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Review Of The Micropalaeontology Of The Seaspray Group, Gippsland Basin, Victoria.

Christian, Louise R.

Honours Degree, 1995

University of Adelaide

Abstract

Foraminifera have been widely utilised in the petroleum industry since the 1920s, as a means to determining age correlative surfaces, and for making depositional palaeoenvironment determinations.

The focus of this project was the foraminifera of the mid-Miocene to Pleistocene section of the Seaspray Group, Gippsland Basin.

In 1966, Taylor developed an initial biostratigraphic framework for the Gippsland Basin. This was modified in 1975, however, since this time, very little work has been carried out on the foraminifera1 micropalaeontology of the region.

In this study, the biostratigraphic zonations applied by Taylor to samples from three offshore wells, Mackerel #1 (1971), Hapuku #1 (1975b) and Fortescue #1 (1979), have been revised.

In total, forty-eight sidewall core and cuttings samples were analysed, the planktonic component of the foraminifera forming the basis of the biostratigraphic evaluation. Approximately 300 foraminiferal specimens were picked from each sample, and were sorted according to species (planktonics) and genera (benthics). Reference slides were produced, and SEM photography of selected key species carried out.

Ana1ysis of two seismic profiles from a 3-D data set (Fortescue #1 to Mackerel #2, and Mackerel #2 to Hapuku #1), has proven a valuable tool for aiding in the correlation of zone boundaries between wells.

The biostratigraphy of Hapuku #1 was deemed largely analogous with that of Taylor (1975b) - discrepancies regarding zone boundaries were minor.

The zonation of samples from Mackerel #1 delineated in this study were found to vary markedly from those of Taylor (1971), with the zones of Taylor significantly higher than those outlined here. The application of seismic data to this study has enhanced the understanding of sub-surface geology, and as the zone boundaries of Hapuku #1 and Mackerel #1 correlate entirely, the boundaries delineated in this study are deemed legitimate.

The zonation of Fortescue #1 remains an anomaly, as neither the boundaries of Taylor (1979), nor those delineated here correlate with surrounding wells. The seismic data available do not provide an answer to the problem, and as such, the biostratigraphy of Fortescue #1 is enigmatic.

Palaeoenvironmental determinations, based upon recovered faunal assemblages and derived indices - plankton diversity, plankton to benthos (P:B) ratios and infauna to epifauna I:E) ratios, were integrated with seismic data.

Micropalaeontological data provided an understanding of climatic conditions, trophic resources, and degree of oceanic stratification, along with an indication of depositional position on the continental margin.

Seismic interpretation, both from profile and from time-slices of selected interval, generally confirmed the environment of deposition. Sequence stratigraphic concepts after Van Wagoner (1987) were utilised. Extensive canyon-cutting was evident within a number of zones, karst topography due to sub-aerial exposure of the shelf was delineated, while progrades at the top of the section were shown to move off the shelf-edge with decreasing time.

The section studied from Fortescue #1 to Hapuku #1 displayed an overall shallowing trend, both from micropalaeontological and seismic analyses.

Finally, micropalaeontological analysis and seismic interpretation were integrated with the eustatic sea-level curve of Haq et al. (1987), and with d18 O curves from DSDPs 563 and 289. Correlations between micropalaeontological analyses, seismic data and these curves proved significant.

Australian School of Petroleum
THE UNIVERSITY OF ADELAIDE

SA 5005 AUSTRALIA

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