TERMINOLOGY
The terminology used in this report is intended to represent
standard definitions and usage of the oil and natural gas industry
and the resource-assessment community. No attempt has been made to
include a detailed listing of common industry definitions; however,
several definitions that are essential to the proper understanding
of this report are presented. The definitions are intended to be
generally explanatory rather than strictly technical.
Undiscovered resources.-Resources postulated from geologic
information and theory to exist outside of known oil and (or) gas fields.
Technically recoverable resources.-Resources in accumulations
producible using current recovery technology but without reference to
economic profitability. These are oil and natural gas resources that
may be produced at the surface from a well as a consequence of natural
pressure within the subsurface reservoir, artificial lifting of oil
from the reservoir to the surface, and the maintenance of reservoir
pressure by fluid injection. (This definition is modified from that of
the National Petroleum Council.) These resources are generally conceived
as existing in accumulations of sufficient size to be amenable to the
application of existing recovery technology.
Measured (proved) reserves.-That part of the identified economic
resource that is estimated from geologic evidence supported directly by
engineering data. Measured reserves are demonstrated with reasonable
certainty to be recoverable in future years from known reservoirs under
existing economic and operating conditions. Resources in this category
are not a principal focus of this assessment. Data reported are from
the Energy Information Administration (Energy Information Administration, 1994).
Conventional accumulation.-A discrete deposit, usually bounded by a
downdip water contact, from which oil, gas, or NGL can be extracted
using traditional development practices, including production at the
surface from a well as a consequence of natural pressure within the
subsurface reservoir, artificial lifting of oil from the reservoir to
the surface where applicable, and the maintenance of reservoir pressure
by means of water or gas injection.
Continuous-type deposit.-A hydrocarbon accumulation that is pervasive
throughout a large area, that is not significantly affected by
hydrodynamic influences, and for which the standard methodology for
assessment of sizes and numbers of discrete accumulations is not
appropriate.
Unconventional accumulation.-A broad class of hydrocarbon deposits
of a type (such as gas in "tight" sandstones, gas shales, and coal-bed gas)
that historically has not been produced using traditional development
practices. Such accumulations include most continuous-type deposits.
Field growth (inferred reserves).-That part of the identified
resources over and above measured (proved) reserves that will be added
to existing fields through extension, revision, improved recovery
efficiency, and the addition of new pools or reservoirs.
Inferred reserves.-For this report, inferred reserves is the
difference between proved reserves in known fields and the remaining
recoverable resources in known fields-this definition of inferred
reserves includes two resource categories used in previous USGS oil
and gas assessment documents (e.g., Mast and others, 1989): "indicated
reserves" and "inferred reserves."
Indicated reserves.-That part of identified oil resources in
known productive reservoirs in existing fields in addition to measured
reserves that are expected to respond to improved recovery techniques.
For this report, indicated reserves are included as part of inferred
reserves.
Barrels of oil equivalent (BOE).-Gas volume that is expressed in
terms of its energy equivalent in barrels of oil. For this assessment,
6,000 cubic feet of gas equals 1 barrel of oil equivalent (BOE).
Gas-oil ratio (GOR).-Average ratio of associated-dissolved gas
to oil; a point estimate of the volume of gas (in cubic feet) dissolved
in oil or otherwise associated with a barrel of oil in known or postulated
oil accumulations. As in the most recent National Assessment (Mast and
others, 1989), an accumulation with a GOR in excess of 20,000 is considered
a gas accumulation.
NGL to non-associated gas ratio.-The volume of natural gas liquids
(in barrels) contained in 1 million cubic feet of gas in a known or
postulated gas accumulation.
NGL to associated-dissolved gas ratio.-The volume of natural gas
liquids (in barrels) contained in 1 million cubic feet of associated-
dissolved gas in a known or postulated oil accumulation.
Field.-An individual producing unit consisting of a single pool or
multiple pools of hydrocarbons grouped on, or related to, a single
structural or stratigraphic feature.
Accumulation.-A single oil or gas deposit as defined by the trap,
charge, and reservoir characteristics of the play.
Play.-A play is a set of known or postulated oil and (or) gas
accumulations sharing similar geologic, geographic, and temporal
properties, such as source rock, migration pathway, timing, trapping
mechanism, and hydrocarbon type.
Play area.-The two-dimensional plan extent over which a play concept
is considered to be valid and within which all known accumulations and
potential for undiscovered accumulations or other additions to reserves
within the play exist.
Play attributes.-Geologic characteristics that describe principal
properties of and necessary conditions for the occurrence of oil and
(or) gas accumulations of the minimum size (1 MMBO [million barrels of
oil] or 6 BCFG [billion cubic feet of gas]) within the defined parameters
of a play. Although many combinations of individual underlying elements
are possible, three attributes were considered in the evaluation of play
risk in this assessment. These attributes are as follows:
1. Charge.-The occurrence of conditions of hydrocarbon generation and
migration adequate to cause an accumulation of the minimum size.
Included in this attribute are subsidiary elements, including
existence of source rocks with sufficient organic matter of the
appropriate composition, appropriate temperature and duration of
heating to generate and expel sufficient quantities of oil and (or)
gas, and timing of expulsion of oil and gas from source rocks
appropriate for filling available traps.
2. Reservoir.-The occurrence of reservoir rocks of sufficient quantity
and quality to permit the containment of oil and (or) gas in volumes
sufficient for an accumulation of the minimum size.
3. Trap.-The occurrence of those structures, pinch-outs, permeability
changes, and similar features necessary for the entrapment of oil
and (or) gas in at least one accumulation of the minimum size.
Included in this attribute are existence of seals sufficient for
entrapping hydrocarbons and capable of holding oil and gas
accumulations during appropriate ranges of geologic time.
Play probability.-Play probability represents the product of the
probabilities of the three play attributes considered in this
assessment (charge, reservoir, and trap). It is an estimate, expressed
as a decimal fraction, of the chance that oil or natural gas exist
within the particular play. For recoverable resources, the play
probability represents the likelihood that technically recoverable
quantities of oil or natural gas exist in at least one accumulation of
the minimum size (1 MMBO or 6 BCFG) in the area being assessed.
Conditional estimates.-Sizes, numbers, or volumes of oil or natural
gas that are estimated to exist in an area, assuming that they are present.
Conditional estimates, therefore, do not incorporate the risk that the
area may be devoid of oil or natural gas.
Risked (unconditional) estimates.-Resources that are estimated to
exist, including the possibility that the area may be devoid of oil or
natural gas. Statistically, the risked mean may be determined through
multiplication of the mean of a conditional distribution by the related
probability of occurrence. Resource estimates presented in this report
are risked estimates.
Cumulative probability distributions for resource estimates.-Graphical
depictions of estimated resource volumes presented with associated cumulative
probabilities of occurrence. These distributions are used to derive the 95
percent, 5 percent, and mean resource levels reported in this publication: a
low case, with a 95 percent probability of that amount or more occurring
(a 19 in 20 chance); a high case, with a 5 percent probability of that amount
or more occurring (a 1 in 20 chance); and a mean case representing an arithmetic
average of all possible resource outcomes weighted by their probabilities.