The Gaea50 probabilistic exploration prospect appraisal program

Introduction

This program follows the logic of the Petroleum System as far as the required information is available and uses optionally the results of a world-wide calibration effort. With more scanty information the system allows the user to make subjective guesses of probabilities and quantitified parameters. In a sense it is an
"expert system" as it attempts to encapsulate a large amount of geological information and theory. In that way it supports the explorationist with more background knowledge than he may personally possess.

Exploration prospect apparaisal is a combination of experience and statistical procedures. The result, to be useful in decision making consists of numbers, such as probabilities and volumes of hydrocarbons. To translate the geological/geophysical and other information into numbers a model is required that breaks up the problem into modules for which input parameters have to be estimated or guessed. The elements of the petroleum system, such as generation, migration, entrapment, retention and recovery have each their specific parameters. Depending on the sophistication of the model the number of parameters totals from 10 to some 60 for the Gaea50 program. For most of the inputs a probabilistic approach is used in the form of a " User-defined Distribution" or UDD" to specify the uncertainty about an input parameter.

The GAEA50 model has been set up slightly different from the earlier GAEAPAS model. We keep a more stringent separation between "VOLUME" and "CHANCE". Therefore the reult is a volume distribution and a probability of success (POS). As the probabilities are estimated separately (as different from GAEAPAS) the total number of Monte Carlo cycles (10,000) is used for the non-zero outcomes to obtain better resource volume-precision.

The results of the program are presented in such a way that addition of resource estimates, economic evaluations and ranking of exploration opportunities are facilitated.