Publication Name: PESA's Eastern Australasian Basin Symposium II
Authors: S.J. Gallagher, I.R. Duddy, P.G. Quilty, A.J. Smith, M.W. Wallace, G.R. Holdgate and P.J. Boult
Date Published: December 2004
Number of Pages: 28
Reference Type: Book Section
Abstract:
While palynomorph and conodont colour indices are widely used as thermal indices in rocks of various age,enhancing more quantitatively understood analyses such as vitrinite reflectance and fission track data, it is not generally known that agglutinated foraminiferal colouration changes in a consistent manner with increasing temperature. The foraminiferal assemblages in the Late Cretaceous Sherbrook Group in the Otway Basin show a systematic colouration change down hole, which is not specifically related to the colour of the host lithology. Analyses of over 7,000 agglutinated foraminiferal specimens of Haplaphragmoides in core, sidewall core and ditch cuttings from Voluta-1, Port Campbell-2 and Flaxmans-1 wells show a similar foraminiferal colour variation to that observed in wells drilled in Canada studied by McNeil et al. (1996). McNeil et al. experimentally calibrated these changes to increasing temperature and established a Foraminiferal Colouration Index (FCI) for evaluation of thermal histories in Mesozoic to Cenozoic well sections. Our data from the three wells have been directly compared and correlated to existing VR data. There is a high correlation between the FCI, VR data and the variation is consistent down hole whether in core, sidewall core or ditch cuttings. Based on the predicted maturity profiles for each well from basin modelling we propose a preliminary correlation that directly
relates VR to FCI values. The advantages of including FCI analysis in foraminiferal studies are: 1) The FCI data is quick and easy to collect; 2) FCI is a most effective thermal indicator in the early stages of hydrocarbon generation; 3) FCI can be used to determine the amount of caving and reworking in samples and therefore can be collected from ditch cuttings; and 4) FCI can be applied in marine sequences where vitrinite is either rare or absent. These facts make FCI analyses a potentially useful tool for thermal history analyses of fine-grained marine strata. Future studies will be directed towards verifying and refining the correlation between the FCI and VR data.