Page 286 - Standard Handbook Petroleum Natural Gas Engineering VOLUME2

Reserve Estimates 253

that the objective formation is laterally continuous and contains commercially
recoverable hydrocarbons at locations beyond direct offsets.
Reserves that can be produced through the application of established improved
recovery methods are included in the proved classification when: (1) successful
testing by a pilot project or favorable production or pressure response of an
installed program in that reservoir, or one in the immediate area with similar
rock and fluid properties, provides support for the engineering analysis on
which the project or program is based and (2) it is reasonably certain the project
will proceed.
Reserves to be recovered by improved recovery methods that have yet to be
established through repeated commercially successful application are included
in the proved category only after a favorable production response from the
reservoir from either (1) a representative pilot or (2) an installed program that
provides support for the engineering analysis on which the project or program
is based.
Unproved Reserves. These are based on geologic and/or engineering data
similar to that used in estimates of proved reserves, but technical, contractual,
or regulatory uncertainties preclude such reserves being classified as proved.
Estimates of unproved reserves may be made for internal planning of special
evaluations, but are not routinely compiled.
Unproved reserves are not to be added to proved reserves because of different
levels of uncertainty. Unproved reserves may be divided into two subclassifica-
tions: probable and possible.
Probable Reserves. These reserves are attributed to known accumulations and
are less certain to be recovered than proved reserves. In general, probable
reserves may include (1) reserves that appear to exist a reasonable distance
beyond the proved limits of productive reservoirs, where fluid contacts have
not been determined and proved limits are established by the lowest known
structural occurrence of hydrocarbons, (2) reserves in formations that appear
to be productive from core and/or log characteristics only, but that lack
definitive tests or analogous producing reservoirs in the area, (3) reserves in a
portion of a formation that has been proved productive in other areas in a field,
but that is separated from the proved area by faults, (4) reserves obtainable by
improved recovery methods and located where an improved recovery method
(that has yet to be established through repeated commercially successful opera-
tion) is planned but not yet in operation, and where a successful pilot test has
not been performed but reservoir and formation characteristics appear favorable
for its success, (5) reserves in the same reservoir as proved reserves that would
be recoverable if a more efficient primary recovery mechanism were to develop
than that assumed in estimating proved reserves, (6) incremental reserves
attributable to infill drilling where closer statutory spacing had not been
approved at the time of the estimate, and ('7) reserves that are dependent for
recovery on a successful workover, treatment, retreatment, change of equipment,
or other mechanical procedures, when such procedures have not been proved
successful in wells exhibiting similar behavior in analogous formations.
Possible Reserves. These are associated with known accumulations and are
less certain to be recovered than probable reserves. In general, possible reserves
may include (1) reserves indicated by structural and/or stratigraphic extrapola-
tion from developed areas, (2) reserves located where reasonably definitive geo-
physical interpretations indicate an accumulation larger than could be included