Assessments of undiscovered oil and gas potentials for a group of geologically related, untested prospects can be effectively made from an estimate of the possible ranges in number and size of potential fields, assuming that the play exists, coupled with an evaluation of geologic risks that it might not exist. Field size distributions are constructed from known-field reserves in geologically similar plays, from assessments of representative prospects in the play, or from simulations of distributions of the play's prospect areas, reservoir parameters, and potential hydrocarbon relations. The field-size distributions are truncated at both ends, at a practical minimum and at the largest size reasonably expected in the play. The possible range of numbers of potential fields is estimated from counted and postulated numbers of untested prospects in conjunction with a success ratio, or from look-alike field densities. The chance that the play exists is the chance that there is at least one field of at least the minimum size assessed. The final assessment curves, developed in a Monte Carlo simulation, portray exceedance probability versus the range of possibly recoverable hydrocarbon potentials.