112
Statistical Methods for Estimating Petroleum Resources
If we defi ne u ri = P [R i |G] and u r = P [R |G], then the probability of
hydrocarbons being present is defi ned as
P [a prospect containing hydrocarbon]
= P [G ʝ R ] = P [R | G ] × P [G ]
(5.4)
= u r × u g
If the geological factors are independent, then the prospect-level
geological factor is defi ned as
k
r
r i
i
= ∏
u
u
(5.5)
If the risk factors are not independent, then the rule of multiplication of the conditional probability rule must be applied as follows:
u r = P [R 1 ∩ R 2 ∩ · · · ∩ R k ]
(5.6)
Integrating information obtained from tested wells with data from
adjacent wells can identify the presence or absence of a particular prospect-level geological factor. For example, the presence or absence of
closure can be recognized by reviewing stratigraphic or seismic correlations after drilling. The existence of reservoir facies can be identifi ed
from mechanical logs. Adequacy of seal can be established by examining (1) the presence or absence of cap rock, (2) the quality of the seal,
and (3) possible leakage of the closure. Adequate source and migration
factors mean that oil has migrated into the trap. Therefore, if a potential reservoir is shown from drill stem tests to contain either oil, oil
shows, or oil traces, then the factor is considered to be present.
Marginal Probability Distribution
Figure 5.2A displays a probability distribution for the geological factor
of adequate maturation. The assumption used here is that either the
sample size is large enough to represent the play (population), or it is
a random sample from the play (population). We also assume that the
geochemical interpretations are valid.
The distribution suggests a 70% chance that the percentage of
hydrocarbons extracted from the play in question would range from
Statistical Methods for Estimating Petroleum Resources
If we defi ne u ri = P [R i |G] and u r = P [R |G], then the probability of
hydrocarbons being present is defi ned as
P [a prospect containing hydrocarbon]
= P [G ʝ R ] = P [R | G ] × P [G ]
(5.4)
= u r × u g
If the geological factors are independent, then the prospect-level
geological factor is defi ned as
k
r
r i
i
= ∏
u
u
(5.5)
If the risk factors are not independent, then the rule of multiplication of the conditional probability rule must be applied as follows:
u r = P [R 1 ∩ R 2 ∩ · · · ∩ R k ]
(5.6)
Integrating information obtained from tested wells with data from
adjacent wells can identify the presence or absence of a particular prospect-level geological factor. For example, the presence or absence of
closure can be recognized by reviewing stratigraphic or seismic correlations after drilling. The existence of reservoir facies can be identifi ed
from mechanical logs. Adequacy of seal can be established by examining (1) the presence or absence of cap rock, (2) the quality of the seal,
and (3) possible leakage of the closure. Adequate source and migration
factors mean that oil has migrated into the trap. Therefore, if a potential reservoir is shown from drill stem tests to contain either oil, oil
shows, or oil traces, then the factor is considered to be present.
Marginal Probability Distribution
Figure 5.2A displays a probability distribution for the geological factor
of adequate maturation. The assumption used here is that either the
sample size is large enough to represent the play (population), or it is
a random sample from the play (population). We also assume that the
geochemical interpretations are valid.
The distribution suggests a 70% chance that the percentage of
hydrocarbons extracted from the play in question would range from