The time-calibrated composite - A powerful tool in basin exploration

The Taranaki Basin has a complex geological history and biostratigraphic data are key to resolving many stratigraphic and structural problems facing the explorationist. Biostratigraphic data, however, are themselves complex and contradictory; inconsistencies in fossil ranges can be caused by incomplete sampling, misidentification, sedimentary reworking, and real variations in the distribution of species. Because of these factors, the problem of correlation Is enormous when dealing with large numbers of fossil species in many wells. Quantitative stratigraphic methods address these problems by using a variety of objective or semi-objective mathematical approaches. A quantitative biostratigraphic analysis of eight Taranaki exploration wells demonstrates that a substantial amount of new high resolution biostratigraphic information can be extracted from existing data. The range tops of 87 taxa, representing foraminifers, nannofossils, pollen, spores, and dinoflagellates, in cuttings samples have been used to derive a precise correlation of the eight wells that compares favourably with seismic correlations. The PC program, Constrained Optimisation (CONOP), which employs the principles of both graphic correlation and unitary association, has been used, and the method extended to derive a composite section that is calibrated against the time scale. Using this "time-calibrated composite", the depositional history of the well sequences can be determined, and the stratigraphic position, age and duration of unconformities can be determined. From the eight wells studied, it appears that during the Paleocene through Oligocene interval, there were short pulses of relatively rapid deposition separated by periods of nondeposition. A regional unconformity at the base of the Eocene is revealed. The time-calibrated composite promises to be a powerful tool in basin analysis, particularly for detecting sequence boundaries.