Glossary For Peak Oil and Gas, Energy Terminology

in Peak Oil by


The Association for the Study of Peak Oil and Gas
Cumulative production
The cumulative production is the sum of all oil that has ever been produced until a specific date. Cumulative production can be given for a field, oil basin, country or the world.
Decline rate
The decline rate refers to production only. It is defined as the negative relative change of production over a time period. Often a period of a year is used. The decline rate can be expressed as a fraction or as percent.
( Last year’s production – This year’s production ) / Last year’s production
Assume a production of 1 Gb in year 2000 and 0.95 Gb in year 2001. The decline rate for year 2001 would then be (1 – 0.95)/1 = 0.05 = 5%. If the production is rising, the decline rate becomes negative.
Depletion rate
The depletion differs from the decline rate in that it takes into account the amount of oil that is left. The depletion rate is defined as this year’s production divided by the amount of oil that is left
Depletion rate = This year’s production / Oil left at start of this year
The amount of oil left is calculated by taking the URR minus last year’s cumulative production. The depletion rate depends on the estimated amount of oil left. As more oil is produced, less oil is left. At a constant production the depletion rate grows while the decline rate is zero. The depletion rate can never become negative.


Geological basin
A large geological area in which sedimentation is occurring or has occurred. Certain parts of the basin might therefore have the required geological conditions to trap oil. Consists of many oil fields.
Hubbert peak theory
A synonym for Peak oil theory



In context of oil production, the definition from BP Statistical Review (B.4) is used: crude oil, shale oil, oil sands and NGLs (natural gas liquids – the liquid content of natural gas where this is recovered separately).
In context of oil consumption, the definition from World Energy Outlook (A.6) is used: crude oil, condensates, natural gas liquids, refinery feedstocks and additives, other hydrocarbons and petroleum products (refinery gas, ethane, LPG, aviation gasoline, motor gasoline, jet fuels, kerosene, gas/diesel oil, heavy fuel oil, naphtha, white spirit, lubricants, bitumen, paraffin waxes, petroleum coke and other petroleum products).
Oil-in-place / Recovery factor
Oil-in-place is the estimated total amount of oil that is in the ground before production has started. For various reasons far from all of this oil can be recovered. Oil-in-place is usually calculated on a field basis and in an early stage. The oil-in-place value is multiplied by a valuecalled recovery factor and results in an estimated URR for a single field (see below). Later in a field’s production phase the URR is usually calculated with other techniques
Oil reservoir
A subsurface porous and permeable rock body that contains oil, gas or both.
Oil field
An area consisting of a single reservoir or multiple reservoirs all grouped on, or related to, the same individual geological structural feature or stratigraphic condition.
Production refers to the amount of oil that is produced during a certain time period (most often a day or a year). The following units are common in litterature:
kb/d (1 000 barrels per day)
Mb/d (1 000 000 barrels per day)
Gb/y (1 000 000 000 barrels per year)
1 Mb/d = 365/1000 = 0.365 Gb/y
1 Gb/y = 1000/365 = 2.74 Mb/d


Recoverable Reserves (Estimated future production from known fields)
The recoverable reserves are an estimate of how much recoverable oil is still left in the already found oil fields. It can only be an estimate since it’s impossible to know exactly how much oil is still in the ground.
Because of this uncertainty, reserves are calculated with a certain probability. A reserve estimate followed with, for instance, ‘P90’ means that there is a 90% chance that there is at least as much recoverable oil as the reserve estimate claims.


Ultimately Recoverable Resources (URR)
URR is a concept with many names: Total Recoverable Reserves, Ultimately Recoverable Reserves (shortened to URR) or simply “Ultimate”
The URR is an estimate of the total amount of recoverable oil that exists in the ground before the production starts. In case of a single oil field, URR is defined as:
URR (single field) = cumulative production + recoverable reserves
When talking about a region or a country, the URR refers to the total amount of oil that will ever be produced from that region/country including yet-to-find fields:
URR (region, country) = cumulative production + recoverable reserves + yet-to-find
A wildcat is an exploration borehole drilled when searching for oil or gas.


Yet-to-find is an estimate of the amount of recoverable oil that exists in fields that have not yet been found.

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