Abstract

In applied oilwell micropaleontology zonations are used to identify relative age of rocks (biostratigraphy), while comparison of composition of fossil assemblages with modern fauna and flora distribution helps to determine depositional environments (biofacies). Since few explorationists are sufficiently familiar with this discipline to evaluate results and do re-interpretations without violating basic data, this paper attempts to review the various microfossil groups currently used in Indonesia, the facies ranges in which they can be applied successfully, and some common pitfalls.

Although local centers of micropaleontological activity have contributed little or no modifications to the "worldwide, low-latitude zonations" published more than 15 years ago, new time scales and data published outside Indonesia justify an updated version of the Indonesian biostratigraphic correlation chart.

Errors or inaccuracies in, "paleo" reports may result from the type of samples used, sample spacing, "geological mixing" of fossils, laboratory or field contamination, biased picking of samples by technicians, misidentification of species or insufficient time spent on looking for marker species.

A significant new development that relies heavily on micropaleontological data is sequence stratigraphy. By combining biofacies trends, lithologies, well log patterns and age information, micropaleontologists are in a position to pick depositional sequences in well sections. If done correctly, this improves understanding of geological history, interpretation of fossil-poor zones, allows prediction of lateral facies changes, and, since most seismic reflectors are sequence boundaries or "condensed intervals", facilitates integration with seismic data.

Analysis of microfossils is standard procedure in most wildcat wells in Indonesia. Since detailed biofacies analysis aids in recognition of depositional systems and high-frequency cyclicity ("parasequences"), microfauna or flora analyses should also be considered in field development wells for reservoir mapping and prediction studies.