Australian North West Shelf Research Articles

Leeuwin current observations on the Australian North West Shelf, May–June 1993

Observations of the Leeuwin Current, the poleward flow of warm, low salinity water off the west coast of Australia, are described from acoustic Doppler current profiler (ADCP), temperature and salinity (CTD) and moored current meter measurements from the Australian North West Shelf made in May and June 1993. Two cross-shelf and slope transects of ADCP and CTD measurements were completed at latitudes of approximately 17 and 19S. After the removal of the tidal signal, the ADCP measurements reveal the poleward flowing Leeuwin Current. The current is broad (250 km at 17°S), deep (at least 440 m) and relatively weak (typically 0.2 m S −1), but the poleward transport of approximately 4 × 10 6 m 3 s −1 is similar to values reported in the literature for the more intense part of the Leeuwin Current, between approximately 22 and 35°S, where a maximum of approximately 7 × 10 6 m 3 s −1 has been observed. There is a significant flow reversal or undercurrent, of magnitude similar to the poleward transport, at the northern transect, but only a weak undercurrent is observed at the 19°S transect. In the CTD data the current is also seen as a low salinity core with salinity less than 35.2 but also with patches less than 35.0, possibly a result of a series of eddies. A current meter record from mid-way along the 19°S transect shows the poleward flow to be relatively persistent over all of May, but slightly stronger towards the end of May. There is only one significant reversal of flow towards the equator lasting a few days. Current meter data obtained between August 1983 and March 1985 from 120 m depth in the Timor Sea (approximately 12°S) are also reported and reveal a weak poleward flow for much of the year and strongest from January to April. For some of this time (March–April) the poleward flow is supported by the South East Trade winds. However, the poleward current weakens and turns equatorward in May/June to flow into the persistent winds from the east. Simultaneous current meter measurements from the shelf at 20°S show similar strength poleward flows from December to March.

A study of the design of inclined wellbores with regard to both mechanical stability and fracture intersection, and its application to the Australian North West Shelf

Knowledge of the in-situ stress field can be applied both in planning most stable drilling trajectories and also in maximizing intersection of teh wellbore with open, natural, and hydraulically-induced fractures in the reservoir. Inclined wells drilled with the optimum drilling direction (azimuth) and deviation (from the vertical), at which the shear stress anisotropy around the wellbore wall is minimized, may be more mechanically stable than vertical wells in various stress regimes. Such mechanically stable trajectories may also maximize open fracture intersection. Combining the objectives of maximum mechanical stability and maximum open fracture intersection • • wells in an extensional stress regime should be highly deviated from vertical (at about 55–70°) and drilled along the azimuth of the least horizontal princial stress; • • wells in a strike-slip stress regime should be horizontal and drilled at 55–70° with respect to the major horizontal principal stress; • • wells in a compressional stress regime should be slightly deviated from vertical (at about 20–35°) and drilled along the azimuth of the major horizontal principal stress. Application of this study to the Wanaea/Cossack field of the Australian North West Shelf, where the in-situ stress field has been constrained, suggests that the most stable drilling direction and that which maximizes potential intersection with any open fractures in the reservoir is horizontal, in the azimuth of the least horizontal principal stress, 005–010°N.

Observations of Internal Tide Propagation on the Australian North West Shelf

Abstract The propagation of internal tides (12-h period internal waves) is observed over a continental shelf break region on the Australian North West Shelf. The observed phase speeds of the waves, calculated from thermistor chain and current meter data, are shown to agree closely with simple linear theory for phase velocity Of freely propagating waves. Even over the relatively steep bathymetry of a continental shelf break region (ratio of bathymetric slope to slope of internal wave characteristics reaching 0.27), a simple modal model for phase speed is found to be accurate, providing that slowly varying depth is taken into account. The direction of wave propagation is determined from the orientation of baroclinic current ellipses derived from current meter data; a two-dimensional, horizontal field of phase information; and visual observations of surface slicks. 0bservations also demonstrate that substantial temporal variations in phase are a result of variations in the thermal stratification of the water...

A three-dimensional numerical model of the response of the Australian North West Shelf to tropical cyclones

AbstractA three-dimensional barotropic and baroclinic model is developed to simulate currents and temperature changes induced by tropical cyclones traversing the continental shelf and slope region of the Australian North West Shelf. The model is based on a layered, explicit, finite difference formulation using the nonlinear primitive equations with an embedded entrainment scheme; a mixed-surface-layer interface is defined, which is allowed to shift from one interface to another, depending on the strength of a storm. The model has been tested by simulating the currents and temperature changes induced by tropical cyclones Orson and Ian. The modelled currents and temperatures agreed well with the available measured records except near the seabed. It has been found that the pre-storm currents have very little influence on the peak of the storm-induced currents and the currents in the wake of a tropical cyclone. The model contained no coefficients which must be calibrated for a particular application and clearly illustrated the importance of the baroclinic effects on the storm-induced response over the North West Shelf of Australia.

Determination of Matrix Thermal Conductivity from Dry Drill Cuttings

This paper reports experimental data relating the observed dry-bulk thermal conductivities of powdered minerals to the porosities of the powdered aggregates. The mineral samples were obtained by crushing quartz, calcite, and feldspar crystals. Experiments also were conducted on clays from various sources. The experimental data suggest an exponential relationship between the dry-bulk thermal conductivity and the aggregate porosity. The thermal conductivities of the original quartz, calcite, and feldspar crystals were determined in a dry state to estimate their matrix thermal conductivities. An empirical relationship was determined among dry-bulk thermal conductivity, aggregate porosity, and matrix thermal conductivity. This empirical relationship provided estimates of matrix thermal conductivity that were consistent with theoretical mix formulae for 50:50 mixtures of combinations of the minerals quartz, calcite, and feldspar, and also a sample of Flag Sandstone from the North West Shelf, Australia. The method was used to estimate heat flows at the sites of two Australian North West Shelf wells: Dampier 1 and Tryal Rocks 1, which are 47.4 mW/m2 and 79.7 mW/m2, respectively. These values are less than sea floor estimates of heat flow in the same region, and suggest nonconductive heat-transfer processes within the North West Shelf sedimentary section.

Hydrocarbon Prospect Mapping Using Balanced Cross Sections and Gravity Modeling, Onin and Kumawa Peninsulas, Irian Jaya, Indonesia: ABSTRACT

Exploration field geology mapping and acquisition of gravity data has been conducted on approximately 650 line kilometres of26 surveyed traverse lines across the Onin and Kumawa Peninsulas of western Irian Jaya. Karstified New Guinea Limestone with a maximum thickness of 2,150 m is the predominant surface outcrop, and precludes use of seismic. However, integrated use offield geology data, balanced cross-sections, and gravity modelling has enabled us to identify two giant hydrocarbon prospects. The Onin and Kumawa Peninsulas lie at the margin of Jurassic age faulting associated with the Australian Northwest Shelf. Jurassic rift sands of the Lower Kembelangan Formation are the primary reservoir target. During the Plio-Pleistocene, collision of the Australian plate. and the Banda Arc inverted sections of the rift system, including the Onin and Kumawa Peninsulas. A better understanding of the regional structure was gained by integrating the Mobil Oil gravity data (of 1992) and that collected by Shell Oil (in the 1950's) in the structurally less deformed Bomberai region east ofOnin and Kumawa. Bouguer reduction was carried out using 2.4 glcm3 density, GRS 1967 and IGSN 1971. A sequence of gravity maps were generated, including Bouguer, regional, residual, downward continued, and second derivative. Spectral analyses indicate that basement is about 3 km depth at Onin and about 6 km in the Bomberai area. The Bomberai and Onin-Kumawa regions are separated from one another by a steep gravity gradient which has a SE-NW strike direction. This gravity gradient may represent a change of lithology. Prospect definition was obtained by evaluating traverse profile data. Balanced cross-sections were constructed, using detailed biostratigraphy to determine formation thicknesses and amount of fault offsets. Where possible, onshore balanced section profiles were tied to offshore seismic profiles and wells. Two dimensional forward gravity models were calculated, using formation densities from well data. The calculated profiles were then compared to profile observed values. Differences between calculated and observed profiles were resolved by adhering to the exploration model, which required that the main thickness changes would be in the Lower Kembelangan Formation rift section. Density values for the seven formations were held constant. Constraining the variables to one (Lower Kembelangan thickness) in the gravity profile modelling maintains credibility of the technique. Errors inherent in the structural maps derived from the cross­ sections are likely to be vertical shift which would be approximately constant across the prospects. The interactive work between geologists and geophysicists was effective in producing a logical representation of subsurface geology, which in turn allowed selection of drill sites with some degree of confidence. Subsequent to completing the modelling, offshore seismic data across the western plunge of the Onin Peninsula was obtained. The structural style demonstrated in the modelling and the seismic data are comparable. This enhances our confidence in the modelling results.

A Three-Dimensional Barotropic Model of the Response of the Australian North West Shelf to Tropical Cyclones

A three-dimensional barotropic model using a -coordinate or depth transformation, and forced with tropical cyclone wind and atmospheric pressure fields has been applied to the continental shelf and slope regions of the Australian North West Shelf. The atmospheric pressure field of the cyclone causes a nearly isostatic sea level response and weak currents. In an unbounded ocean the wind field would produce currents that circulate around the center of the cyclone and eject water, thus tending to reduce sea level. The coastline forms a boundary for the ejected water resulting in a large sea level rise and strong currents between the cyclone path and the coastline. The model provides a good description of the vertical current shear as observed at a number of moorings during two different cyclones. Sea level rises are accurately modeled and a reasonable description of observed current strengths is achieved. However, the predicted currents and sea level changes at offshore stations are sensitive to the track followed by the cyclone and often this not known with sufficient accuracy.

Stratigraphic correlation across an arc‐continent collision zone: timor and the Australian northwest shelf

The Kolbano area of southern West Timor comprises the subaerially exposed upper part of an imbricate stack at the leading edge of the Timor arc‐continent collision zone. Rocks of Permian to Plio‐Quaternary age are exposed in the Kolbano Complex. Detailed stratigraphic correlation between the Kolbano area and the Northwest Shelf of Australia (as recorded from petroleum exploration wells) demonstrates that the Kolbano area has close stratigraphic links with the Northwest Shelf, in particular with the Ashmore Block and Sahul Platform. Reconstruction of Jurassic‐Neogene palaeoenvironments for the Kolbano area indicates that the Kolbano area represents the outermost edge of the precollisional Australian passive margin. The region has been considerably shortened by Pliocene tectonism, but retains a predeformational distribution of facies with respect to Australia. The northern half of Timor is composed largely of Australian continental margin material. As this now lies to the north of the Kolbano area, interpretations of Timor involving primarily vertical crustal movements would seem to be inadequate. A new model for the evolution of Timor invokes a significant component of wrench faulting in the deformation.

A NORTH WEST SHELF TRIASSIC REEF PLAY: RESULTS FROM ODP LEG 122

Site 764 of the Ocean Drilling Program (ODP), drilled during Leg 122 in the Exmouth Plateau region, cored 200 m of Upper Triassic (Rhaetian) reef complex. This site, on the northern Wombat Plateau (northernmost Exmouth Plateau) represents the first discovery of Triassic reefal material near the Australian North West Shelf. Seismic reflection data through Site 764 show that the reef itself corresponds predominantly to a seismically poorly reflective zone. A number of regional unconformities appear to correspond, however, to traceable seismic horizons which pass with reduced amplitude through the reef, indicating stages of reef growth separated by erosion or non- deposition. Seismic facies around the edges of the reef are consistent with the deposition of wedges of prograding reef- derived detritus.Application of the seismic criteria for reef recognition established at ODP Site 764, to other seismic reflection data on the Wombat Plateau, demonstrates that a major Upper Triassic reef complex fringes the margins of the Wombat Plateau. The Wombat Plateau lies at the western end of the North West Shelf, which was part of the southern margin of a warm Tethys Ocean in the Late Triassic, at a palaeolatitude of 25- 30°S. Upper Triassic reefs are known from southeast Indonesia and Papua New Guinea, and now the Wombat Plateau, and may be common elsewhere along the outer margin of the North West Shelf. Upper Triassic reef complexes, with their associated reservoir, source and seal facies, could represent an exciting new petroleum exploration play for the entire North West Shelf. Facies analysis suggests that they are likely only on the outer shelf and slope. Shallow Triassic reef complexes are clearly identifiable using high resolution seismic reflection data. Seismic reflection data of lower resolution may well reveal the associated detrital carbonate wedges, which are more laterally extensive than the reefal core, deeper in the section.

A new genus and five new species of Callianassidae (Crustacea: Decapoda: Thalassinidea) from Northern Australia

A new genus, Neocallichirus and five new species, N. darwinesis and N. horneri (type species) from around Darwin , Northern Territory, Australia, and Callianassa acutirostella, C. parvula and Neocallichirus caechabitator from Australian North West Shelf, of Callianassidae (Crustacea: Decapoda: Thalassinidea) are described. Callianassa praedatrix de Man, originally reported from Indonesia, is recorded for the first time from Australia. A key to Australian callianassid species is provided.

Two new palaemonid shrimps (Crustacea: Decapoda) from the Australian North West Shelf

Two palaemonid shrimps, from about 80 and 50 m on the Australian North West Shelf, are described and illustrated. A new genus, Carinopontonia, is designated for the single incomplete specimen of C. paucipes sp. nov. The new genus is most closely related to the genus Dasycaris Kemp, 1922. A single specimen of a second shrimp can be referred to Apopontonia Bruce, 1977, but to neither of the known species, and it is now described as A. tridentata sp. nov.

Hydraulic Jump and Undular Bore Formation on a Shelf Break

Abstract The evolution of the semidiurnal internal tide as it propagates across the Australian North West Shelf is discussed analytically. As the tide is of long wavelength and small amplitude, this evolution is described by a perturbed extended Korteweg-de Vries equation. It is found that the flow is dominated by nonlinearity, and hence is hydraulic, except in the neighborhood of any shocks predicted by hydraulic theory. Hydraulic theory predicts the formation of two shocks in each period of the tide. The inclusion of dispersion in boundary layers around the shocks results in one of these shocks breaking up into an undular bore. Good agreement is found with observations by Holloway of the semidiurnal internal tide on the Australian North West Shelf.

A numerical model study of internal tides on the Australian Northwest Shelf

A numerical model study of internal tides on the Australian Northwest Shelf

Climatology of internal tides at a shelf-break location on the Australian North West Shelf

The concept of defining an internal tide climate is used as a means of providing an assessment of the amplitude of semi-diurnal vertical displacements of density interfaces and of horizontal baroclinic currents at a particular location. The analysis uses current meter and thermistor chain observations from North Rankin, a location just seaward of the shelf break on the Australian North West Shelf, spanning a period of 28 months. Contributions from both principal lunar (M2) and principal solar (S2) period internal waves are considered. The final climatological averages (monthly values) show the baroclinic currents to be comparable to or stronger than the semi-diurnal barotropic currents at the same location for the majority of the year (October through to May). The temporal variability closely follows the variability in the stratification with very weak baroclinic motion during the winter months (June to September).

Typton nanus sp. nov., a new commensal shrimp (Crustacea: Decapoda: Palaemonidae) from the Australian North-West Shelf

A new species of commensal shrimp, Typton nanus sp. nov., (Decapoda: Palaemonidae) from the Australian North-west Shelf, is described and illustrated. The single example was collected from 40-46m from an unidentified sponge host. Seven species of Typton Costa are now known from the Indo-West Pacific region, all also occurring in Australian waters. The present report represents the greatest depth record for an Indo-West species of the genus.

Internal hydraulic jumps and solitons at a shelf break region on the Australian North West Shelf

A large‐amplitude, shoreward propagating, semidiurnal internal tide has previously been observed at a shelf break region on the Australian North West Shelf. This paper discusses detailed observations of the wave shape as the internal tide shoals over the shelf slope, near the shelf break. The leading face of the internal tide is seen to steepen and form an internal hydraulic jump. The properties of the jump are well described by a two‐layer hydraulic jump model in which the nearly uniform background temperature (density) profile is strongly distorted by the internal tide to an approximately two‐layer stratification just before a jump. The height of the jumps reaches 60 m in 123 m water depth, and the jumps contribute significantly to the strong dissipation of the internal tide that has previously been observed in this region. Following some of the jumps, packets of ∼ 12 internal waves of 20 min period are observed. These are described as solitons, and it is found that a second‐order theory (in wave amplitude) is necessary to describe the wavelength and phase speed. These waves also play an important role in the energy dissipation of the internal tide.

Factors influencing the standing crop of phytoplankton on the Australian Northwest Shelf seaward of the 40 m isobath

An investigation was made to identify the physical factors that influence the distribution in space and time of the phytoplankton standing crop on the Australian Northwest Shelf (18°–22°S, 114°–121°E). During the 18 months of the investigation, 8 cruises were made to the study area. On each of these, there were two oceanographic transects across the shelf, from approximately 40 m, to waters deeper than 1000 m. At various points along each transect, profiles were obtained of temperature, salinity, nitrate-N, oxygen saturation, and in vivo chlorophyll fluorescence. Photosynthetically active radiation was monitored from dawn till sunset. In addition to these “seasonal transects”, special transects were made when tidal amplitude was at a maximum (spring tide) and minimum (neap tide). The standing crop of photoplankton was of the order of 20–40 mg of chlorophyll a per m 2, which is much more than that of the oligotrophic Central Indian Ocean Gyre, and much less than in eutrophic parts of the Arabian Sea and coastal waters of Somalia. The nutrient source for the Northwest Shelf water column is not runoff from the hinterland, nor upwelling, but slope water washing up onto the shelf in summer when the (southerly) Leeuwin Current is no longer flowing. The phytoplankton concentration that results from this enrichment is located at the base of the pycnocline, immediately above the bottom mixed layer, and its depth rises and falls with the rise and fall of the pycnocline. Because incident solar illumination in this area is so high, both in summer and in winter, there is a significant amount of phytoplankton (0.5 μg l −1) underneath the pycnocline over a considerable part of the shelf (∼40–100m); tidal mixing makes this readily available to benthic filter feeders. No surface phytoplankton concentrations or surface temperature fronts were encountered seaward of the 40 m isobath, presumably because surface waters are warmed so much by the sun that their buoyancy can withstand tidal mixing from below.

87:6092 Internal hydraulic jumps and solitons at a shelf break region on the Australian north west shelf: Holloway, P.E., 1987. J. geophys. Res., 92(C5):5405–5416

87:6092 Internal hydraulic jumps and solitons at a shelf break region on the Australian north west shelf: Holloway, P.E., 1987. J. geophys. Res., 92(C5):5405–5416

Factors influencing the standing crop of phytoplankton on the Australian northwest shelf seaward of the 40 m isobath : Tranter, D.J. and G.S. Leech, 1987. Continent. Shelf Res., 7(2):115–133

Factors influencing the standing crop of phytoplankton on the Australian northwest shelf seaward of the 40 m isobath : Tranter, D.J. and G.S. Leech, 1987. Continent. Shelf Res., 7(2):115–133

Life History Strategies and Recent Changes in Population Structure in the Lizardfish Genus, Saurida, on the Australian Northwest Shelf

Aspects of the life history of four sympatric species of the lizardfish genus, Saurida, were examined based on material obtained by demersal trawling on the Northwest Shelf of Australia. The four vary widely in age and size at sexual maturity and in maximum age and length: two species (Saurida undosquamis and Saurida sp. 1) reach large size and live for 6-7 yr, whereas two others (S. longimanus and Saurida sp. 2) reach sexual maturity within a few months of settling to the substratum and may rarely live more than a year. The minimum generation time for Saurida sp. 2 is shorter than that yet reported for any other species of tropical demersal fish species. These life history features are discussed in relation to recent changes in the lizardfish population on the Northwest Shelf and compared to those of other tropical demersal fishes. Limited data suggest that very short generation times may not be unusual for such demersal fishes, which may have important implications for both population dynamics and management of tropical fish species.