A discovery-process model that had been devised previously to estimate the size distribution of oil and gas fields remaining to be discovered in a single producing horizon was adapted to a basin containing spatially connected multiple plays and producing horizons. This model used the concept of the area of influence of wells to estimate search effort. Other variables estimated from the historical drilling and discovery data are the effective basin size and the exploration-drilling efficiency. Data from the Midland Basin are used to illustrate how the model works. ?? 1980 Plenum Publishing Corporation.