A numerical rating system can be instrumental in deriving the percentage chance for success of a new prospect, regardless of geographic location, the same way each time, by any geologic analyst. The percentage chance of geologic success for a new prospect is most vital in computing the economics on a new prospect. The parameters in the rating system define the percentage chance that a prospect has a trap, hydrocarbon accumulation, and the composite chance factor. Graphs simplify the analysis of a prospect and indicate whether it is drillable and economic. The statistical chance for failure due to bad luck (Law of Gambler's Ruin) is calculated and integrated graphically with Composite Chance Factor, Profit to Risk Investment, and minimum number of wells necessary for any chance factor, to eliminate speculation. A simple formula permits computing a grade for a prospect. Grades so derived result in an order of preference of drillable prospects. A revised well classification system is the natural evolution of prospect quantification.