ABSTRACT

In geological play assessment, it is necessary to estimate the size distribution of undiscovered fields. Exxon currently uses four modelling approaches:

1. fitting of discovered sizes within the play, honouring trends in size with discovery sequence
2. compositing of assessments of untested prospects within the play
3. convolution of statistical models for measured closure sizes with play-wide reservoir and fillup models
4. use of in-house tables relating the potential field size distribution to the expected size of the largest prospect

Size distributions from geological analogs are occasionally used where no discovery history is present and data are too sparse to identify any prospects. Each method has some unique strengths and weaknesses. Confidence can be gained by finding agreement between different methods. Moreover, the size distribution of appropriately screened fields can be used to tune play parameters for prospect-based methods, which may then be extended with more confidence from mature areas into frontier areas.

Modelling progresses in four steps:

1. geological definition of plays and classification of known fields and prospects
2. choice of the minimum size of fields and prospects to be included in the count, and culling of fields and prospects to be excluded from the first step
3. fitting mathematical models to the selected populations
4. assessing the potential contribution from smaller fields

The assessment process is assisted with an interactive graphics computer program to extract field and prospect data from on-line databases, and cull and fit models to the selected populations. One important output of this system shows composited projections of future field size distribution for aggregations of plays, with overlays of the distributions of known fields and inventoried prospects.

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