During April/ May 1988 the Bureau of Mineral Resources will conduct a 22 day geochemical research programme in the Otway and Gippsland Basins. This programme follows previous preliminary studies (January-February 1987) which examined the distribution and molecular composition of light hydrocarbon gases within deep water surficial sediments in the Otway Basin.The major thrust of the forthcoming cruise will be to obtain shallow water (continental shelf) geochemical data from both the Otway and Gippsland Basins, to test ideas generated from the previous programme, and, inparticular, to integrate the results with in house BMR geohistory and maturation modelling studies. The objectives are, therefore:

1.To develop new information on source rocks, maturation, and hydrocarbon migration on the Mussel and Crayfish Platforms and Voluta Trough in the Otway Basin.

2.Specifically, to test the relationship between variations in sourcerock maturation (as indicated by well data and geohistory analyses) andthe surface hydrocarbon gas composition and distribution within sediments from the Otway and Gippsland Basins.

The majority of the programme will be focussed on the Otway Basin, with the final 4-5 days of the cruise spent in the Gippsland Basin. Sampling will becarried out along previously shot BMR multi-channel seismic lines, and will consist of vibracoring in shallow water (<150m) and combinations of piston and gravity coring in deeper water.

In the Otway Basin, four key profiles will be sampled (see Fig.1). The two western profiles (A & B) run from the Crayfish Platform into the VolultaTrough, whereas the eastern profiles (C & D) run from the Mussel Platform into the central Voluta Trough. On all of these profiles the basal Early Cretaceous, considered the section with the best source potential, becomes progressively deeper buried (towards A" etc.), with maturity increasing from VR =1.0-1.3 (peak oil generation) on the Platform margins to over mature (VR = 3.0-4.0) in the central Voluta Trough. Similarly, the Late Cretaceous sequences (eg. Belfast Mudstone), which are immature on the Platform margins, are ideally placed within the oil window in the central Voluta Trough. The molecular compositions of the thermogenic gases withinthe surface sediments along these profiles may thus provide important cluesto the nature of the principal source horizons within the Otway Basin. For example, a dry gas signature in the central Voluta Trough would argue against a Late Cretaceous source, and support the contention that the Early Cretaceous is the major source horizon.

In addition to the above profiles, specific areas will be studied indetail. These include previously defined hydrocarbon anomalies andpotential seafloor seeps off Port Douglas (see Fig.1).

The Gippsland Basin programme will consist principally of sampling from the southern shelf, through the Central Deep, and onto the Lakes Entrance Platform (see Fig.2). Latrobe Group source rocks vary from immature toovermature along this transect, and the objective is to determine whetherthis change in maturity is reflected in the composition of the surface hydrocarbon gases. The Gippsland programme will allow a comparison of theresults obtained from a frontier exploration area, the Otway Basin, withthose from a major oil/gas producing province.

In both the Otway and Gippsland Basins, high resolution seismic data willbe collected over selected hydrocarbon anomalies, in order to better definenear-surface structure, particularly faulting. This data will be acquiredwith either a 15 or 80 cubic inch water gun.