Assessment Unit (AU):
A mappable volume of rock within the total petroleum system
(TPS) that encompasses fields (discovered and undiscovered) sharing
similar geologic traits and socioeconomic factors. The fields
within an AU should constitute a sufficiently homogeneous population
that the chosen methodology of resource assessment is applicable.
A TPS might equate to a single AU, but in other cases a TPS can be divided
into two or more AUs, depending on differences in the distribution and
ages of source rocks and reservoirs, so that resources can then be assessed
for each AU individually.
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Associated/dissolved
gas: Natural gas that occurs in an oil field, either
as a free gas cap or in solution; synonymous with gas in oil fields.
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Barrels of oil equivalent
(BOE): A unit of petroleum volume in which the gas portion
is expressed in terms of its energy equivalent in barrels of
oil. For this assessment, 6,000 cubic feet of gas equals 1 BOE.
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Field: A production
unit consisting of a collection of oil and gas pools that, when
projected to the surface, form an approximately contiguous area
that can be circumscribed.
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Gas field: A
field with a gas to oil ratio of 20,000 cubic feet/barrel or
greater.
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Gas in gas fields:
Gas volumes in gas fields.
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Gas in oil fields:
Gas volumes in oil fields.
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Gas to oil ratio
(GOR) : Ratio of gas to oil (in cubic feet/barrel) in
a field. In this assessment, GOR is calculated using known gas
and oil volumes at surface conditions.
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Geologic province:
A USGS-defined area typically covering hundreds to thousands
of square kilometers that encompasses a natural geologic entity
(for example, sedimentary basin, thrust belt, delta) or some combination
of contiguous geologic entities.
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Natural gas liquids
(NGL): Petroleum that occurs naturally as a gas in the
reservoir, but as a liquid under surface pressure conditions.
Natural gas liquids are typically reported separately from crude
oil.
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Nonassociated gas:
Natural gas in a gas field. Synonymous with gas in gas fields.
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Oil field: A
field with a GOR less than 20,000 (in cubic feet/barrel).
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Oil in gas fields:
Oil volumes in gas fields. For this assessment, oil in gas
fields was calculated along with other liquids rather than separately.
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Oil in Oil fields
: Oil volumes in oil fields.
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Petroleum: A
collective term for oil, gas, natural gas liquids, and tar.
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Total Petroleum System
(TPS): A mappable entity encompassing genetically
related petroleum that is in seeps, shows, and accumulations (discovered
or undiscovered) that have been generated by a pod or by closely
related pods of mature source rock, together with the essential
mappable geologic elements (source, reservoir, seal, and overburden
rocks) that controlled fundamental processes of
generation, migration, entrapment, and preservation
of petroleum.
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Undiscovered petroleum
resources: Resources postulated from geologic information
and theory to exist outside of known oil and gas fields.
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USGS assessed petroleum
volumes: The quantities of oil, gas, and natural gas
liquids that have the potential to be added to reserves within
some future time frame. For this assessment, the time frame is
30 years. The USGS assessed petroleum volumes include those from undiscovered
fields, whose sizes are greater than or equal to the stated minimum
field or pool sizes, and from the reserve growth of fields already discovered.
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