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Statistical Methods for Estimating Petroleum Resources
total organic carbon content in excess of 0.5. The marginal probability
for adequate source is interpreted as 0.7—in other words,
P [TOC ≥ 0.5%] 5 0.7
where TOC is total organic carbon.
Dependence in Prospect-Level Geological Factors
Traditionally, exploration risk is an expression of the products of marginal probabilities of geological factors, such as the presence of closure,
reservoir facies, adequate source, and adequate seal. The statistical
assumption presumed in such a product operation is that risk factors
are independent. The assumption of independence of risk factors has
been challenged using exploratory well data obtained from the HuangHua Basin of eastern China.
Data from 242 exploratory wells in a sandstone play in the HuangHua Basin were analyzed to determine why a particular well had failed.
The presence or absence of closure and of reservoir facies, as well as
the adequacy of source and seal, were recorded for each well (Lee et al.,
1989). In Table 5.3, the number one indicates that a factor is present,
whereas zero indicates that a factor is absent.
First, if we assume that these factors are independent of each other,
then the overall prospect-level geological factor is the product of
184 /242, 220 / 242, 185 / 242, and 228 / 242, which equals 0.50.
Second, the geological factors were analyzed using the following
conditional probability formula:
P [Closure ʝ Reservoir Facies ʝ Source ʝ Seal ]
= P [Closure ] ×
P [Reservoir Facies | Closure ] ×
P [Source | Closure ʝ Reservoir Facies ] ×
(5.7)
P [Seal | Closure ʝ Reservoir Facies ʝ Source ]
= 184 / 242 × 127 / 184 × 111 / 127 × 109 / 111 = 0.45
The difference between these two approaches is 0.05. This example
demonstrates that geological factors might not be independent. The
dependency between any two factors was studied further by using
Statistical Methods for Estimating Petroleum Resources
total organic carbon content in excess of 0.5. The marginal probability
for adequate source is interpreted as 0.7—in other words,
P [TOC ≥ 0.5%] 5 0.7
where TOC is total organic carbon.
Dependence in Prospect-Level Geological Factors
Traditionally, exploration risk is an expression of the products of marginal probabilities of geological factors, such as the presence of closure,
reservoir facies, adequate source, and adequate seal. The statistical
assumption presumed in such a product operation is that risk factors
are independent. The assumption of independence of risk factors has
been challenged using exploratory well data obtained from the HuangHua Basin of eastern China.
Data from 242 exploratory wells in a sandstone play in the HuangHua Basin were analyzed to determine why a particular well had failed.
The presence or absence of closure and of reservoir facies, as well as
the adequacy of source and seal, were recorded for each well (Lee et al.,
1989). In Table 5.3, the number one indicates that a factor is present,
whereas zero indicates that a factor is absent.
First, if we assume that these factors are independent of each other,
then the overall prospect-level geological factor is the product of
184 /242, 220 / 242, 185 / 242, and 228 / 242, which equals 0.50.
Second, the geological factors were analyzed using the following
conditional probability formula:
P [Closure ʝ Reservoir Facies ʝ Source ʝ Seal ]
= P [Closure ] ×
P [Reservoir Facies | Closure ] ×
P [Source | Closure ʝ Reservoir Facies ] ×
(5.7)
P [Seal | Closure ʝ Reservoir Facies ʝ Source ]
= 184 / 242 × 127 / 184 × 111 / 127 × 109 / 111 = 0.45
The difference between these two approaches is 0.05. This example
demonstrates that geological factors might not be independent. The
dependency between any two factors was studied further by using