SEQUENCE BIOSTRATIGRAPHY, PETROLEUM EXPLORATION AND A. CINCTUM

Abstract

Sequence biostratigraphy is a relatively new discipline that has rapidly expanded in parallel with the development of sequence stratigraphy. Sequence bios- tratigraphic concepts have resulted in significant improvements in our ability to calibrate biozones, correlate and determine ages of sedimentary units, and to estimate environments of deposition. Significant advances in the development of integrated biostratigraphic methods and knowledge during the past 20 years are now being rapidly integrated into the physical framework provided by depositional sequences. Sequence stratigraphy provides a physical framework consisting of a predictable hierarchy of correlation surfaces, ranging from sequence boundaries to parasequence boundaries, within which biostratigraphic observations may be placed. These correlation surfaces define chronostratigraphic units with varying degrees of lateral extent that can be used to assess, using time-distance grids, the relative position of biozone 'tops' or 'bases'. They also provide a physical link between the open-ocean planktonic microfossil chronozones and chronozones developed in paralic and non-marine strata. In addition, the delineation of large, apparently sudden, water depth changes across downlap surfaces, associated with condensed sections, has resulted in more accurate and precise paleobathymetric estimation in exploration wells. Recognition and biostratigraphic 'fingerprinting' of major water depth changes are essential for correlation through intensely faulted areas.

The rate of return from biostratigraphic and geochemi- cal sampling is generally poor primarily because of the lack of emphasis on the importance of developing and maintaining well planned sampling strategies and programs throughout an exploration drilling program. The design of biostratigraphic and geochemical sampling strategies has been improved by sequence stratigraphic concepts. Biostratigraphic sample quality (high) is inversely proportional to sedimentation rate (low). Sequence biostratigraphy provides a consistent, predictable, method of recognising low sedimentation rate units in the subsurface using a variety of tools, ranging from seismic to well log facies analysis.

Some of the basic principles of sequence biostratigraphy are illustrated using an example from the Carnarvon Basin. The Barremian A. cinctum andM. australis dinocyst Acme and Oppel zones respectively, appear to be strongly associated with distinct environments. Consequently, it is difficult to calibrate them to the AGSO timescale and to use them regionally as reliable zones to subdivide the Barremian. Abundant numbers of A. cinctum appear to be restricted to specific regions of the Carnarvon dominated by shallower marine conditions and associated with the infilling of major incised river systems. Further biostratigraphic subdivisions within the Early Cretaceous and specifically the M. australis - A. cinctum interval are warranted, especially in light of the number of plays and prospects defined and discoveries made within this interval of the Carnarvon Basin. More detailed biostratigraphic work coupled with regional sequence stratigraphy and a more focussed sampling strategy should produce a high quality age-model for this prospective interval that had not received significant attention until recently.