This page contains a non-exhaustive list of terms used throughout the website. The references for this glossary are provided at the bottom of this page.

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A mixture of hydrocarbons (mainly pentanes and heavier) that exist in the gaseous phase at original temperature and pressure of the reservoir, but when produced, are in the liquid phase at surface pressure and temperature conditions. Condensate differs from NGLs in two respects: (1) NGL is extracted and recovered in gas plants rather than lease separators or other lease facilities, and (2) NGL includes very light hydrocarbons (ethane, propane, or butanes) as well as the pentanes-plus that are the main constituents of condensate.

A natural mixture of hydrocarbons and other compounds that have not been refined. Produced crude oil commonly contains solution gas and closely resembles the original oil from the reservoir rock. Hydrocarbons are the most abundant compounds in crude oils, but they also contain nitrogen, sulphur, and/or oxygen (NSO) compounds and variable concentrations of trace elements, such as vanadium and nickel.

Hydrocarbons are chemical compounds consisting wholly of hydrogen and carbon molecules.

The hydrocarbon-water contact (oil-water contact, OWC, or gas-water contact, GWC) is the lowest elevation at which mobile hydrocarbons occur. The gas-oil contact (GOC) is the elevation above which gas is the produced hydrocarbon phase.

In hydrocarbon accumulations there may be a transition zone in which water is coproduced with hydrocarbons.

Portion of petroleum that exists either in the gaseous phase or is in solution in crude oil in a reservoir, and which is gaseous at atmospheric conditions of pressure and temperature. The largest component of natural gas is methane (CH4) with modest amounts of heavier hydrocarbons such as propane, plus carbon dioxide and other inert gases.

Biogenic gas is formed at shallow depths and low temperatures by anaerobic microbial decomposition of sedimentary organic matter, readily observable in bogs, swamps and marshes, but also in landfills and shallow sediments. Consequently, it is often referred to as marsh gas or microbial gas.

In contrast, thermogenic gas is formed at deeper depths by: (1) thermal cracking of sedimentary organic matter into hydrocarbon liquids and gas (this gas is co-genetic with oil, and is called "primary" thermogenic gas), and (2) thermal cracking of oil at high temperatures into gas ("secondary" thermogenic gas) and pyrobitumen.

Biogenic gas is very dry (i.e. it consists almost entirely of methane). In contrast, thermogenic gas can be dry, or can contain significant concentrations of "wet gas" components (ethane, propane, butanes) and condensate (C5+ hydrocarbons).

A mixture of light hydrocarbons that exist in the gaseous phase in the reservoir and are recovered as liquids in gas processing plants. NGLs differ from condensate in two principal respects: (1) NGLs are extracted and recovered in gas plants rather than lease separators or other lease facilities, and (2) NGLs include very light hydrocarbons (ethane, propane, or butanes) as well as the pentanes-plus that are the main constituents of condensates.

Associated gases such as nitrogen, carbon dioxide, hydrogen sulphide, and helium that are present in naturally occurring petroleum accumulations.

Defined as a naturally occurring mixture consisting of hydrocarbons in the gaseous, liquid, or solid phase. Petroleum may also contain non-hydrocarbon compounds, common examples of which are carbon dioxide, nitrogen, hydrogen sulphide, and sulphur. In rare cases, non-hydrocarbon content of petroleum can be greater than 50%.

A mixture of hydrocarbons derived by upgrading (i.e., chemically altering) natural bitumen from oil sands, kerogen from oil shales, or processing of other substances such as natural gas or coal. SCO may contain sulphur or other non-hydrocarbon compounds and has many similarities to crude oil.

Petroleum plays

See the Play analysis lexicon for glossary terms specific to petroleum plays.

Diagram of conventional and unconventional play types

Figure 1. Conventional and unconventional play types

A natural system that links an active or once active source rock to all the geological elements and processes that are essential for a hydrocarbon accumulation to exist in time and space regardless of economics, such as carrier beds, reservoir rocks, seals and overburden, trap formation, migration, accumulation and preservation.

Petroleum systems have two processes:

  • Trap formation
  • Generation–expulsion–migration–accumulation of hydrocarbons.

A petroleum play consists of leads, prospects, and possibly producing fields with primary elements of similar geological history of hydrocarbon charge, reservoir/seal pairs and trap controls on their occurrence.

A prospect is a technically mature subsurface exploration opportunity, with the potential to contain commercial quantities of hydrocarbons, that consists of one or more untested potential traps or objectives in one or more plays.

Conventional oil and gas resources exist in discrete petroleum accumulations related to localised geological structural features and/or stratigraphic conditions, typically with each accumulation bounded by a downdip contact with an aquifer, and which is significantly affected by hydrodynamic influences such as buoyancy of petroleum in water.

Conventional natural-gas accumulations are those in reservoirs with a permeability high enough to allow gas to flow at rates for economic production.

A reservoir that cannot be produced at economic flow rates nor recover economic volumes of natural gas unless the well is stimulated by a large hydraulic fracture treatment or produced by use of a horizontal wellbore or multilateral wellbores.

Tight gas formations are generally divided into (1) gas reservoirs that occur in low-permeability, poor quality reservoir rocks in conventional structural and stratigraphic traps and (2) basin-centred gas accumulations (BCGA).

Unconventional resources exist in petroleum accumulations that are pervasive throughout a large area and that are not significantly affected by hydrodynamic influences (also called “continuous-type deposits”).

Unconventional gas resources have been divided into two broad types – those in which gas has been generated by natural thermogenic and/or biogenic processes, and those in which gas (synthesis gas) is synthetically generated underground or in a surface plant with a thermo-chemical process.

Examples of thermogenic gas include coal seam gas (CSG), basin-centred gas, shale gas, gas hydrate, natural bitumen (tar sands) and oil shale deposits.  Typically, such accumulations require specialized extraction technology (e.g. dewatering of CSG, fracture stimulation programs for shale gas, steam and/or solvents to mobilize bitumen for in-situ recovery and in some cases mining).

Natural gas from unconventional reservoirs is chemically the same as that from conventional reservoirs. Geology differentiates conventional from unconventional reservoirs. In some instances, the extracted petroleum may require significant processing prior to sale.

Global exploration focuses onshore for unconventional reservoirs rather than offshore due to the unfavourable economics of developing some types of unconventional reservoirs offshore.

Types of unconventional oil and gas plays

Basin-centred gas

An unconventional natural gas accumulation that is regionally pervasive and characterised by low permeability, abnormal pressure, gas saturated reservoirs and lack of a down-dip water leg.

Continuous gas accumulation

A gas accumulation that is pervasive throughout a large area and which is not significantly affected by hydrodynamic influences, so generally lacks a well-defined OWC or GWC.  Such accumulations are included in unconventional resources. Examples include “basin-centred” gas, coal seam gas, shale gas and gas hydrates.3

Coal seam gas (CSG)

Natural gas contained in coal deposits. The gas is primarily methane, and may also contain variable amounts of inert or non-inert gases. The term CSG is used in Australia, while elsewhere it may be termed coalbed methane (CBM) or natural gas from coal (NGC).

Underground coal gasification (UCG)

A technique by which solid coal underground is converted in situ into a gaseous product known as synthesis gas, or syngas. Also referred to as In-situ Gasification (ISG).

Oil shales

Shale, siltstone and marl deposits that are highly saturated with in situ kerogen (organic material from buried plants or animals). Whether extracted by mining or in situ processes, the material must be extensively processed such as by heating to yield a marketable product (synthetic crude oil). Sometimes referred to as kerogen shale.

Shale oil

Oil trapped in very low permeability mudrock which forms shale when deeply buried. The shale needs to be fracture stimulated to release the oil.

Shale gas

Shale gas is gas produced from organic-rich mudrock, which serve as the source and reservoir for the gas. Shales have very low matrix permeabilities (hundreds of nanodarcies), and therefore large natural or hydraulically induced fracture networks are required to produce the gas at economic rates.

Gas hydrates

Naturally occurring crystalline substances composed of water and gas, in which a solid water lattice accommodates gas molecules in a cage-like structure, or clathrate. At conditions of standard temperature and pressure (STP), one volume of saturated methane hydrate will contain as much as 164 volumes of methane gas. Due to this large gas-storage capacity, gas hydrates are thought to represent an important future source of natural gas. Gas hydrates are included in unconventional resources, but the technology to support commercial production has yet to be developed.

Production and processing techniques

Conventional recovery methods (primary and secondary) typically extract approximately one-third of the original oil-in-place in a reservoir. EOR techniques are used to recover the oil remaining as a residual oil saturation after conventional recovery has finished. EOR requires the injection of chemical compounds dissolved in water, the injection of steam, or the injection of a gas that is miscible with the oil in place.

Gas to liquids projects use specialised processing (e.g. Fischer-Tropsch synthesis) to convert natural gas or syngas into liquid petroleum products. These projects are applied to large natural gas accumulations where a lack of adequate infrastructure or local markets would make conventional natural gas development uneconomic. These projects can also use syngas as a feedstock, produced by gasification of coal deposits (mined or in-situ).

Liquefied natural gas (LNG) projects use specialised cryogenic processing to convert natural gas into liquid form for tanker transport. LNG is about 1/614 the volume of natural gas at standard temperature and pressure.

Syngas or synthesis gas is generated at the surface via coal to liquids (CTL) mining and surface processing plants or in situ (in position) by underground coal gasification (UCG - the coal gasification process uses the reaction of coal with oxygen and steam to create syngas). Syngas produced by coal gasification is typically a combustible mixture of predominantly carbon monoxide and hydrogen, with lesser amounts of carbon dioxide, methane and other gases (e.g. nitrogen, steam and gaseous hydrocarbons), depending on the composition of the coal.

Integrated gasification combined cycle (IGCC) plants combust syngas in a combined-cycle turbine system to produce electricity. Syngas is also widely used as a feedstock for the production of chemicals and liquid fuels.

Coal can be turned into gases and liquids that can be used as fuels or processed into chemicals to make other products. These gases or liquids are sometimes called synthetic fuels or synfuels, made by heating coal with hydrogen and a solvent in large vessels. The product is then further refined to achieve high grade fuel characteristics. These fuels produce fewer air pollutants when burned than burning coal directly.


The following were used to help define terms in this Glossary:

American Association of Petroleum Geologists (AAPG). AAPG Wiki

Holditch, S.A. 2006. Tight Gas Sands. J Pet Technology 58 (6): 86-93. SPE-103356-MS

Rice, D.D. and Claypool, G.E., 1981: Generation, Accumulation, and Resource Potential of Biogenic Gas; AAPG Bull., v. 65, pp 5-25

Society of Petroleum Engineers, 2011: Guidelines for the Application of the Petroleum Resources Management System.

Society of Petroleum Engineers, 2018: Petroleum Resources Management System (SPE-PRMS)

South Australian Department for Energy and Mining, 2018. Underground Coal Gasification in South Australia; Brochure 027

US Energy Information Administration (EIA). Natural gas explained, and Coal explained

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