Peak Oil Definitions and Terms

It’s important to realize that when you hear total oil production in barrels per day (around 85 million today), this isn’t actually crude oil only, it’s several components, all known as ‘liquids’: crude oil (light); bitumin, heavy crudes (tar sands, Orinoco); liquid natural gas (LNG); biofuels (biodiesel and ethanol); and Condensates:

Natural gas condensate is a low-density mixture of hydrocarbon liquids that are present as gaseous components in the raw natural gas produced from many natural gas fields. It condenses out of the raw gas if the temperature is reduced to below the hydrocarbon dew point temperature of the raw gas. The natural gas condensate is also referred to as simply condensate, or gas condensate, or sometimes natural gasoline because it contains hydrocarbons within the gasoline boiling range.

At the raw natural gas processing plant, the gas will be dehydrated and acid gases and other impurities will be removed from the gas. Then the ethane (C2), propane (C3), butanes (C4) and C5 plus higher molecular weight hydrocarbons (referred to as C5+) will also be removed and recovered as byproducts.

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. Assume a production of 1 Gb in year 2007 and 0.95 Gb in year 2008. The decline rate for year 2008 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.

Distillates (Fuel Oils)
The oil industry’s term for diesel and heating oil. As opposed to gasoline, for example.

Fuel oil in the United States is classified into six classes, numbered 1 through 6, according to its boiling point, composition and purpose. The boiling point, ranging from 175 to 600 °C, and carbon chain length, 20 to 70 atoms, of the fuel increases with fuel oil number. Viscosity also increases with number, and the heaviest oil has to be heated to get it to flow. Price usually decreases as the fuel number increases. No. 1 fuel oil, No. 2 fuel oil and No. 3 fuel oil are variously referred to as distillate fuel oils, diesel fuel oils, light fuel oils, gasoil or just distillate. For example, No. 2 fuel oil, No. 2 distillate and No. 2 diesel fuel oil are almost the same thing (diesel is different in that it also has a cetane number limit which describes the ignition quality of the fuel). Distillate fuel oils are distilled from crude oil. Gas oil refers to the process of distillation. The oil is heated, becomes a gas and then condenses. It differentiates distillates from residual oil (RFO). No. 1 is similar to kerosene and is the fraction that boils off right after gasoline. No. 2 is the diesel that trucks and some cars run on, leading to the name “road diesel”. It is the same thing as heating oil. No. 3 is a distillate fuel oil and is rarely used. No. 4 fuel oil is usually a blend of distillate and residual fuel oils, such as No. 2 and 6, however, sometimes it is just a heavy distillate. No. 4 may be classified as diesel, distillate or residual fuel oil. No. 5 fuel oil and No. 6 fuel oil are called residual fuel oils (RFO) or heavy fuel oils. As far more No. 6 than No. 5 is produced, the terms heavy fuel oil and residual fuel oil are sometimes used as synonyms for No. 6. They are what remains of the crude oil after gasoline and the distillate fuel oils are extracted through distillation. No. 5 fuel oil is a mixture of No. 6 (about 75-80%) with No. 2. No. 6 may also contain a small amount of No. 2 to get it to meet specifications. Residual fuel oils are sometimes called light when they have been mixed with distillate fuel oil, while distillate fuel oils are called heavy when they have been mixed with residual fuel oil. Heavy gas oil, for example, is a distillate that contains residual fuel oil. The ready availability of very heavy grades of fuel oil is often due to the success of catalytic cracking of fuel to release more valuable fractions and leave heavy residue.

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.

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.

Peak Oil
The term Peak Oil refers to the maximum rate of the production of oil in any area under consideration, recognising that it is a finite natural resource, subject to depletion.

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.

Based on information in

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