Page 201 - Hydrocarbon

188 Expressing Uncertainty

1 + K i 2
1.12

1.08

1.04

1.00
thickness
area N/G phi S o B o RF

Input variable
Figure 7.12 Ranking of impact of variables on uncertainty in reserves.

in reserves. This approach can thus be used for
planning data gathering activities
planning how to mitigate the effects of downside in key variables
planning how to take advantage of upside in key variables.

7.2.6. Three-point estimates: a short-cut method
If there is insufficient data to describe a continuous probability distribution for a
variable (as with the area of a field in an earlier example), we may be able to make a
subjective estimate of high, medium and low values. If those are chosen using the
p85, p50 and p15 cumulative probabilities described in Section 7.2.2, then the
implication is that the three values are equally likely, and therefore each has a
probability of occurrence of 1/3. Note that the low and high values are not the
minimum and maximum values.
To estimate the product of the two variables shown in Figure 7.13, a short-cut
method is to multiply the low, medium and high values in a matrix (in which
numbers have been selected).
Note that the low value of the combination is not the absolute minimum (which
would be 4, and is still a possible outcome), just as the high value is not the
maximum. The three values (which are calculated by taking the mean of the three
lowest values in the matrix, etc.) represent equally likely outcomes of the product
A B, each with a probability of occurrence of 1/3.
This short-cut method could be repeated to include another variable, and could
therefore be an alternative to the previous two methods introduced. This method