Evaluation Models
25
variables is an important element to be considered in resource evaluation; otherwise, the mean and variance of a pool-size distribution may
be over- or underestimated (see Chapter 5).
Mixed Populations
Figure 2.12A is a lognormal probability plot of all discovered Keg
River reefs currently known from the Black Creek basin of the Western
Canada Sedimentary Basin. The plot shown in Figure 2.12B displays
the reefs from the Keg River shelf basin–Rainbow play, a subbasin
within the Black Creek basin. Most of the data in Figure 2.12B follow
a straight line, but the plot tends to be slightly convex upward. This
convex-upward phenomenon may be the result of both dependent and
biased sampling, because of the selective nature of the discovery process (i.e., large pools have higher probabilities of being discovered).
Therefore, the nonlinearity in Figure 2.12A may be indicative of a
mixed population.
The lack of linearity in the plot may be indicative of any one or all of
the following circumstances:
The data set chosen is not from a lognormal population. Figure
1.
2.13, for example, shows Pareto and Weibull data sets plotted on
the log probability plot, which exhibits a serpentine pattern.
The data set was not chosen randomly (see Chapter 3).
2.
There is more than one population in the data set (Fig. 2.12A).
3.
The sample size is too small, as shown in Figure 2.14, which
4.
displays probability plots for a simulated lognormal distribution with different sample sizes. It is apparent that the plots
become straighter when the sample size increases. The impact
of mixed populations from lognormal, Pareto, and Weibull
populations on the uncertainty of estimations will be discussed in Chapter 4.
From this overview of the nature of geological populations, we now
move on in the next chapters to a discussion of how to apply these statistical models in petroleum resource evaluation.
25
variables is an important element to be considered in resource evaluation; otherwise, the mean and variance of a pool-size distribution may
be over- or underestimated (see Chapter 5).
Mixed Populations
Figure 2.12A is a lognormal probability plot of all discovered Keg
River reefs currently known from the Black Creek basin of the Western
Canada Sedimentary Basin. The plot shown in Figure 2.12B displays
the reefs from the Keg River shelf basin–Rainbow play, a subbasin
within the Black Creek basin. Most of the data in Figure 2.12B follow
a straight line, but the plot tends to be slightly convex upward. This
convex-upward phenomenon may be the result of both dependent and
biased sampling, because of the selective nature of the discovery process (i.e., large pools have higher probabilities of being discovered).
Therefore, the nonlinearity in Figure 2.12A may be indicative of a
mixed population.
The lack of linearity in the plot may be indicative of any one or all of
the following circumstances:
The data set chosen is not from a lognormal population. Figure
1.
2.13, for example, shows Pareto and Weibull data sets plotted on
the log probability plot, which exhibits a serpentine pattern.
The data set was not chosen randomly (see Chapter 3).
2.
There is more than one population in the data set (Fig. 2.12A).
3.
The sample size is too small, as shown in Figure 2.14, which
4.
displays probability plots for a simulated lognormal distribution with different sample sizes. It is apparent that the plots
become straighter when the sample size increases. The impact
of mixed populations from lognormal, Pareto, and Weibull
populations on the uncertainty of estimations will be discussed in Chapter 4.
From this overview of the nature of geological populations, we now
move on in the next chapters to a discussion of how to apply these statistical models in petroleum resource evaluation.