There are two ordinarily acceptable means of approach to the problem of estimating oil reserves: decline curves and the "volumetric" method. The former has had the field of its effectiveness reduced by the artificial production methods created by proration.

Of the many "volumetric" factors bearing upon this problem, the basic ones, (1) thickness and character of reservoir rocks, (2) porosity and degree of saturation, (3) contained gases, (4) temperature, and (5) contained moisture, relate to the quantities of original oil within the reservoir. Each of these presents difficulties of interpretation, and errors, if cumulative, may account for the discrepancy between a high initial estimate and a low actual recovery.

Proration, because it greatly prolongs a stage comparable with the "flush" period in older fields, encourages a growing tendency to overestimate the reserves of oil in new fields. This tendency may be partially overcome by a more thorough analysis of those factors employed in such appraisals and by a recognition of the artificial character of the conditions governing production today.

It has become the practice to give to "recoverable oil percentages" a definite value, but, without an exact knowledge of the amounts of original oil, it is obvious that this factor is an approximation, at best. Whether or not the value of this percentage is predictable, it is probable that the influence of certain aids to recovery, such as the "water drive," has been much exaggerated.

The writer offers no fool-proof method of overcoming the difficulties inherent in this problem but calls attention to the harmful effects of overestimation and suggests a recognition of all of the factors having a bearing on such calculations and the adoption of a more conservative attitude toward their evaluation.