The outlined procedure ties both the absolute level and the relative high grading of assessed prospect chances of success to the regional chances of the surrounding oil and gas play (prospect group). These estimated chances strongly influence exploration economic analyses. The average prospect chance is established from the group vs. the individual risk aspects of the play, then the specific prospects are assigned higher or lower chances according to relative merit. Maintaining the average level keeps prospect and play assessments entirely compatible. Group and individual risk factors are documented fully to help make geologically proper summations and to calculate the proper expected value for economic evaluation. Any dependencies between risk factors are analyzed carefu ly to avoid risking the same basic factor twice. Chances are consistently related to exceeding a practical, uniform minimum field size for the play. The chosen minimum may range from very small to very large, depending on the focus of the assessor, data availability, and maturity of the area. Postdrilling checks against actual experience include determining if the sum of the predrill risked-mean hydrocarbon volumes approximately equals the sum of the discoveries. The basic approach also works for assessing and aggregating different possible reservoir zones in a single prospect. The system can make fullest use of play maps in locating and quantifying the potentially most rewarding areas for exploration, and can be used by independents, major companies, and governments. Systematic geologic risk assessments should help explorers find oil and gas more profitably.