A glossary of terms used on the website

American Petroleum Institute; API gravity, usually expressed as °API, is a measure of how heavy or light a petroleum liquid is compared to water.

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 and commonly referred to as “decommissioning’.

Annex B
Operator’s development plan for an offshore installation. It requires government approval before it can be implemented.

Appraisal well
A well drilled as part of an appraisal drilling programme to better understand and estimate the physical extent, reserves and likely production rate of a field.

Argillaceous limestone
A limestone containing a significant proportion of clay minerals.

Associated gas
Natural gas produced with crude oil from the same reservoir.

Barrel, commonly abbreviated to bbl
A unit of volume measurement used for petroleum and its products (around 7.3 barrels = 1 ton, 6.29 barrels = 1 cubic metre).

Billion cubic feet; 1 bcf = 0.83 million tonnes of oil equivalent.

Billion cubic metres (1 cubic metre = 35.31 cubic feet).  

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.

Oil, condensate and gas, as the case may be, are produced simultaneously, without any pressure maintenance in the reservoir.

Blow-out preventer (BOP)

BOPs are high-pressure wellhead safety valves, designed to shut off the uncontrolled flow of hydrocarbons in an emergency situation. They are deployed during drilling activities.

In the very unusual and unlikely event that well pressure exceeds the ability of the wellhead valves or drilling mud to control it, oil and gas could "blow wild" up to the surface.

Barrels of oil equivalent per day.

Barrels of oil per day.

The hole as drilled by the drill bit.

British thermal unit (BTU)
A measure of the heating value of a fuel.

Capital expenditure.

Casing and casing string
Metal pipe inserted into a wellbore and cemented in place to protect both subsurface formations (such as groundwater) and the wellbore. A surface casing is set first to protect groundwater. The production casing is the last one set. The production tubing (through which hydrocarbons flow to the surface) will be suspended inside the production casing.

Common Data Access is a not-for-profit subsidiary of Oil & Gas UK, set up in 1994 to provide data management services to its members and to the UK oil industry in general.

Chemical precipitation of ions carried in groundwater chemically precipitating to form new crystalline material between sedimentary grains.

Chance of Success (COR, or Probability of Success, POS)
The probability that an exploration well will encounter flowable hydrocarbons from a prospect with a described volumetric probability distribution. The COS or POS represents the combined probability of the presence of key elements of a petroleum accumulation such as: the presence of mature source rock, effective hydrocarbon migration and charge, the presence of an effective reservoir and seal couplet and the presence and definition of a hydrocarbon trapping mechanism or trap.

Rocks composed of broken pieces of older rocks.

Containing coccoliths, which are the skeletal remains of calcareous algae/plankton as found ubiquitously in the Chalk of NW Europe.

Commercial field
An oil and/or gas field judged to be capable of producing enough net income, at sufficiently low risk, to make it economic for development.

The installation of permanent wellhead and downhole equipment for the production or injection

An engine used to increase the pressure of natural gas so that it will flow more easily through a pipeline or from a reservoir.

Hydrocarbons which are in the gaseous state under reservoir conditions and which become liquid when temperature or pressure is reduced, such as when reservoir fluids flow up production tubing to surface. Typically, a mixture of pentanes (C5s) and higher hydrocarbons.

Contingent resources
Contingent resources are those quantities of petroleum estimated, as of a given date, to be potentially recoverable from known accumulations, but the applied project(s) are not yet considered mature enough for commercial development due to one or more contingencies. Contingent resources may include, for example, projects for which there are currently no viable markets, or where commercial recovery is dependent on technology under development, or where evaluation of the accumulation is insufficient to clearly assess commerciality. Contingent resources are further categorised in accordance with the level of certainty associated with the estimates and may be sub-classified based on project maturity and/or characterised by their economic status.1C = Low estimate scenario of contingent resources.2C = Best (Most Likely, Mid) estimate scenario of contingent resources.3C = High estimate scenario of contingent resources.

Conventional oil
Oil found or expected to be present within a conventional reservoir. The term can be applied to trapped petroleum defined by a discovered discrete petroleum accumulation or play related to localised geological structural features and/or stratigraphic condition, typically with the accumulation bounded by a down-dip contact with an aquifer, and which is significantly affected by hydrodynamic influences such as the buoyancy of petroleum in water.

Core and coring
A continuous cylindrical sample of rock from the wellbore, normally taken in 30 ft sections by means of a special “core barrel” tool.

Creaming Theory
A statistical technique which recognises that in any exploration province after an initial period in which the largest fields are found, success rates and average field sizes decline as more exploration wells are drilled and knowledge of the area matures.

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.

An engine used to increase the pressure of natural gas so that it will flow more easily through a pipeline.  


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

Development well
A well drilled within the proven area of an oil or gas reservoir to the depth of a stratigraphic horizon known to be productive; a well drilled in a proven field for the purpose of completing the desired spacing pattern of production; development wells can also be used for the injection of water to maintain reservoir pressure, or to safely dispose of produced water deep underground.

A discovery is a petroleum accumulation for which one or several exploratory wells have established through testing, sampling and/or logging the existence of a significant quantity of potentially moveable hydrocarbons.

A term used to describe tools, equipment, and instruments used in the wellbore, or conditions or techniques applying to the wellbore.

When referring to the oil and gas industry, this term indicates the refining and marketing sectors of the industry. More generically, the term can be used to refer to any step further along in the process from “upstream”crude oil and natural gas production.

Drill mud
Drilling fluid used to clean and lubricate the drilling process, recover samples of the sub-surface formations to surface and to counteract the natural pressure of the formation.

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. Drilling rigs are also used onshore for land-based drilling activities.

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.

The Department for Energy Security and Net Zero


Abbreviation for exploration and appraisal.

Abbreviation for exploration and production. The ‘upstream’ sector of the oil and gas industry.

Electric logs
Tools used within the wellbore to measure the rock and fluid properties of surrounding rock formations.

Enhanced oil recovery (EOR)
A secondary production process whereby oil is recovered other than by the natural pressure in a reservoir. Refers to a variety of processes to increase the amount of oil removed from a reservoir, typically by injecting a liquid (e.g. water, surfactant) or gas (e.g. natural gas, nitrogen, carbon dioxide).

Exploration well
Drilling carried out to determine whether hydrocarbons are present in a particular area or structure. Sometimes known as a ‘wildcat well’, particularly in areas where little drilling has taken place previously.


When a company acquires an interest in a licence by taking over all or part of the financial commitment for drilling an exploration well.

Fault block
A very large subsurface block of rock, created by tectonic and localised stresses.

Field Development Plan
Operator's development plan for an oil or gas field, onshore or offshore. It requires government approval (by the NSTA) before it can be implemented.

Flow test
A flow test or well test involves testing a well by flowing hydrocarbons to surface, typically through a test separator. Key measured parameters are oil and gas flow rates, downhole pressure and surface pressure. The overall objective is to identify the well's capacity to produce hydrocarbons at a commercial flow rate.  

Fisheries Legacy Trust Company; formed in 2007 by Oil & Gas UK, Scottish Fishermen’s Federation (SFF) and National Federation of Fishermen’s Organisations (NFFO) to enhance the safety of fisherfolk by ensuring the provision in perpetuity of information relating to oil and gas seabed structures and equipment in UK waters.

Formation damage
The reduction in permeability in reservoir rock due to the infiltration of drilling or treating fluids into the area adjacent to the wellbore.

Fractured and fracturing (fracking)
Containing a crack or surface of breakage within rock; fractures can enhance permeability of rocks greatly by connecting pores together; fracturing is the creation of fractures to break down rock by pumping of fluids at high pressure. The objective is to increase production rates from a reservoir.


Gas field
A field containing natural gas, but no or limited producible oil.

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

Gas in place (GIP), Gas initially in place (GIIP)
The quantity of gas that is estimated to exist originally in naturally occurring accumulations before any extraction or production.


Hot shale
A shale rock displaying average initial TOCs normally exceeding 2% and represented by a high gamma-ray electric log reading.

Horizontal well
A high-angle well (with an inclination of generally greater than 85°) drilled to enhance reservoir performance by placing a long wellbore section within the reservoir.


The Industry Mutual Hold Harmless indemnity regime is a scheme run by LOGIC to address the contractual gap which traditionally exists between contractors working on the UKCS.

Integrated company
Indicates a firm that operates in both the upstream and downstream sectors (from exploration through to refining and marketing).

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

Irish Sea.

Industry Technology Facilitator; an internationally recognised champion for technology innovation within the oil and gas industry acting as a conduit between technology innovators and the industry.  


Licence or Lease
A legal document conveying the right to drill for oil and gas, or the tract of land on which a licence or lease has been obtained where the producing wells and production equipment are located.

Lifting cost
The operating costs of producing oil or gas from a well, licence or lease.


Thousands of barrels.

Measured depth.

The probability-weighted average of all possible values and is a measure of the central tendency either of a probability distribution or of the random variable characterised by that distribution.

Metric tonne
Equivalent to 1,000 kilogrammes, 2,204.61 lb; 7.5 barrels.

A term sometimes used to refer to those industry activities that fall between exploration and production (upstream) and refining and marketing (downstream). The term is most often applied to pipeline transportation and storage of crude oil and natural gas.

Millions of barrels.

Million barrels of oil equivalent.

Millions of cubic feet per day.

Maximising Economic Recovery


Northern North Sea.

Non-associated gas
Natural gas produced from a reservoir that does not contain significant quantities of crude oil.

The North Sea Transition Authority.


Oil field
An accumulation, pool or group of pools of oil in the subsurface, containing producible oil. An oil field consists of a reservoir in a shape that will trap hydrocarbons and that is covered by an impermeable or sealing rock. It may also contain associated gas.

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

Operating expenditure.


A 10% probability that a stated volume will be equalled or exceeded.

A 50% probability that a stated volume will be equalled or exceeded.

A 90% probability that a stated volume will be equalled or exceeded.

Possible reserves (P3)
Those reserves which at present cannot be regarded as 'probable' but are estimated to have a 10% or less chance of being technically and economically producible.

Probable reserves (P2)
Those reserves which are not yet proven but which are estimated to have a 50% chance of being technically and economically producible.

A project associated with a potential oil or gas accumulation that is sufficiently well defined to represent a viable drilling target.

Proven field
An oil and/or gas field whose physical extent and estimated reserves have been effectively determined.

Proven reserves
Those reserves which on the available evidence are virtually certain to be technically and economically producible (i.e. having a better than 90% chance of being produced).

A depleted well or dry hole that has been (typically) filled with cement, with all surface equipment removed.

An offshore structure that is permanently fixed to the seabed.

Primary recovery
Recovery of oil or gas from a reservoir purely by using the natural pressure in the reservoir to force the oil or gas out of the wells and up to the surface.


Recoverable resources
Those quantities of petroleum estimated, as of a given date, to be potentially recoverable from known accumulations.

Those quantities of petroleum anticipated to be commercially recoverable by application of development projects to known accumulations from a given date forward under defined conditions; reserves must further satisfy four criteria: they must be discovered, recoverable, commercial and remaining (as of the evaluation date) based on the development project(s) applied; reserves are further categorised in accordance with the level of certainty associated with the estimates and may be sub-classified based on project maturity and/or characterised by development and production status.1P = proven reserves (both proved developed reserves + proved undeveloped reserves.2P = 1P (proven reserves) + probable reserves, hence "proved AND probable”.3P = the sum of 2P (proven reserves + probable reserves) + possible reserves, hence "proven AND probable AND possible”.

Drill crew members who work on the derrick floor, screwing together the sections of drillpipe when running or pulling a drillstring.


Secondary recovery
Recovery of oil or gas from a reservoir by artificially maintaining or enhancing the reservoir pressure by injecting gas, water or other substances into the reservoir rock.

Use of reflected and refracted sound waves generated at the surface to ascertain the nature of the subsurface geological structures. 2D seismic records a two dimensional cross-section through the subsurface collected using the two-dimensional common depth point method.

A production hiatus during which the platform ceases to produce while essential maintenance work is undertaken (a planned shutdown), or as a result of an automatic safety valve closure (an unplanned shutdown).

Southern North Sea.

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

Standard agreements
State-of-the-art solutions for oil and gas industry agreements and recommended for use by all UKCS licensees. They are user-friendly and easy to implement. In helping simplify operational and transactional procedures, they focus resources and save costs.

Step Change in Safety
The UK based partnership with the remit to make the UK the safest oil and gas exploration and production province in the world. It was founded in 1997 by the oil and gas industry trade associations with the aim of reducing the UK offshore oil and gas industry injury rate by 50%.  

The shapefile format is a popular geospatial vector data format for geographic information system (GIS) software.


Trillion cubic feet.

Tight oil
Oil found, or expected to be present, within a reservoir with low permeability, i.e. a tight reservoir. The term, in the case of the Kimmeridge limestone, is applied to a play where trapped petroleum accumulations are expected to be pervasive throughout a large area and that are not significantly affected by hydrodynamic influences (also called resource play or continuous-type deposits).

The superstructure of a platform.

TVDSS (or TVDss, or tvdss)
True vertical depth below a subsea datum.


The exploration and production portions of the oil and gas industry.

United Kingdom Continental Shelf, the offshore waters of the UK Sector.

Those quantities of petroleum, which are estimated, as of a given date, to be contained within accumulations that have not been tested by drilling.


Vertical seismic profile, recording of seismic waves directly at the borehole to enable seismic two-way travel time reflectors to be accurately correlated with formation depths encountered by the well.


Well datastore
Run by CDA, one of the largest shared online stores of digital well report and log data in the world.

Well simulation
An operational well intervention performed on an oil or gas well to increase production by improving the flow of hydrocarbons from the drainage area into the well bore.

Wildcat well
A well drilled in an area where no current oil or gas production exists.

West of Shetland Isles


Xmas tree
The assembly of fittings and valves on the top of the casing which control the production rate of oil.


2D seismic
Seismic data collected using the two-dimensional common depth point method.