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Statistical Methods for Estimating Petroleum Resources
addition or subtraction of the variance from the pool-size distribution.
If a lognormal distribution were adopted, the variance and covariance
could be adjusted. The Beaverhill Lake play was used to describe the
roles of Equations 5.11 and 5.12 in the pool-size equation.
Figure 5.6A displays the positive correlation between the random
variables of pool area and average net pay. Figure 5.6B demonstrates
the impact of the covariance to the pool-size distribution. The solid line
of Figure 5.6B shows the pool-size distribution derived by omitting the
covariance, whereas the circles show the pool-size distribution derived
by including the covariance. The former distribution has a mean of
13 × 10
6 m
3 and the latter has a mean of 32 × 10
6 m
3
. The difference is
more than double.
Figure 5.7 demonstrates the impact of negative covariance on the
pool size. The solid line shows the pool-size distribution derived by
omitting the negative covariance between the pool area and average
net pay, whereas the circles show the pool-size distribution derived by
including the covariance. The difference of the two means is more than
double.
Figure 5.5. (A, B) Log-linear relationships between water saturation and
porosity of Bashaw play (A) and Cardium play (B), Western Canada Sedimentary
Basin.