Abstracts of Talks

Quantitative Palynological Review Of Well Potoroo-1
: A Sequence Biostratigraphic Approach And Its Impact On The Seismic Stratigraphy Of The Ceduna Sub-basin In The Great Australian Bight, South Australia
By Huibert van den Brink, Centre of Excellence in Petroleum Geology, Curtin University of Technology
Presented at PESA WA Branch Luncheon meeting, May 24th, 2001

A quantitative palynological review of the well Potoroo-1, has been carried out. The main scope of this review is to derive a fully integrated sequence stratigraphic breakdown of the Tertiary to Cretaceous sequence of Potoroo-1, calibrated with wireline logs and seismic data. Potoroo-1 is located in the Great Australian Bight Basin at the northern flank of the Ceduna Sub-basin, offshore South Australia (Figure 1, below).

The palynological review is based on a new set of quantitative palynological analyses of ditch cutting samples complemented with an existing set of semi-quantitative analyses of ditch cutting and sidewall core samples, resulting in a densely spaced (10 to 30 m) sample set. It provided an integrated palynological zonation for Potoroo-1. The zonation for the upper section, from the Eocene to the Cenomanian, mainly refers to the Perth Basin palynology as described in Marshall's unpublished PhD thesis (1984). The Helby et al. (1987) regional palyno-zonation is applicable in the lower section from the Cenomanian to the Albian. The integrated palynological zonation is mainly based on first downhole occurrences (FDO's) of key microplankton taxa. Some marker taxa are illustrated in figure 2.

Microplankton diversity peaks have been correlated with potential marine flooding surfaces in the marginal to nearshore marine upper section of Potoroo-1 (Eocene to Cenomanian). Potential transgressive trends are illustrated in the shelfal to offshore marine lower section of the well of Albian age by increasing upward trends in microplankton diversities (Figure 3). Potential sequence boundaries are expressed as breaks in microflora composition. A section of Maastrichtian to Campanian age containing a poor sporomorph flora (base at 1310 m) has been detected on top of a section of Santonian age containing a moderately rich microplankton and rich sporomorph flora (top at 1322 m). A section of Cenomanian age containing a lean microplankton flora (base at 2370 m) has been detected on top of a section of Albian age containing a rich and diverse microplankton flora (top at 2382 m). Added to this is a biostratigraphic hiatus with the absence of the top of the X. asperatus Zone.

The proposed sequence biostratigraphy of Potoroo-1 has been calibrated with the gamma-ray (GR) log to arrive at proper depth figures. Microplankton diversity peaks, correlated with marine flooding surfaces, have been linked to the nearest GR high (slightly higher in the section) to take minimal caving effect into account. GR breaks from a low to a high reading have been used to pinpoint the sequence boundaries within the 12 m intervals.

The two marine flooding surfaces at 948 m and 1417 m and the two sequence boundaries at 1312 m and 2377 m at the Potoroo-1 well location are clearly expressed on seismic. The marine flooding surfaces correspond to two seismic horizons characterised by pronounced amplitude anomalies. At the level of the two sequence boundaries clear onlap features are expressed 10 to 20 km in a basinward direction.

The sequence biostratigraphical metho-dology, applied in this study, is well established in high resolution biostrati-graphical studies; decimetre to metre scale, requiring (sidewall) core samples. At the present scale, it has not been applied so far despite being relatively easy to implement. The only extra costs of the present study are incurred by the analyses of the 'infill' ditch cutting samples.

Biography
Huibert van den Brink received an MSc in Quaternary Geology from the Free University in Amsterdam, Holland in 1980. He was employed by Shell International from 1981 to 1999 as exploration geologist/stratigrapher in a range of areas including UK, Holland, Oman, Malaysia, Nigeria and Australia. In 1999 he joined the Centre of Excellence in Petroleum Geology at Curtin University with the rank of senior lecturer, specialising in palyno-stratigraphy. His present research is focused on Neogene microplankton recovered from deep water Shell wells offshore North Borneo. He teaches petroleum geology and related courses to under- and postgraduate students.

He originated the Potoroo-1 project in 1999, whilst still working for the Shell/Woodside alliance.

References
Helby, R. J., Morgan, R. P. and Partridge, A. D. (1987). A Palynological Zonation of the Australian Mesozoic, Mem. Ass. Australas. Palaeontols. 4, 194.
Marshall, N. G. (1984). Late Cretaceous dinoflagellates from the Perth Basin, Western Australia. unpubl. thesis, University of Western Australia.