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Statistical Methods for Estimating Petroleum Resources
Some geological factors such as source, maturation, and migration,
for example, would normally be present throughout a play, but at an
early stage of exploration we cannot determine whether these factors
are in place. PETRIMES provides ways of handling this type of uncertainty. However, we fi rst need to explore the concepts of play-level and
prospect-level geological factors.
Play-Level Geological Factor
The play-level geological factor measures the chance that a geological factor is common to all prospects within a play, and is a regional
phenomenon across an entire play. The occurrence of a play-level geological factor is denoted by G (global); the marginal probability of this
event is represented by u g . White (1980) referred to the play-level geological factor as a play chance or group risk (Gehman et al., 1981; White
and Gehman, 1979).
If a play contains hydrocarbons, then all geological factors are
present. Let these factors or events be denoted by G 1 , G 2 , . . . , G j . The
probability of a play having hydrocarbons is then
u gi = P [G i ]
= P [a play has factor G i ]
= P [a geological factor G i is satisfi ed for all prospects within
the play, i = 1, … , j ]
For example, G 1 = [adequate source], G 2 = [adequate preservation], … .
If all play-level geological factors exist, then
u g = P [G 1 ʝ G 2 ʝ · · · ʝ G j ]
= P [play possessing all factors]
(5.2)
If any of these G i values do not occur, then the play does not contain
hydrocarbons. If G 1 , G 2 , … , G j are statistically independent, then the
probability of having all play-level geological factors simultaneously is
defi ned as follows:
j
g
g i
i
= ∏
u
u
(5.3)
(5.1)