During April and May 1991 the Bureau of Mineral Resources conducted a combined deep crustal seismic and Direct Hydrocarbon Detection (DHD) survey (Rig Seismic Survey 100; Figure la) in the Bonaparte Basin, which is located in the Timor Sea off northwestern Australia. This survey is one of three combined seismic and DHD surveys (Surveys 97, 99 and 100) which have been conducted in the Timor Sea (Figure lb). Survey 100 collected approximately 2540 line-km of DHD, together with approximately 2100 line-km of deep crustal seismic, gravity, and magnetic data. The DHD data from this survey complements that obtained in the same general area during Survey 99 (Bickford et al., 1992). Several bottom-water light hydrocarbon anomalies were detected during the survey, mostly in the Petrel Sub-basin. The strongest anomalies were detected over the Petrel gas/condensate accumulation, in the vicinity of the Petrel-1 wellhead. Weak but aerially extensive anomalies were associated with the Tern gas/condensate accumulation. The Petrel anomalies differed in character from those found over Tern, in that they were strong, up to two orders of magnitude above background, and were confined to a small area. In contrast, the Tern anomalies were weak, generally less than two-fold above background, but extended over a large area. A cross-plot model of percent hydrocarbon wetness versus methane has been used as a tool to predict the potential 'source' (oil prone, gas/condensate or dry gas) of bottomwater anomalies. The data from the anomalies detected over the Petrel and Tern gas/condensate accumulations show wetness trends from background (less than 1%) to levels of about 3.4%, with increasing methane concentrations up to 272 ppm (over Petrel). The crossplot model trends are consistent with the hydrocarbon compositions in these gas/condensate accumulations. Several other hydrocarbon anomalies were detected away from exploration wells. These anomalies were typically weak, and usually of gas or gas/condensate 'source' (according to the crossplot model). However, one strong anomaly, detected in the southern Petrel Subbasin, had a maximum percent hydrocarbon wetness value greater than 16%, and an oilprone 'source' according to the crossplot model.

During February-March 1991 the BMR conducted a combined Direct Hydrocarbon Detection (DHD) and high resolution seismic survey (Rig Seismic Survey 99; Fig. 1) in the Bonaparte Basin. This survey is one of three combined seismic and DHD surveys which were conducted in the Timor Sea (Fig. 2). Survey 99 collected approximately 3466 km of DHD, high resolution seismic, gravity, magnetics and side-scan sonar data. The data from this survey complement those obtained from Survey 100 (Bishop et al. 1992), which was conducted in the same general area. Three vibracores were taken in the northwestern corner of WA-217-P, while two grab sample sites were occupied in the vicinity of the Petrel gas field. Several bottom-water light hydrocarbon anomalies were detected during the survey, with most of these being located within the Petrel Sub-basin. A single, strong bottom-water anomaly, and several weaker hydrocarbon anomalies were found in the vicinity of the Petrel gas field, while weak, but aerally extensive anomalies were associated with the Tern gas field. No significant hydrocarbon anomalies were found in the Sahul Syncline. There were no strong C1 - C4 anomalies in the Malita Graben, however, one strong butane anomaly was found in the western end of the Malita Graben (principally composed of n-butane). Slightly elevated concentrations of light hydrocarbons were detected over an area of about 30 km in the eastern end of the Malita Graben (in the vicinity of the Heron 1 well). Cross-plots of percent hydrocarbon wetness versus methane indicate that the water column anomalies detected in the vicinity of the Tern and Petrel gas fields were sourced from a gas/condensate source, which is consistent with the known composition of the reservoired hydrocarbons at Petrel and Tern. The anomalies detected over the Tern and Petrel fields probably represent seepage directly from those accumulations. The general lack of hydrocarbon anomalies in both the Sahul Syncline and the Malita Graben may indicate that these source rock "kitchens" are no longer actively expelling hydrocarbons. A vertical profile conducted over one of the bottom-water anomalies near the Petrel gas field showed that the anomaly could be detected 40 metres above the seafloor. This highlights the fact that the DHD fish should be towed as close to the seafloor as possible if weak bottom-water anomalies are to be detected.

Transglobal Environmental Geosciences (TEG) and the Bureau of Mineral Resources,as part of the Joint Research Agreement into hydrocarbon seepage around theAustralian Continental margin, conducted a calibration survey over some knownhydrocarbon accumulations in parts of the Barrow Sub-basin and the Ranldn Platformduring 1989. This survey was conducted at the end of a larger and proprietarygeochemical 'sniffer' survey conducted by TEG (for Amoco Production Co., USA) inthe Perth Basin. Approximately 220 line km of Direct Hydrocarbon Detection (DHD) data werecollected during the survey in the vicinities of known hydrocarbon accumulations,Saladin, South Chervil, Chervil, North Herald and South Pepper in the inshore part ofthe Barrow Sub-basin. Additional survey lines were run in the deeper water part of thebasin, on the landward side of the rift, in the vicinities of Chinook and Griffin, andfurther to the north, on the southern extension of the Rankin Platform (Alpha Arch)over the Gorgon gas/condensate field. Light hydrocarbon anomalies were detected in bottom-waters in the vicinities ofSaladin, Chervil, South Chervil, North Herald and South Pepper, although theanomalies were generally weak (< five-fold background). No anomalies were detectednear Chinook and Griffin, nor Gorgon, although problems with the tow-cable and tow-fish resulted in the tow-fish being too high above the seafloor to detect any significantseepage.

During February-March 1990 the BMR conducted a combined seismic and DHD (Direct Hydrocarbon Detection) survey in the Arafura Sea of northern Australia. This survey was the second to employ the 'geochemical sniffer' (DHD) aboard Rig Seismic, and represented the first deployments of the geochemical equipment from amidships Rig Seismic. The purpose of these deployments were to develop the capability to collect underway, bottom-water geochemical data simultaneously with seismic reflection, gravity and magnetic data. The bottom-water geochemical data hence would complement existing remote sensed methods to aid in offshore exploration for hydrocarbons, and to provide new insights into concepts of hydrocarbon generation and migration. As such, the deployment of the equipment amidships Rig Seismic was successful and will become a permanent installation. However, some mechanical (deployment) and data collection difficulties resulted in some gaps and artefacts in the bottom-water DHD, and these are noted in the text. Some parts of the data have been heavily edited.

The Bureau of Mineral Resources (BMR) collected 1430 line-km of bottom-water Direct Hydrocarbon Detection (DHD) data during a survey aboard R.V. Rig Seismic in the Durroon Sub-basin, the Otway Basin, the Torquay Sub-basin, and the Gippsland Basin, during late September and early October of 1991. No significant bottom-water anomalies were detected in the Durroon Sub-basin. Anomalous concentrations of light C2+ hydrocarbons were detected in the eastern Otway Basin. The anomalies were not extensive, comprising only a few data points representing a few kilometres in extent. One anomaly (of methane, ethane and propane) was accompanied by high levels of the biogenic hydrocarbons, ethylene and propylene, suggesting in-situ biogenic activity in the water column. However, anomalous concentrations of C7 and C8 hydrocarbons were also found here and at three other locations, and are from an unknown 'source'. A weak bottom-water anomaly was detected in the Torquay Sub-basin in the same location as an anomaly detected during an earlier survey (Rig Seismic Survey 89), two years previously. The weakness of the anomaly prevents a confident interpretation of the potential 'source' of the hydrocarbon anomaly, but the data suggests it is derived from a gas/condensate, or dry thermogenic gas 'source'. Several strong bottom-water anomalies were detected in the Gippsland Basin. Bottomwater anomalies were found near the Sunfish and Tuna oil/gas accumulations, in similar locations to anomalies found on Rig Seismic Survey 89, two years earlier. However, another previously-detected anomaly (near Barracouta) was not reproduced. Additional anomalies were found near Flathead, and to the west of Wahoo. The anomaly west of Wahoo was weak and in a similar area to that detected on Survey 89. The composition of most bottom-water hydrocarbon anomalies in the Gippsland Basin are indicative of a liquid-prone hydrocarbon 'source', while one anomaly in the northern sector of the survey area is indicative of a gas/condensate 'source'.

The Mount Isa Inlier and Environs Mineral Deposit Database describes 251 of the most significant mineral deposits in the Mount Isa Inlier and environs, out of a total of more than 1800 locations listed in the Australian Geological Survey Organisation (AGSO) mineral occurrence database, 'MINLOC'. The database comprises five commodity-based tables, grouped as follows: copper-gold-silver (187 locations), lead-zinc-silver (22), gold (15), uranium (12) and miscellaneous (including silver, cobalt, tungsten, tin, manganese, iron and beryl; 15 locations). Each of the mineral deposits described either has combined production and reserves equivalent in value to more than 20 tonnes of copper metal, or is a significant prospect typical of a particular deposit type. Mineral deposits described in the database are presented on a 1:500 000 scale metallogenic map of the Mount Isa Inlier and Environs, which is available separately.

As part of its geochemical research program, the Marine Geoscience and PetroleumGeology Group (Australian Bureau Of Mineral Resources) is evaluating the usefulness ofthe Direct Hydrocarbon Detection (DHD) method. The data for this DHD program wereacquired during a co-operative high resolution seismic reflection program with WoodsidePetroleum Pty Ltd in the Dampier Sub-Basin. The data acquisition phase took placebetween October 22-28, 1990, with a total of 531 km (25 lines) of DHD data beingcollected between the Angel gas field in the north-east and the Madeleine 1 well in thesouth-west of Woodside Petroleum exploration permit WA-28-P in the Dampier Sub-Basin, on the North-West shelf, Australia. No significant hydrocarbon anomalies were detected on any of the lines, in spite of the factthat many lines traversed known oil and gas accumulations, such as the Wanaea, Cossackand Angel accumulations. The lack of anomalies indicates that the major reservoirhorizons in this part of the Dampier Sub-Basin are well-sealed, and that little opportunityfor the vertical migration of hydrocarbons exists. While no significant anomalies were detected, very minor increases in total hydrocarbonwere, however, observed over some of the wells/fields. The largest increase in THC wasobserved over the Montague 1 well location, where the value increased from abackground level of 16ppm to a high of 22.8ppm. There was no increase in any of thelight hydrocarbon gases.

In October/November 1990 the Australian Bureau of Mineral Resources (BMR) carried out an 18 day combined water column geochemical and high resolution seismic survey on the Vulcan Sub-basin region of the Timor Sea. This report presents the results of the water column geochemical (direct hydrocarbon detection or DHD) aspects of that program. During the program, 2730 km of DHD data were obtained along 44 lines over the Vulcan Sub-basin, the Ashmore Platform and the Londonderry High. Ten water bottom hydrocarbon anomalies were detected during the program. Seven of these anomalies fell into two distinct groupings, which were associated with: - the Skua field and surrounding fault blocks, - the intersection of the NE-trending Vulcan Sub-basin/Londonderry High Boundary Zone with a prominent NW-trending transfer fault zone. The composition of the hydrocarbon anomalies within the Skua grouping was generally consistent with them having an oil-prone, Late Jurassic source,, and is thus compatible with the known composition of the hydrocarbons in the Skua accumulation. The composition of the other grouping was more consistent with a gas/condensate source; they may have originated from more gas prone Permo-Triassic source rocks on the edge of the Londonderry High. The remaining anomalies were all very weak, and may have been due to biogenic activity. The data indicate that the DHD technique can be useful at a prospect level within the Timor Sea (for example, it did remotely detect the Skua accumulation). The types of accumulations which are most easily detected using DHD are those with a significant gas cap, a relatively shallow (<2000 m) reservoir, and faulting which extends from the reservoir horizon to near the seafloor. Furthermore, the data suggest that transfer fault zones provide important pathways for hydrocarbon migration in this region.

No abstract available

No abstract available