Estimating Future Rates Of Petroleum Discoveries In The Permian Basin
Authors: Drew Lawrence J. , Root David H. , Bawiec Walter J. 
Published: December 2007

Abstract
The search for petroleum in the Permian Basin of western Texas and eastern New Mexico has produced 4,036 discoveries by the drilling of 30,340 exploratory wells through the end of 1974. This historical discovery record can be used to forecast the number of fields of various sizes that will be discovered by a given number of exploratory wells in the future. The economic costs of discovery and production are related to the sizes of the undiscovered fields; therefore, it is important to estimate the size distribution of the undiscovered fields as well as their total hydrocarbon content. The deeper parts of the basin have not been explored as intensively as the shallower parts. Therefore, we divided the basin into 5,000-ft-depth intervals and made separate forecasts for each interval. The larger future discoveries are anticipated to be made in the deeper intervals.
The method of estimating the undiscovered resources and the rate of future discoveries in the Permian Basin is based on a modification of the discovery process model developed by Arps and Roberts.1 The undiscovered recoverable hydrocarbons in the basin after 1974 is estimated to be 6.4 billion bbl of oil equivalent. One-half of this total is expected to be in the 5,000- to 10,000-ft-depth interval, and approximately one-half of the total is estimated to be in 33,000 small fields (less than 1.5 million bbl of recoverable oil equivalent). The future discoveries are estimated for each of 20 1,000-exploratory-well increments, the model forecasts 151 new discoveries, of which 85% will contain less than 1.5 million recoverable bbl of oil equivalent (BOE).

Introduction
This paper forecasts the number and size of future oil and gas discoveries in the Permian Basin of western Texas and eastern New Mexico. These forecasts were made as part of a study on the economics of petroleum exploration at the regional level. Estimates of the sizes and depths of future discoveries in the basin were necessary to make forecasts useful for discovery- and production-cost analysis. The Permian Basin has a long exploration history that includes the drilling of 30,340 exploratory wells and the discovery of 4,036 oil and gas fields through the end of 1974. Because of this extensive history, we were able to forecast future discoveries by extrapolating the past drilling and discovery record.
The method used to forecast the size distribution of future discoveries is based on the well-documented characteristic of the petroleum-discovery process; the large deposits tend to be discovered early in the exploration of an area.1–4 Any reasonable quantitative model of the discovery process must incorporate this qualitative characteristic.
The model used here was proposed by Arps and Roberts.1 This model is derived from the following assumption: the probability that the next exploratory well will find a given field is proportional to the ratio of the area of the field to the area of the basin. Thus, a given large field has a higher probability of being discovered early than a given small field. This model originally was devised to forecast the future discoveries in a basin having only a single productive horizon without large structural relief. Therefore, the model must be modified before it can be used in a basin, such as the Permian Basin, that has many productive strata at widely varying depths. In order to apply the model in the Permian Basin, we divided the basin into four layers, each 5,000 ft thick. Forecasts of future discovery rates then were made independently for each layer.