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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Petroleum plays

Petroleum system1

A petroleum system includes all the geologic elements and processes that are essential if an oil and gas accumulation is to exist:

  • Source rock
  • Reservoir rock
  • Seal rock
  • Overburden rock

Petroleum systems have two processes:

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

Plays and prospects2

A petroleum play consists of one or more geologically related prospects, and a prospect is a potential trap that has been mapped and must be evaluated by drilling. Upon drilling, the site becomes either a dry hole or a producing field.

Conventional oil and gas 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.3

Conventional natural-gas accumulations are those in reservoirs with a permeability high enough to allow gas to flow at rates for economic production).2

Tight reservoir plays1

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 wellbores8. TGFs are generally divided into (1) basin-centred gas accumulations (BCGA) and (2) gas reservoirs that occur in low-permeability, poor quality reservoir rocks in conventional structural and stratigraphic traps.

Unconventional oil and gas plays4

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.

Biogenic gas is formed at shallow depths and low temperatures by anaerobic bacterial decomposition of sedimentary organic matter. 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).

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. It is primarily methane with modest amounts of heavier hydrocarbons such as propane, plus carbon dioxide and other inert gases. Geology differentiates conventional from unconventional reservoirs. In some instances, the extracted petroleum may require significant processing prior to sale e.g. bitumen upgraders.5

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

Diagram of conventional and unconventional play types
Figure 1. Conventional and unconventional play types

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.4

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

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, it may also be termed coalbed methane (CBM) or natural gas from coal (NGC).3

UCG, also referred to as ISG, is a technique by which solid coal underground is converted in-situ into a gaseous product known as synthesis gas, or syngas.6

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 (e.g. heating) to yield a marketable product - synthetic crude oil.4

Shale oil is oil trapped in very low permeability mudrock which forms shale when deeply buried. The shale needs to be fracture stimulated to release the oil.2

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.2

Gas hydrates are 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. Because of 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.4

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Condensates are a mixture of hydrocarbons (mainly pentanes and heavier) that exist in the gaseous phase at reservoir temperature and pressure, but when produced, are in the liquid phase at surface pressure and temperature conditions. Condensate differs from natural gas liquids (NGL) in two principal 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, butanes) as well as the pentanes-plus that are the main constituents of condensate.

Crude oil2

Crude oil is natural mixture of hydrocarbons and other compounds that have not been refined. 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.


Hydrocarbons are chemical compounds consisting wholly of hydrogen and carbon.

Natural gas

Natural gas is the portion of petroleum that exists either in the gaseous phase or is in solution in crude oil in natural underground reservoirs, and which is gaseous at atmospheric conditions of pressure and temperature. Natural gas may include some amount of non-hydrocarbons.

Natural gas liquids (NGL)

Natural gas liquids (NGL) are a mixture of light hydrocarbons that exist in the gaseous phase and are recovered as liquids in gas processing plants. NGL differs from condensate in two principal 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, butanes) as well as the pentanes-plus that are the main constituents of condensate.

Non-hydrocarbon gas3

Naturally occurring associated gases such as nitrogen, carbon dioxide, hydrogen sulphide and helium.


Petroleum is 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 could be greater than 50%.

Synthetic crude oil (SCO)4

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.

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Production and processing technologies

Enhanced oil recovery (EOR)2

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

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).

Liquified natural gas (LNG)

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 to liquids (CTL)7

The coal to liquids (CTL) process is a direct conversion process in which coal is reacted with hydrogen and a solvent under high temperature and pressure to produce liquid synfuels. This product is then further refined to achieve high grade fuel characteristics.


1 From the Guidelines for the Application of the Petroleum Resources Management System SPE, AAPG, WPC and SPEE, 2011

2 American Association of Petroleum Geologists (AAPG). AAPG Wiki

3 Modified from the Glossary of Terms appended to the Petroleum Resources Management System, SPE, AAPG, WPC and SPEE, 2007

4 From the Glossary of Terms appended to the Petroleum Resources Management System, SPE, AAPG, WPC and SPEE, 2007

5 AAPG, 2015. The most common kind of gas is "unconventional"

6 South Australian Department for Energy and Mining, 2018. Underground Coal Gasification in South Australia

7 US Energy Information Administration (EIA). Coal explained

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

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