A method determines the performance index of subterranean rock. In one embodiment, a performance index method determines a performance index for subterranean rock of an area. The area includes a well. The method includes determining a time period during producing the well. The method also includes determining the performance index from data of the time period from the equation PI=(q/dd)*(cum./GPI). The term PI is the performance index, and the term q is the average daily rate of the well for the time period. The term dd is the average drawdown per day of the well for the time period, and the term cum. is the cumulative production of the well for the time period. The term GPI is the gross perforated interval of the well for the time period.