Page 12 - Well Logging and Formation Evaluation

2 Well Logging and Formation Evaluation

• HWC = hydrocarbon/water contact; the depth below which the forma-
tion is water bearing as encountered in a particular well. Likewise,
OWC for oil and GWC for gas
• GOC = gas oil contact; the depth below which any gas in the reservoir
will be dissolved in the oil
• Gross thickness = the total thickness of the formation as encountered
in a particular well
• Net thickness = the part of the gross thickness that contains porous rock
subject to given cutoff criteria
• Pay thickness = the part of the net thickness that is considered to be
capable of producing hydrocarbons in a particular well

Because of inherent uncertainties in all the parameters used to deter-
mine STOIIP or GIIP, geologists will usually develop probabilistic
models, in which all the parameters are allowed to vary according to dis-
tribution functions between low, expected, and high values. The resulting
static models may then be analyzed statistically to generate the following
values, which are used for subsequent economic analyses:

• P 50 STOIIP: the value of the STOIIP for which there is a 50% chance
that the true value lies either above or below the value
• P 15 STOIIP: the value of the STOIIP for which there is only a 15%
chance that the true value exceeds the value. Often called the high case.
• P 85 STOIIP: the value of the STOIIP for which there is an 85% chance
that the true value exceeds the value. Often called the low case.
• Expected STOIIP: the value of the STOIIP derived by taking the
integral of the probability density function for the STOIIP times the
STOIIP. For a symmetric distribution, this will equal the P 50 value.

Similar terminology applies to GIIP.
In order to predict the hydrocarbons that may be actually produced from
a field (the reserves), it is necessary to construct a dynamic model of the
field. This will generate production profiles for individual wells, subject
to various production scenarios. Additional terminology that comes into
play includes:

• Reserves = the part of the STOIIP or GIIP that may be actually pro-
duced for a given development scenario. Oil companies have their own
rules for how reserves are categorized depending on the extent to which
they are regarded as proven and accessible through wells. Terms fre-