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Glossary of Oil & Gas terms


Abandon To cease work on a well which is non-productive, to plug off the well with cement plugs and salvage all recoverable equipment . Also used in the context of field abandonment.

Appraisal Well

A well drilled as part of an appraisal drilling programme which is carried out to determine the physical extent, reserves and likely production rate of a field.

Associated Gas

Natural  gas associated with oil accumulations, which may be dissolved in the oil at reservoir
conditions or may form a cap of free gas above the oil.


A unit of volume measurement used for petroleum and its products (7.3 barrels = 1 ton:
6.29 barrels = 1 cubic metre).


A North Sea acreage sub-division measuring approximately 10 x 20 kms, forming part of a
quadrant. e.g. Block 9/13 is the 13th block in Quadrant 9.

preventers (BOPs)

Are high pressure wellhead valves, designed to shut off the uncontrolled flow of hydrocarbons.


When well pressure exceeds the ability of the wellhead valves to control it. Oil and gas "blow wild" at the surface.


The hole as drilled by the drill bit.

Casing string

The steel tubing that lines a well after it has been drilled. It is formed from sections of steel tube screwed together.

Christmas tree 
(Subsea tree)

The assembly of fittings and valves on the top of the casing which control the production rate of oil.                                                          
CNS Central North Sea.

Commercial field

An oil and/or gas field judged to be capable of producing enough net income to make it worth developing.

The installation of permanent wellhead equipment for the production of oil and gas.


Hydrocarbons which are in the gaseous state under reservoir conditions and which become
liquid when temperature or pressure is reduced. A mixture of pentanes and higher hydrocarbons.


Taking rock samples from a well by means of a special tool -- a "core barrel".

Crane barge

A large barge, capable of lifting heavy equipment onto offshore platforms. Also known as a
"derrick barge".

Rock chippings cut from the formation by the drill bit, and brought to the surface with the mud. Used by geologists to obtain formation data.


The tower-like structure that houses most of the drilling controls.

Development phase   

The phase in which a proven oil or gas field is brought into production by drilling production
(development) wells.

Drilling rig A drilling unit that is not permanently fixed to the seabed, e.g. a drillship, a semi-submersible or a jack-up unit. Also means the derrick and its associated machinery.

Dry Gas Natural gas composed mainly of methane with only minor amounts of ethane, propane and butane and little or no heavier hydrocarbons in the gasoline range.

Dry hole

A well which has proved to be non-productive.

E&A Abbreviation for exploration and appraisal.


Abbreviation for exploration and production.

Exploration drilling

Drilling carried out to determine whether hydrocarbons are present in a particular area or structure.

Exploration phase

The phase of operations which covers the search for oil or gas by carrying out detailed geological and geophysical surveys followed up where appropriate by exploratory drilling.

Exploration well

A well drilled in an unproven area. Also known as a "wildcat well".


A geographical area under which an oil or gas reservoir lies.


Floating storage and offloading system, often a ship or barge-shaped floating hull
incorporating tanks for storage of produced oil, and a method of loading the oil into offtake
tankers. These installations do not have any production or processing facilities.


Floating production, storage and offloading vessel which includes, in addition to its storage
and offloading capability, facilities for receiving crude oil from producing wells and processing
it for export by seperating water and gas.


Floating storage unit; a floating facility intended only for storage of oil. Export may be by pipeline to an onshore facility rather than offloaded to shuttle tankers.

Gas field A field containing natural gas but no oil.

Gas injection

The process whereby separated associated gas is pumped back into a reservoir for conservation purposes or to maintain the reservoir pressure.


A compound containing only the elements hydrogen and carbon. May exist as a solid, a liquid or a gas. The term is mainly used in a catch-all sense for oil, gas and condensate.

Injection well

A well used for pumping water or gas into the reservoir.


The lower section, or "legs", of an offshore platform.

Lay barge

A barge that is specially equipped to lay submarine pipelines.

Liquified natural
gas (LNG)
Oilfield or naturally occurring gas, chiefly methane, liquified for transportation.

petroleum gas

Light hydrocarbon material, gaseous at atmospheric temperature and pressure, held in the liquid state by pressure to facilitate storage, transport and handling. Commercial liquified gas consists
essentially of either propane or butane, or mixtures thereof.


A subsea mainifold is structure consisting of pipework and valves used to transfer hydrocarbons
from a subsea well.


An aperture in the centre of a drillship or semi-submersible drilling rig, through which drilling anddiving operations can be conducted.

Natural gas

Gas, occurring naturally, and often found in association with crude petroleum.


Natural gas liquids. Liquid hydrocarbons found in association with natural gas.


A mixture of liquid hydrocarbons of different molecular weights.

Oil field

A geographic area under which an oil reservoir lies.


The company that has legal authority to drill wells and undertake production of hydrocarbons are
found. The Operator is often part of a consortium and acts on behalf of this consortium.


The property of a formation which quantifies the flow of a fluid through the pore spaces and into the wellbore.


A generic name for hydrocarbons, including crude oil, natural gas liquids, natural gas and their

Pipeline crossing

Where two or more pipelines cross over one another on the seabed.


Pipeline end manifold, a system which transfers products to offtake tankers.


An offshore structure that is permanently fixed to the seabed.


The underground formation where oil and gas has accumulated It consists of a porous rock to hold the oil or gas, and a cap rock that prevents its escape.

Riser (drilling) A pipe between a seabed BOP and a floating drilling rig.


 The section of pipework that joins a seabed wellhead to the Christmas tree.


A production hiatus during which the platform ceases to produce while essential maintenance
work is undertaken.


Single point mooring. A buoy anchored offshore that serves as a mooring point for offloading
and connecting for tankers loading or offloading oil or gas.


The operation of drilling the first part of a new well.

Suspended well

A well that has been capped off temporarily.

Template A metal structure fixed to the seabed to form a base for subsea equipment.


The superstructure of a platform.


United Kingdom Continental Shelf.

Wildcat well A well drilled in an unproven area. Also known as a "exploration well".


West of Shetland Isles.