Cretaceous times the basin was opened to oceanic circulation in the Late Cretaceous, when its western margin subsided. Prograding Tertiary shelf carbonates subsequently covered the old basin and formed the present continental shelf. /P> Exploration of the basin and environs began in 1964 when license areas totaling some 63,000 mi2 (164,000 km2) were awarded to the Northwest Shelf Joint Venture, a consortium of companies presently comprising the Woodside Group, together with Hematite Petroleum, Shell Development Australia, California Asiatic and BP Petroleum Development Australia. During the 11-year period to 1975, some 13,700 mi (22,000 km) of seismic were shot and 11 exploration wells drilled, resulting in the discovery of gas at Scott reef. At the end of the initial permit period some 62% of the total area was relinquished. The remaining permit areas, totaling 24,162 mi2 (62,579 km2) were renewed for the first renewal period of 5 years. During this renewal period a furt er 4,100 mi (6,700 km) of seismic were acquired and 8 wells drilled, resulting in 2 additional gas discoveries (Brecknock and Brewster) and one encouraging oil show (Caswell). In 1980, following statutory relinquishment, remaining permit areas, totaling 10,842 mi2 (28,083 km2), were renewed for a further 5-year period. Initial obstacles to exploration were water depth and the considerable thickness of multiple-generating Tertiary carbonates covering much of the basin. These have been largely overcome by advances in drilling technology and seismic data processing. Lost circulation zones and geopressured claystone intervals continue to cause problems with drilling. Geochemical and maturation studies indicate that mature source rocks are present in the central basin, parts of which have been generative since the Late Cretaceous. Structural analysis suggests that the timing of trap formation is favorable with respect to likely hydrocarbon migration throughout much of the central basin, but is less favorable in the north, where a very late phase of structural growth is evident. End_of_Article - Last_Page 958------------

A REGIONAL INTERPRETATION OF THE BROWSE BASIN

The Browse Basin, off the northwest coast of Australia, originated as an intracratonic basin. It resulted from tensional movements and developed into an Atlantic-type continental margin. Sedimentation, which commenced in the Late Carboniferous and extended through the Middle Jurassic, is confined mostly to a linear, northeast-trending depocentre located between the onshore Kimberley Block and the Scott Plateau to the west. This sedimentary series is characterised by rift valley type deposits laid down between successive episodes of tectonism. Tectonism near the end of the Middle Jurassic was associated with breakup of the continent along what developed as the seaward edge of the Scott Plateau. This geological event marks an important change in the depositional environment of the basin from pre-breakup, mainly paralic and fluviodeltaic to post-breakup, transgressive marine conditions. The post-breakup series was formed under relative tectonic quiescence, except for some local structural rejuvenation and regional subsidence of the outer continental margin which eventually resulted in the deposition of Cretaceous and Tertiary sediments across the Scott Plateau. A fully open marine environment, however, was not achieved basinwide until during the Late Cretaceous, when the Scott Plateau had subsided sufficiently to allow unrestricted ocean circulation. Evolution of the basin was completed by a seaward prograding, mainly carbonate Tertiary wedge.Geophysical data show several sub-parallel, structurally high Mesozoic trends, oriented towards the northeast. The Scott Reef gas/condensate discovery lies on one such trend. The structural elements were interpreted as being mostly initiated prior to breakup by episodes of block faulting which produced a horst and graben topography. The horst blocks were subsequently onlapped and draped by the post-breakup sediments. The hydrocarbon accumulation at Scott Reef is restricted to the horst block reservoirs, but hydrocarbon indications elsewhere within the basin in the post-breakup series suggest that accumulations could also exist in this section.

Distribution of Cainozoic foraminiferida in the Scott Reef No. 1 well, Western Australia

Abstract Micropalaeontological investigation of samples from Scott Reef No. 1 has revealed a thick Cainozoic carbonate sequence. Rich planktonic faunas have enabled the identification of Lower Miocene to Oligocene (N6 to P19) and Lower Eocene to Lower Palaeocene intervals (P6 to Pic). The remainder of the succession has been dated on benthonic evidence and spot age‐determinations on isolated planktonic occurrences. The palaeoenvironmental history of the Cainozoic sequence appears to be regressive from a bathyal situation in the Palaeocene through indeterminate marine Eocene, bathyal to inner shelf Oligocene to Middle Miocene, to reefal conditions which have persisted from the Middle Miocene to the present day.

PETROLEUM IN WESTERN AUSTRALIA: A REVIEW OF EXPLORATION, PRODUCTION, AND FUTURE PROSPECTS

Modern petroleum exploration has been in progress in Western Australia since 1952, and has been concentrated mainly in the Perth, Carnarvon, Canning, and Bonaparte Gulf Basins. Two large onshore fields have been developed, the Barrow Island oilfield in the Carnarvon Basin (found in 1964), and the Dongara gasfield in the Perth Basin (found in 1966). Small gasfields have also been developed at Mondarra, Gingin, and Walyering in the Perth Basin, but Gingin and Walyering are now virtually depleted.Major gas-condensate fields have been found offshore. These are the North Rankin, Goodwyn, West Tryal Rocks, and Angel fields in the northern Carnarvon Basin, and the Scott Reef field in the Browse Basin. They were found during the period 1971 to 1973, but none has yet been developed.Since 1968 the accent has been on offshore exploration, and this reached a peak in 1972. Exploration activity, both onshore and offshore, is currently declining, owing to the lack of recent success and the unfavourable exploration climate prevailing in Australia today.Original reserves in the Dongara gasfield amounted to about 13 billion cubic metres, of which nearly 2.1 billion have now been produced. Current gas production from Dongara and the small adjoining Mondarra field is about 2.2 million cubic metres per day, and production will continue at about this rate until 1981, after which it will begin declining. Production will fall steeply in 1987, when existing contracts expire. At that time about 90% of the reserves will have been depleted.The original in-place reserves of the Barrow Island oil-field amounted to some 750 million barrels, and it is expected that about 240 million will be recovered. Current oil production is around 37,000 barrels per day, compared with the peak of 48.000 barrels per day reached in 1970. Nearly 43% of the original reserves have now been produced.Total reserves of the major fields in the offshore northern Car-narvon Basin (in the proved and probable categories) are more than 345 billion cubic metres of gas and 320 million barrels of condensate. Of these amounts more than 220 billion cubic metres of gas and 180 million barrels of condensate are in the North Rankin field, which is the largest gasfield in Australia and is a giant by world standards. This is followed by Goodwyn (about 65 billion cubic metres of gas and 90 million barrels of condensate), West Tryal Rocks (more than 30 billion cubic metres of gas) and Angel (about 30 billion cubic metres of gas and 50 million barrels of condensate).Further drilling will be required before gas reserves of the Scott Reef field can be estimated, but the results of the first well and the size of the structure indicate that they could be very large. It is clear that future exploration in Western Australia will be mainly concentrated offshore, in the Carnarvon, Browse, Bonaparte Gulf, and Perth Basins. However, there are still some prospective onshore areas in the Perth, Carnarvon, and Canning Basins.The chances of finding giant oilfields in Western Australia have declined markedly in recent years, as It seems that the generative sequences are mainly gas prone, and most of the obvious structures have now been drilled. However, the prospects are good for further large gas discoveries, and there is a reasonable chance that significant oil reserves will also be found.

PLATE TECTONICS AND THE EVOLUTION OF THE TIMOR SEAR NORTHWEST AUSTRALIA

The Browse and Bonaparte Gulf Basins in northwestern Australia contain sedimentary sections which range in age from early Palaeozoic to Recent. These basins developed as a result of the rifting and break up of Gondwanaland into two continents. The sedimentary and tectonic histories of these basins clearly reflect the relative movements of these continents.In pre-Permian times, the earliest postulated rifting may have been early Palaeozoic associated with the north-south separation of Gondwanaland from Eurasia which produced the Tethyan Sea. This was followed by Middle Palaeozoic rifting which affected the Bonaparte Gulf Basin, Fitzroy Graben and perhaps also the Browse Basin.A Late Carboniferous-Early Permian rift valley developed between the Kimberley Block and a western landmass. Alternate marine and fluvio-deltaic deposits characterised sedimentation in this rift valley until Early Jurassic time. Marine transgressions inundated from the north where the rift valley opened into the Tethyan Sea.During the Late Jurassic and Early Cretaceous, the western landmass was detached from the Australian continent along a major right lateral wrench fault. At this time the Indian Ocean Basin appeared as a distinct morphological unit and inundated the Browse Basin. Associated with these movements were the emergence of the Ashmore-Sahul Block and Northeast Londonderry Ridge and the development of the Scott Reef-Buccaneer trend. Marine volcanics at Scott Reef and Ashmore Reef may be related to the formation of new ocean crust.At the end of the Early Cretaceous, tectonic events associated with the complete separation of the western landmass from Australia, resulted in a regional northwesterly tilt of the basin and a major marine transgression.During the Tertiary, Australia moved north, away from Antarctica, and eventually collided with the Southeast Asian and West Pacific oceanic plates. Timor uplifted as a Tertiary melange of Australian sediments behind a north-dipping sub-duction zone along the Timor Trough. The Timor Sea remained relatively stable and was the site of carbonate shelf sedimentation.

Exploration on Continental Shelf Off Northwest Australia

Development of the Northwest shelf area into a proven hydrocarbon-bearing province has been relatively fast and successful thanks to recent advances in seismic technology and to close geologic-geophysical coordination. Work to date has located several geologic provinces and subbasins. The Dampier subbasin is especially attractive because of thick sandstone reservoirs and excellent shale source rocks, particularly in the Jurassic and Triassic sections. The Rankin trend has three separate accumulations, with separate gas and/or oil-water contacts proved by the drilling of North Rankin No. 1, Rankin No. 1, and Goodwyn No. 1. Thick sandstone sections in these wells had a westerly source. Similar features and environmental conditions are expected to extend into the adjoining Beagle Trough. The Browse basin, offshore from the ancient Precambrian-Proterozoic Kimberley block, contains more than 30,000 ft of Tertiary, Cretaceous, Jurassic, and Triassic sedimentary rocks. The Scott Reef gas discovery is on the outer rim of this basin and source sands of the productive Jurassic-Triassic section are indicated to have come from the northwest. Many upthrust-block trends within the basin are untested. Twelve wells drilled in the offshore Bonaparte basin resulted in significant evidence of hydrocarbons, although reservoir quality was poor. Outer margins of the Bonaparte basin may have well-developed reservoir sands, possibly from a northern source. The only area which lacks evidence of hydrocarbons is the offshore Canning basin; 40 wells have been drilled onshore without success or good evidence of hydrocarbons. The offshore Canning area is, however, very large and little is known of the detailed pre-Jurassic stratigraphy. Much more exploration is required and possibly the deeper Paleozoic sediments will be demonstrated to be objective targets.

Exploration on Continental Shelf Off Northwest Australia: ABSTRACT

Exploration of a 144,000-sq mi rank wildcat area in offshore northwestern Australia began in 1964 with analog seismic surveys. Each year additional surveys have been carried out with a steady improvement in data quality due to advantage being taken of technological advances of the industry. Interpretation of these data and incorporation of available drilling results have permitted structural and depositional models to be rationalized for post-Paleozoic time-rock sequences. Improvements in drilling technology have allowed locations to be sited in greater water depths confirming depositional and structural models. End_Page 637------------------------------ To overcome seismic data problems associated with the high-speed Tertiary section, navigational problems due to the size of the area, drilling problems associated with geo-pressured shales and the highly porous Tertiary beds, new techniques constantly were required and introduced. To integrate the interpretation of the large volume of data and to apply the maximum amount of geologic rationalization require the cooperation and mutual support of competent geologists and geophysicists. Large areas to be mapped and steadily increasing volume of data from this area required the utilization of computer-assisted mapping. By mid-1971 approximately 40,000 mi of seismic data were in the files. At the same time wells 11 (Scott Reef No. 1) and 12 (North Rankin No. 1) were drilling and located what promise to be commercial accumulations of hydrocarbons. However, feasibility studies still are being evaluated and step-out wells have yet to be drilled. Since completion of these wells, other wildcats have been drilled resulting in several discoveries. Future potential in the area looks very bright with source, reservoir, and cap rocks in Triassic, Jurassic, and Cretaceous rocks. The potential of the Paleozoic section is as yet virtually unknown. End_of_Article - Last_Page 638------------

Cartier Furrow, a Major Structure along the Continental Margin of North-western Australia

A COMPILATION of recent aeromagnetic surveys in offshore north-western Australia1–4 (Fig. 1) reveals a furrow in the magnetic basement, which I propose to call the Cartier Furrow, after Cartier Island. The furrow is at least 1,250 km long, and 6 km deep, and is filled with sediment. It underlies the upper continental slope except where it crosses the Londonderry Rise, which is a spur in the continental shelf. The furrow is bounded on its south-eastern side by the offshore Bonaparte Gulf Basin in the north, by the offshore Canning Basin in the south, and, in between, by a prism of non-magnetic rocks which thin south-eastward towards the Pre-Cambrian Kimberley Block. The north-western side of the furrow is marked by the atolls of the Rowley Shoals, Scott Reef, and Seringapatam Reef, which rise from the continental slope; by the outer part of the Londonderry Rise, which is crowned by the reefs of Ashmore Island, Cartier Island and Hibernia Reef; and by the south-east slope of the Timor Trough. From a comparison with onshore surveys in the Bonaparte Gulf Basin, Veevers and van Andel5 identified the magnetic basement beneath the Sahul Shelf as either a layer or layers high in the Pre-Cambrian of the Kimberley Block, or Lower Cambrian volcanics. The magnetic basement in the landward part of the Canning Basin6, as shown by drill holes, is crystalline Pre-Cambrian rock (granite or metamorphics), so that the magnetic basement offshore probably coincides also with the surface of Pre-Cambrian rocks.