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  • Gas cap

    a layer of natural gas above the oil in an oil reservoir

    Gas cap drive

    a recovery mechanism whereby the oil from the reservoir is displaced by the downward expansion of an overlying gas cap.

    Gas cycling or re-cycling

    a process in which produced gas is re-injected into the reservoir after removal of the condensate. This is to maintain the reservoir pressure and prevent condensate from 'condensing' in the reservoir and then becoming difficult to recover. This is called retrograde condensation.

    Gas field

    a field containing natural gas but insignificant quantities of oil.

    Gas gathering system

    a central collection point for offshore gas fields, with pipelines from a number of fields, often owned by a number of different companies. From there, the gas is transported to a central processing system onshore

    Gas grid

    the term used for the network of gas transmission and distribution pipelines in region or country, through which gas is transported to industrial, commercial and domestic customers.

    Gas injection

    see gas cycling

    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 bottomhole pressure of the well, increasing or sustaining the flow rate of the well.

    Gas liquefaction

    the process of cooling natural gas to a temperature of -162°C, thereby reducing its volume by a factor of 600, and making it liquid. The resulting LNG is then transportable by purpose-designed ships (LNG carriers) or may be stored in tanks.

    Gas oil

    the medium distillates from the oil refining process intermediate between light lubricating oils and kerosene; used to produce diesel fuel and burned in central heating systems.

    Gas pipeline

    a pipeline used to transport gas between two places, either offshore or onshore.

    Gas processing

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

    Gas to oil ratio (GOR

    the volume of gas at atmospheric pressure produced in association with a unit volume of oil.

    Gas treatment

    removal of impurities, condensate, hydrogen sulphide and any liquids from the raw natural gas contained in a gas field.

    Gas well

    a borehole sunk into the ground sunk into the ground with the objective of bringing natural gas to the surface.

    Gas/condensate field

    a reservoir containing both natural gas and oil, with a greater proportion of gas. Condensate appears when the gas is drawn from the well, and its temperature and pressure change sufficiently for some of it to become liquid petroleum.

    Gas/condensate ratio

    1) for a gas/condensate reservoir this is the ratio of the condensate to the gas. As for oil it can be measured in scf per bbl. Alternatively the inverse is used and the typical units are bbl per mmscf, 2) for a dry gas field only the inverse is normally used. Typical units are again bbl per mmscf, but grammes per cubic metre may well be used.

    Gas/condensate reservoir

    a reservoir in which neither natural gas nor crude oil is the predominant production stream. To increase the recovery of the condensate, the gas may be re-cycled for the early years and produced at a later date.

    Gaseous fuels

    the light, highly volatile distillates from the refining process; used to manufacture motor spirit, aviation gasoline and camping gas


    the process whereby gaseous fuel is manufactured from a solid or liquid fuel.


    the fuel used in cars and motorcycles etc (also known as petrol). Naturally occuring gasoline is known as condensate

    Geological time scale


    the detectors used in seismic surveys to pick up sound waves reflected from sub-surface strata.


    a large sedimentary basin in which extremely deep layers of sediments have accumulated over a long period of time.

    Geothermal gradient

    a measure of the rise of rock temperature with increasing depth below the Earth's surface.

    Going in hole

    lowering the drill string into the wellbore

    Gravimetric survey

    a geological survey method in which measurements of the variations in the surface gravitational field are used to determine the nature of the underlying rocks.

    Gravity platform

    a platform, generally made of reinforced concrete, which sits on the sea bed and is kept in position by its own weight, no piling being used.

    Greenhouse Gases (GHG

    gas that contributes to the formation of an insulating blanket around the earth. Water vapour, carbon dioxide, methane, nitrous oxide and halogenated hydrocarbons are the main GHGs.

    Gross tonnage

    the volume of the interior of a crude oil tanker including all spaces which are permanently closed in (but excluding the double bottom), expressed in tons per 100 cubic feet.

    Gun perforator

    a device fitted with a vertical series of explosive charges that is lowered down a well on a wireline to perforate the formation; also known as a perforating gun.

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