Gas cap

A free gas phase within a reservoir that overlies an oil zone.

Gas/condensate ratio

For a gas condensate reservoir, the ratio of gas to condensate is reported in cf per barrel. The inverse ratio (condensate-gas ratio, CGR) is also used, and is reported in barrels per mcf.

Gas condensate reservoir

A reservoir initially containing natural gas that will precipitate hydrocarbon liquid (retrograde condensate) during pressure depletion. To increase the recovery of the condensate, gas may be re-cycled in early years and produced at a later date.

Gas cycling

Process in which produced gas is re-injected into the reservoir after removal of condensate in order to maintain reservoir pressure and prevent condensate from condensing in the reservoir (retrograde condensation) and becoming difficult to recover. Also known as gas recycling.

Gas day

A period of 24 consecutive hours for measuring gas activity starting at a set time, which is 6:00AM in the UK.

Gas-distribution line

A gas pipeline, normally operating at pressures of 60 pounds per square inch (psi) or less, which transports gas from high-pressure transmission lines to end-users.


A field or group of reservoirs of hydrocarbons containing natural gas, but insignificant quantities of oil.

Gas-gathering system

A system for collecting gas production from different sources for delivery by pipeline to a central point, such as a platform or processing facility. The gas sources could be individual wells, smaller gathering systems, field facilities and platforms.

Gas grid

1) the system of pipelines from the wellhead to the city gate. 2) the network of gas transmission and distribution pipelines in a region or country, through which gas is transported to industrial, commercial and domestic users.

Gas imbalance

A discrepancy between a transporter’s receipt and deliveries of natural gas for a shipper.

Gas lift

One of several methods of artificial lift. A mechanical process using the continuous or intermittent injection of a gas into the production conduit (tubing or casing) to aerate or displace the produced fluids. This creates a reduction of the bottom-hole pressure of the well, increasing or sustaining the flow rate of the well.

Gas marketing

Supply of gas to end users by companies, usually by pipeline.  Also known as gas supply.

Gas processing

The separation of oil and gas, and the removal of impurities and NGLs from natural gas.

Gas reserves

Those quantities of gas that are likely to be commercially recovered from known accumulations from a given date forward.

Gas revenue

The product of gas volume times gas price; gross cash flow from sales of gas.

Gas send-out

The total natural gas produced or purchased (including exchange-gas receipts), or the net natural gas withdrawn from underground storage within a specified time interval, measured at the point of production, purchase or withdrawal, adjusted for changes in local storage quantity.

Gas-to-liquids (GTL)

Conversion of natural gas into high quality synthetic gasoline or distillates, usually using the Fischer-Tropsch process. The technology is most economical when there is a local supply of gas, but only a small local market, or where the gas feedstock is abundant and cheap.  

Gas-to-oil ratio (GOR)

The number of standard cubic feet of gas produced per barrel of crude oil or other hydrocarbon liquid. In some parts of the world, the units are cubic metres of gas per cubic metre of liquid produced.

Gas treatment

Removal of impurities, such as sulphur compounds, carbon dioxide and water vapour from natural gas.

Gas-turbine power plant

A power plant in which the prime mover is a gas turbine. A gas turbine typically consists of an axial-flow compressor that feeds compressed air into one or more combustion chambers where liquid or gaseous fuel is burned. The resulting hot gases are expanded through the turbine, causing it to rotate. The rotating turbine shaft drives the compressors as well as the generator, producing electricity.

Gas well

A well drilled and completed that primarily produces natural gas.

Gathering line

Network-like pipeline that transports natural gas from individual wellheads to a compressor station, treating or processing plant, or main trunk transmission line. Gathering lines are generally relatively short in length and smaller in diameter than the gas sales line.

Gigajoule (GJ)

A joule is an international unit of energy defined as the energy produced from one watt flowing for one second. A very small unit of energy, there are 3.6m joules in a kilowatt-hour. For gas, one gigajoule = 960 cf under standard temperature and pressure conditions. Roughly, 1 gigajoule (Gj) = mcf; one terajoule (Tj) = 1 mmcf; one petajoule (Pj) = 1 bcf; one exajoule (Ej) = 1 tcf.

Gigawatt (GW)

A unit of electric power equal to 1 billion watts, 1 million kilowatts or 1,000 megawatts – enough power to supply the needs of a medium-sized city.

Gigawatt hour (GWh)

1 billion watt-hours.

Grandfather clause

A clause in a contract that maintains the prior rule or policy where a new rule or policy would otherwise be applicable.

Greenfield LNG facility

An LNG facility constructed on a new site, rather than on the site of an existing facility.


A network of pipelines through which gas is transported.

Gross freight

Freight cost excluding the expenses relating to the running costs of the ship.

Gross gas withdrawal

The full volume of compounds extracted at the wellhead, including non-hydrocarbon gases and natural gas plant liquids.

Gross tonnage

Common measurement of the internal volume of a ship determined in accordance with prescribed methods and formulas and expressed in units of 100 cf (= 2.83 cm).


Contact by a ship with the bottom while she is moored or anchored or under way.