^PSFossil Age Group Plots: A Rapid Interpretation Technique for Complex Structural Areas*

By

J. A. Crux ¹, I. G. Gard ¹, P. H. Griggs ², B. J. Farrer ², and N. N. Evans ²

 Search and Discovery Article #40161 (2005)

Posted August 9, 2005

*Poster presentation at AAPG Annual Convention, with SEPM, Calgary, Alberta, June 19-22, 2005.

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¹ Biostratigraphic Associates (Canada) Inc. 45, Grandview Court, Hammonds Plains, Nova Scotia B4B 1K4, Canada, [email protected]

² BHP Billiton Petroleum (Americas), 1360 Post Oak Blvd., Houston, TX 77056

Abstract

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. This technique is applied to both planktonic foraminiferal and nannofossil data sets and is also a fast way to interpret legacy data sets.

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. Fossil age group plots have proven to be an invaluable technique for the stratigraphic interpretation of the tectonically complex area located offshore eastern Trinidad.