ABSTRACT

Most geological information leading to the discovery of new oil and gas fields in a play is produced by drilling exploratory wells. The amount of information that each well contributes to an overall understanding of the play's petroleum geology, and to the discovery of the next field in particular, depends on the number and location of previous wells, the complexity of the regional geology, and the quantity and characteristics of the remaining undiscovered fields. In most plays, the information gained per well follows three distinct phases. First is an initial period of gaining insight into regional geology and testing preliminary exploration strategies, characterized by low exploration efficiency, as measured in volumes of hydrocarbons discovered per foot (metre) of exploratory drilling. The second phase begins with discovery of the "key" exploration concept which governs the trapping of most of the hydrocarbons in the play. Exploration efficiency rises in this phase and reaches its maximum. The final phase is controlled by the average size and "visibility" of the fields that remain in the play after the discovery of the few largest and medium size fields that, in most instances, contain two thirds to over three quarters of the play's resources. Exploration efficiency either drops to zero, or in some plays, depending on economic considerations and the average size and characteristics of small fields, reaches a low, stable value.

The fact that the results of exploratory drilling in a play exhibit serial, as well as spatial correlation, suggests a Markovian process, which is susceptible to analysis through the constructs of information theory. In that context, the "state" of the play before drilling a given well is defined by the number of successful and dry holes drilled, and the quantity of resources discovered. There are a finite number of states the play can take after each additional well is drilled. These are defined by the same variables, though their values (e.g., the number of dry and successful wells, volumes discovered) will have changed. If all potential traps in the play were tested in the course of exploration, the ratio of dry to productive wells and quantity of resources discovered will be the same, independent of the order of wells or discoveries. The exploration process is therefore considered ergotic. For ergotic processes, entropy, as a measure of the information content of each exploratory well, can be measured by examining the succession of probabilities of state transitions as a function of cumulative drilling.

Exploitation of information content of exploratory wells in this framework provides a new approach to measurement of exploratory efficiency and to the estimation of future efficiency and the ultimate resources of a play. Such an approach is orthogonal to most of the traditional methods of assessment, and is unique in that it incorporates into a single system of analysis the impacts of geological complexity, field characteristics and exploration technology applied in the play. Examples taken from simulations and the exploration of plays in the United States are used to illustrate this approach to analysis of the exploration process and assessment of undiscovered resources.

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