Abstract

One of the first steps in reserve assessment is to define uncertain parameters (height, area, net to gross ratio, porosity, saturation...) in terms of a probability distribution. This is done by subjective fitting of the expected range of each parameter to a log-normal curve such that:

– the minimum corresponds with the 5% confidence level

– the most likely value corresponds with the mode of the distribution

– the maximum corresponds with the 95% confidence level.

This very general model can be used in the following ways:

1. Lead or prospect evaluation. We show how it is possible to multiply by hand different stochastically distributed parameters (height, length, width, geometric factor, net to gross ratio, porosity, saturation, volume factor) in order to obtain the resulting hydrocarbon volume distribution easily and directly.

2. Field evaluation. The principal geological hypotheses concerning reserve distribution and the various related development schemes can be described by means of a decision tree. Thereafter, each branch of the tree can be evaluated with the above described log-normal methodology.

3. Country, continent or world reserve evaluation. Either identified or unidentified reserves can be described with a log-normal curve. Moreover, for identified reserves, one can associate proven reserves with the 5% confidence level, probable reserves with the mode of the distribution, possible reserves with the 95% confidence level.

Lastly we show how a log-normal distribution with minimum V₁ (5% confidence), most likely V₂ (mode), maximum V₃ (95% confidence), can be summarized into:

– its mean (expected value) m 1/3 (V₁ + V₂ + V₃)

– its dispersion (standard deviation) 0 1/3 (V₃ − V₁)

Thus, the Decision Theory criterion (i.e. risk adjusted value = m − σ²/2P, where P is the maximum bearable loss) can be directly hand-computed from these three scenarios.

For the next fifteen years, more than two thirds of capital investment in the oil and gas industry will be devoted to exploration and production.

Amongst the many risks involved in Exploration and Production, uncertainty about the reserve situation has been and will continue to be one of the most important, especially as future discoveries become smaller.

This explains why, in common with other oil Companies TOTAL is greatly concerned about this risk. In this talk, we indicate how we, in TOTAL, have developed a methodology and classification for reserve evaluation.