Fossil Age Group Plots: A Rapid Interpretation Technique for Complex Structural Areas

Jason A. Crux¹, I. Gunilla Gard¹, Peter H. Griggs², Bruce J. Farrer², and Neil N. Evans^2
1 Biostratigraphic Associates (Canada) Inc, Hammonds Plains, NS
² BHP Billiton (Americas), Houston, TX

The stratigraphy of complex thrust related structures, offshore eastern Trinidad can only be tentatively understood from seismic data before drilling. Biostratigraphy has long been an important tool in the post-drill stratigraphic interpretation and, in particular, for identification of repeat sequences. Uncertainty often remains in these interpretations that are complicated by reworking of older fossils, caving, and contamination during drilling. To improve on the biostratigraphic precision, we have developed a technique of plotting the fossils recorded in the wells by age groups.

The fossils are divided into groups whose stratigraphic ranges approximately correspond to different second order, or tectonic, sequences. Long ranging species that can occur in several second order sequences are assigned to a separate long ranging group. Plots are made of the electric logs together with the species groups highlighted in different colors. Summary plots can be made of the abundance and diversity of the groups.

Stratigraphic repetition is recognized by the repeated influxes of different color groups. This is easily differentiated from the more scattered patterns caused by reworking of older fossils and the caving of younger fossils from above.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005