Learn more about the terms used throughout the site.

API gravity

A measure of the relative density (weight per volume) of a petroleum liquid compared to the density of water. Oils are categorized by their API gravity, as follows: extra-heavy (<15 API); heavy (15–22 API); medium (22–32 API); light (32–40 API); and ultra-light (>40 API, which are included in the OCI test oils but not identified as ultra-light in Phase 1). It is commonly believed that the higher an oil’s API gravity, the lower its GHG emissions. But API gravity is not necessarily a good indicator of an oil’s total GHG emissions.

Associated gas

Methane (natural gas) or other gaseous hydrocarbons that are found in an oil reservoir and may or may not be dissolved in the oil.


A semi-solid hydrocarbon resource that is classified as extra-heavy oil and includes oil sands or tar sands, which requires extensive upgrading and/or deep conversion refining to produce high value petroleum products.

Bunker fuel production

A type of heavy fuel oil (also known as bunker C) that remains after the distillate fuel oils are removed during the refining process. Bunker fuels can be used in marine vessels, electric power, space heating, and various industrial purposes, and they are produced in different amounts depending on the oil characteristics and the refinery configuration.


Part of the downstream portion of the oil supply chain that entails burning petroleum products derived from crude to provide heat or power, such as burning gasoline in an automobile.

Deep conversion refinery

Part of the midstream portion of the oil supply chain, this is a complex refining configuration used to turn the heaviest oils into petroleum products. Techniques can involve using high temperatures and catalysts to separate (or crack) hydrocarbons and then reject the excess carbon (a process called coking) and/or add hydrogen (hydrotreating) in order to increase production of the highest-value petroleum products.

Diesel production

A fuel composed of distillates or distillate blended with residual oil used in trucks’ compression-ignition engines and other motors that is produced in different amounts depending on an oil’s characteristics, the refinery configuration, and global demand.


Bitumen diluted with one or more lighter petroleum products, typically natural-gas condensates such as naphtha, to make it easier to transport, for example, in a pipeline.


The end of the oil supply chain that involves transporting petroleum products to markets and their end use, including but not limited to combustion.

End use

An overarching term for the part of the downstream portion of the oil supply chain that involves the consumption of all petroleum products that are made from a barrel of oil, whether they are combusted, used as feedstock for consumer products, or otherwise marketed.


Part of the upstream portion of the oil supply chain that entails drilling, fracturing, mining, or otherwise accessing oil resources.


The process of burning off associated gas that is produced with oil because equipment does not exist to gather and sell the gas on the market.

Flare rate

The amount of gas flared, measured in standard cubic feet of gas per day.

Flaring-to-oil ratio

An upstream operating decision regarding the amount of gas flared for every barrel of oil produced, measured in standard cubic feet of gas per barrel.

Gas-to-oil ratio

The estimated total amount of gas production associated with the production of one barrel of oil, measured in standard cubic feet of gas per barrel.

Gasoline production

A complex mixture of relatively volatile hydrocarbons used primarily in motor vehicles with spark-ignition engines that is produced in different amounts depending on an oil’s characteristics, the refinery configuration, and global demand.

Greenhouse gases (GHGs)

The long-lived gases that, when emitted, cause global warming and result in climate change, including carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O).

Greenhouse gas emissions (GHG emissions)

Emissions of long-lived climate impacting gases, measured in CO2 equivalent (eq.) units using each GHG’s ability to warm the climate, known as its global warming potential.

Hydroskimming refinery

Part of the midstream portion of the oil supply chain, this is one of the simplest refinery configurations that separates lighter oils into its hydrocarbon components using heat (distillation) and employs other basic treating processes to remove hydrogen gas in order to make high-octane gasoline.

Medium conversion refinery

Part of the midstream portion of the oil supply chain, this is an extension of a hydroskimming refinery that separates medium-gravity oils into their hydrocarbon components using heat and pressure (vacuum distillation) as well as a process (catalytic cracking) that breaks heavier hydrocarbons into gasoline, diesel, and other distillate feedstock.


The middle portion of the oil supply chain that entails refining crude oil or otherwise transforming hydrocarbons into petroleum products.

Offsite emissions

Activities that generate GHG emissions that cross an upstream (or midstream) boundary. In terms of emissions accounting, emission-related activities that enter the site count as additional emissions (for example, natural gas purchases) and those that are sent offsite are credited and count as negative emissions (for example, natural gas sales).

Oil Assay

A chemical analysis of a crude oil that provides extensive, detailed experimental data for refiners to establish, at a minimum, the compatibility of a crude with a particular petroleum refinery. The OCI requires that assays are reported in a consistent, standardized format so that they can be input into the OCI midstream model (PRELIM).

Oil-climate category

An OCI classification scheme that conveys the general characteristics of crude oils: light, conventional, ultra-deep, high gas, heavy, depleted/watery, high steam, high flare, and extra-heavy.

Oil depth

The depth of an oil field, measured in feet to the well bore. For vertical wells, this is equal to the depth below the surface. Offshore oils are measured from the ocean floor. For inclined or horizontal wells, this depth can exceed the vertical depth below the surface.

Petroleum coke

A solid fuel also known as petcoke or coke that is made up predominantly of carbon, delivered from oil upgraders, refinery coker units, or other cracking processes, that is used predominantly in power generation and for industrial purposes.

Petcoke combustion

Petcoke is produced in larger amounts both upstream when extra-heavy oils are upgraded and downstream when heavy and extra-heavy oils are refined. It is produced in smaller amounts when any oil is processed in a deep conversion coking refinery. Burning petcoke for heat or power generation results in higher GHG emissions than is the case with other petroleum products.

Refinery configuration

A midstream decision regarding the types of processing units assembled to transform oils into petroleum products, including hydroskimming for light, sweet oils; medium conversion configurations for medium, sweet and sour oils; and deep conversion configurations for heavy, sweet and sour oils.


Part of the midstream portion of the oil supply chain that entails processing a crude oil into a suite of final petroleum products for use as energy carriers (such as gasoline) or noncombustible products (such as petrochemicals).

Steam-to-oil ratio

An upstream operating decision affecting the amount of steam input for thermal-oil-recovery projects. This ratio measures the rate of steam injection per barrel of oil produced in barrels of water equivalent per barrel of oil.

Sulfur content

Oils are categorized according to the amount of sulfur they contain as follows: sweet (<0.5 percent sulfur) and sour (>0.5 percent sulfur). In the case of synthetic crude oils derived from oil sands, the sulfur content is reported after the bitumen is upgraded into synthetic crude oil.

Synthetic crude oil

An extra-heavy oil, bitumen, or oil sand that, prior to midstream refining, has undergone extensive upstream processing and upgrading to remove a portion of its carbon (in the form of petcoke) and convert it into a synthetic hydrocarbon that simulates conventional oil.


The movement of oil in the supply chain, including after extraction when crude oil is moved to a refinery and after refining when petroleum products are sold. Transportation GHG emissions vary with distance traveled, modes of transport, and transport fuels used.


An upstream process that converts extra-heavy oil, bitumen, and oil sands into synthetic crude oils of varying qualities by primarily removing their excess carbon and converting that into petroleum coke.


The beginning of the oil supply chain that includes resource extraction, production, surface processing and upgrading, waste disposal, and other miscellaneous operations as well as transporting oil to the refinery.

Water injection ratio

The amount of water injected into a well to stimulate oil production, measured in barrels of water injected per barrel of crude oil produced.

Water-to-oil ratio

The total amount of water generated when producing oil, measured in barrels of water produced per barrel of crude oil produced.

Water intensity

An upstream operating decision regarding the amount of liquid water throughput required to produce a barrel of oil, including both water injected and wastewater generated.

Years in production

The time that an oil play has been in active service. Over long periods, this can impact GHG emissions as an oil field depletes, changing its characteristics and the techniques required both upstream and midstream.