Page 43 - Introduction to Petroleum Engineering

RESOURCES AND RESERVES 27
lower‐quality deposits at the lower part of the triangle. A resource deposit at the top of
the triangle has a relatively large concentration of resource that is relatively inexpen-
sive to extract. A resource deposit at the lower part of the triangle has a lower
concentration of resource, so the extraction is more difficult or expensive. For example,
a large, shallow, light oil reservoir that is onshore would be at the top of the triangle,
while a small, deep, heavy oil reservoir in an offshore environment would be at the
base of the triangle. The amount of resource that is both easy to extract and has a high
concentration is expected to be small, while the amount of resource in a low‐
concentration deposit is difficult or expensive to extract and is expected to be large.

2.2.1 Reserves
Resource size tells us how much of a resource is present in a deposit. The amount of
the resource that can be extracted is discussed in terms of reserves. The definition of
reserves is presented in the Petroleum Reservoir Management System maintained by
the Society of Petroleum Engineers (SPE‐PRMS, 2011). Reserves classifications are
summarized in Table 2.1.
The probability distribution associated with the SPE‐PRMS reserves definitions
recognizes that there is a statistical distribution of resource deposits in nature. For
example, oil reservoirs vary in size from reservoirs with relatively small volumes of
oil to reservoirs containing relatively large volumes of oil. If we plot the volume of
oil in the reservoir versus the number of reservoirs with that volume, we can develop
a distribution of reservoirs as a function of size. This distribution can be represented
by a frequency distribution and can be interpreted as a probability distribution
of reservoir size. If we combine reservoir size and recovery factor, we can obtain
a probability distribution of reserves. As an illustration, suppose we assume the
probability distribution of reserves is a normal distribution. Normal distributions

TABLE 2.1 SPE‐PRMS Reserves Definitions
Proved reserves Those quantities of petroleum, which by analysis of geoscience and
engineering data, can be estimated with reasonable certainty to be
commercially recoverable, from a given date forward, from known
reservoirs, and under defined economic conditions, operating
methods, and government regulations
There should be at least a 90% probability (P ) that the quantities
90
actually recovered will equal or exceed the low estimate
Probable reserves Those additional reserves which analysis of geoscience and engineering
data indicate are less likely to be recovered than proved reserves but
more certain to be recovered than possible reserves
There should be at least a 50% probability (P ) that the quantities
50
actually recovered will equal or exceed the best estimate
Possible reserves Those additional reserves which analysis of geoscience and engineering
data suggests are less likely to be recoverable than probable reserves
There should be at least a 10% probability (P ) that the quantities
10
actually recovered will equal or exceed the high estimate