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The world around us has not always been as we see it today. For example, it is estimated that over 99 percent of all known species are extinct (Newman and Palmer 2003).Humans live a relatively short life, so it is hard for us to con­ceptualize the processes that are at work on a geological timescale. Tectonics, volcanism, erosion, redeposition, meteor impacts, cycles of orbital fluctuation, and changes in radiation from the sun all affect the atmosphere, oceans, and other global systems at a range of timescales. Life on the planet has always had its form, behaviour, abundance, and distribution delineated by these global and extraterrestrial forces. Together with evolution, these forces drive a turnover in the species living on the planet through inception and extinction.

Microfossil data from samples taken through sedimentary successions clearly show the evolution, extinction, and abundance changes of species through geological time. The evolution and extinction of a species does not provide an absolute age in millions of years but occurs at the same point in time in datasets from other wells or outcrops. Fossil events and zones created from these can be used to subdivide and correlate stratigraphy between datasets. Models of geological time and stratigraphy (Gradstein et al.2004; Ogg et al.2008) allow biostratigraphic events and subdivisions to be aligned with absolute time.

While working with seismic cross sections seek out biostratigraphic information and plot it at well locations to aid interpretation. The stratigraphic units, ties, picks, and attributes identified on seismic may be relatively isochronous within the study area, in which case the biostratigraphy can be used as a quality check of the model being built. If the biostratigraphy shows that units are crossing time, then the extent to which this fits existing models and sequence stratigraphic simulations should be considered. Significant age differences where a seismic tie passes through a pair of wells might indicate that mis-ties have occurred. In such cases, faults or unconformities might lie between the well locations, not yet resolved or observed in the seismic image.

Biostratigraphic analysis can be used to provide an independent interpretation of the depositional environment, water depth, and relative distance from land. This can be a useful input to those predicting rock types (for example source rocks) from seismic data. Significant shifts in environment over short distances might indicate mis-ties or exceptional depositional settings.

With enough data, a log of geological age can be plotted against depth at well locations and overlain on seismic sections. Jumps in this log represent uncon­formities or faults which might otherwise not be apparent. When depth is on the vertical axis, the slope of this graph is proportional to sedimentation rate, low gradients on the plot representing low rates of sedimentation.

Biostratigraphy can deliver important insights before, during, and after drilling but like seismic interpretation, larger programs of analysis can take time. We suggest you instigate these projects as early as possible to ensure that results are available when you need them.


References

Gradstein, F M, J G Ogg, and A G Smith, editors (2004). A Geologic Time Scale 2004. Cambridge University Press, DOI 10.1017/CBO9780511536045.

Newman, M E J and R G Palmer (2003). Modelling Extinction. Oxford University Press.

Ogg, J G, G Ogg, and F M Gradstein (2008). The Concise Geologic Time Scale. Cambridge University Press.

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