Probabilistic prospect assessment in the modern exploration era
Darrel Norman, GeoKnowledge USA, suggests that developments in exploration technology and assessment methodologies provide new opportunities and challenges. The modern probabilistic prospect assessment must incorporate multiple zones, risk dependencies and volume correlations, discrete contact scenarios, and contact uncertainty integrated across depth-volume functions in order to honour the uncertainties defined by the evaluation. The modern oil exploration business is a business of decisions. The managers of exploration efforts, from the largest majors down to the smallest independents, are continually choosing between alternatives. Where should we deploy our people? What plays should we evaluate? Should we acquire acreage? Which opportunity should we drill? The decisions are usually made based on the evaluation of a prospect or a group of prospects. Even pure frontier plays are judged based on the potential of real or postulated leads. The prospect evaluation attempts to answer the fundamental question of oil and gas exploration: ‘What is the chance that this prospect will be a commercial success?’ To answer that question, we must describe the range of possible recoverable volumes, and the chances of finding those volumes. The process of estimating the range of potential volumes and their associated probabilities is called probabilistic prospect assessment. It involves quantifying geologic risks and uncertainties. Risk and uncertainty are largely matters of perception. They cannot be measured or calculated directly. Our perception of a prospect’s risk and uncertainty is a function of the geoscientist’s evaluation and the confidence that we have in the evaluation. ‘Evaluation’ should not imply a single map or model. The geoscientist’s evaluation of a prospect should include full disclosure of all possible outcomes and their relative likelihood. The systematic quantification of risk and uncertainty is a numeric expression of the evaluation. Over the past 15 years our ability to quantify the geologic risks and uncertainties associated with the typical exploration prospect has improved tremendously. The primary source of the improvement has been the use of 3D seismic and the detailed evaluations that are now possible prior to testing a prospect. In some trends, 3D seismic provides a level of detail that previously could be obtained only with dense well control. Our ability to make detailed evaluations of undrilled prospects has necessitated a change in our probabilistic assessment methods. As our descriptions of the untested prospect’s risk and uncertainties become more detailed, our methods for quantifying those perceptions must also become more detailed. Otherwise, the probabilistic assessment will not support the geologic evaluation (and vice versa). Modern geologic evaluations deserve modern assessments of uncertainty and risk.