Evaluating Conceptual Plays
137
should collect all relevant data and information from similar basins to
address the following questions and concerns.
The fi rst question that might arise in the case of frontier plays
1.
is: What is the probability that a play exists? This issue can be
analyzed in terms of the presence or absence of factors such as
source rocks, maturation, migration, and favorable reservoir
facies. A marginal probability is applied to each factor to indicate the likelihood that the factor exists (geological factors are
listed at the beginning of this chapter).
If a geological random variable in question has an extreme
2.
range of values, then its variance should be relatively large. On
the other hand, if the values are uniform, then the variance
should be small.
Remember that we do not have enough data to compute
3.
covariances between variables. However, positive or negative
covariances are evident from geological data. Therefore, if the
largest estimated pool size is not what we expect, the following
questions should be addressed: Are the mean and variance of the
pool-size distribution adequate? How much covariance exists?
Table 5.12. Format for Entry of Probability Distributions
Geological variable
Unit of measurement
Probability in upper percentile
1.0
0.5 0.02/0.01 0.00
Area of closure of pool
mi.
2 or km
2
Net pay/no. of pay zones
m or ft./no.
Reservoir or formation
thickness
m or ft.
Porosity
Decimal fraction
Trap fi ll
Decimal fraction
Favorable facies
Decimal fraction
Water saturation
Decimal fraction
Oil or gas saturation
Decimal fraction
Shrinkage factor
Decimal fraction
Formation volume factor
Decimal fraction
Reservoir temperature
º Celsius or Fahrenheit
Reservoir pressure
kPa or psi
Recovery factor
Decimal fraction