A play is a group of exploration prospects or potential fields with geologically similar source, trap and reservoir characteristics contained within a continuous stratigraphic interval. The total economic reserves for an untested play can be estimated via a straightforward, albeit subjective, procedure; and, its net present-worth value can be approximated in an analogous manner.

First, assume that the play exists and model it in accordance with the following estimates: (1) the underlying prospect-size distribution-assumed to be equivalent to the field-size distribution (2) the distribution of the number of prospects in the play and the conditional probability that any prospect in the play will contain hydrocarbons - or, more simply, the distribution of the number of fields in the play (3) the practical truncation points for the prospect-size distribution - the minimum (economic) and maximum field sizes for the play. All of these estimates can be based on the analysis of all geochemical, geologic and geophysical data pertinent to the play and/or information collected from one or more similar (look-alike) plays.

Next, these random variables - field size and number of fields - are combined probabilistically, without recourse to Monte Carlo simulation, to permit the approximation of the distribution for the total economic reserves associated with the play. Finally, estimate (4) the probability that the play really does exist - the chance that adequate source, trap and reservoir are present; and, approximate the distribution of the total, geologically-risked, economic reserves for the play.

For the economic assessment, the evaluation procedure used to determine the minimum (economic) field size is extended over the entire doubly-truncated field-size distribution; and, the same combinatorial process is employed to obtain the distribution of the total, unrisked, net present-worth value of the play. Risk is introduced via the probability that the play is valid and the expected cost of exploration; and, the distribution of the play’s total, geologically-risked, net present-worth value is approximated.

This paper discusses the theoretical bases for the proposed procedure and some practical considerations applicable to input information. It also presents a complete example that uses actual field data and compares the results with an independent evaluation of the same data.

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