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  • 3-D seismic

    seismic data that are acquired and processed to yield a three-dimensional picture of the subsurface.

    Accommodation platform/rig

    a platform or semi-submersible rig that has been specially built or adapted to act as a 'hotel' for offshore personnel.


    a stimulation technique whereby acid (generally some form of hydrochloric) is injected into the reservoir rock to remove cement and improve permeability, thereby increasing the flow of oil or gas.

    Acoustic log

    a record of the time taken by an acoustic (sound) wave to travel over a certain distance through the geological formations. Also called a sonic log. See also seismic survey.


    area covered by a lease granted for oil and gas exploration and for possible future production.

    Air gun

    chamber from which compressed air is released to produce shock waves in the earth. The air gun array is the most common technique used for seismic surveying at sea.


    the simplest homologous series of saturated aliphatic hydrocarbons, consisting of methane (CH4), ethane (C2H6), propane (C3H8), etc., also known as the paraffins.


    the homologous series of aliphatic hydrocarbons containing one double (unsaturated) carbon-to-carbon bond, consisting of ethene (C2H4), propene (C3H6), butene (C4H8),etc., also known as olefins.


    the name given to the annular space between the drill string and the well bore.

    Anticlinal tap

    a hydrocarbon trap formed by the upward bowing of strata into a dome or arch.


    a fold in layered rocks originating below the surface in the form of an elongated dome. Anticlines make excellent drilling prospects since any oil in the deposit will naturally rise to the highest point of the structure because oil has a lower specific gravity than water. See tectonic process.

    API gravity

    the universally accepted scale adopted by the American Petroleum Institute for expressing the specific gravity of oils: API gravity = _________141.5________ _ 131.5 specific gravity at 60 deg F Water at 4 deg C has an API gravity of 10. Crude oil gravities generally range from 10 - 50 deg.

    Appraisal drilling

    drilling carried out to determine the physical extent, reserves and likely production rate of a field.

    Appraisal well

    a well drilled as part of an appraisal drilling programme.


    watery. Usually applied to solutions indicating water is present.


    an underground zone of permeable rock saturated with water under pressure. For gas storage applications, an aquifer will need to consist of a permeable lower layer of rock and an impermeable upper layer (or 'cap') with a cavity for storing gas.


    the mixture of bitumen and aggregate used for road surfacing.


    property, inventory or stock, plant, money, etc. owned by a company.

    Associated gas

    natural gas found in association with oil, either dissolved in the oil at reservoir conditions (solution gas) or as a cap of free gas above the oil (gas cap).

    Associated liquids

    liquid hydrocarbons found in association with natural gas


    the action of securing a section of drill-pipe while another is unscrewed from it.


    unit of pressure. One bar is equal to .987 standard atmospheric pressure, or 14.5 pounds per square inch


    a non-self-propelled vessel used as a base for drilling equipment, to carry cranes, support facilities, accommodation modules, etc., to lay underwater pipelines or to transport crude oil or its products over short distances.

    Barrel (bbl)

    the unit of volume measurement used for petroleum and its products; 1 barrel = 42 US gallons, 35 imperial gallons (approx), or 159 litres (approx): 7.3 barrels = 1 ton (approx): 6.29 barrels = 1 cubic metre.

    BCF (bcf)

    billion cubic feet of natural gas.

    BCM (bcm)

    billion cubic metres.


    barrels of condensate per day (from a gas well)


    below drill floor. Depth reference


    a geological term describing a stratum (layer of sediments or sedimentary rock) of considerable thickness and uniform composition and texture.


    the simplest aromatic compound with a ring of six carbon atoms; one of the most important feedstocks for the chemical industry.


    bottom hole pressure


    see drill bit.


    the subdivision of a nation's exploration and production acreage. Blocks are generally defined in terms of latitude and longitude, at one-degree intervals.

    Block number

    the number assigned to a particular licence block or subdivision thereof in a given national sector of the continental shelf.


    a method of producing a gas/condensate reservoir by letting the reservoir depressure over time without re-injecting any gas. With this method of production some condensate may condense within the reservoir, where its recovery is no longer a practical proposition.


    the situation that occurs when gas, oil or salt water escapes in an uncontrolled manner from a well due either to a release of pressure in the reservoir rock that the various containment systems fail to check or to the failure of the containment systems during production.

    Blowout preventor (BOP)

    a hydraulically operated wellhead device designed to ensure that a blowout cannot occur.

    BOE (Barrel oil equivalent)

    a term frequently used to compare gas with oil and to provide a common measure for different quality gases. It is the number of barrels of stabilised crude oil which contains approximately the same amount of energy as the gas: for example: 5.8 tcf (of lean gas) approximates to 1 billion boe.

    BOPD (bopd)

    in production terms, the number of barrels of oil produced from a well over a 24 hour period, normally an average figure from a longer period of time.


    the wellbore, with or without casing. See open hole.

    Bottomhole assembly (BHA)

    the lower end of the drill string comprising the drill bit, drill collars, heavyweight drill pipe and ancillary equipment.

    Bottomhole money

    Contribution to a joint-venture well, payable only if a specified horizon is reached.

    Bottoms up

    One complete circulation. When a drilling break is seen it may be decided to circulate 'bottoms up' (ie., all sample out of the hole) prior to proceeding.

    Bourdon gauge

    an instrument for measuring the pressure of steam or other gases.

    Brent blend

    a blend of North Sea crudes, used as an international marker for crude oil pricing.

    Bridge plug

    Packer run into the hole on drillpipe or wireline and left in the hole to seal it. Can be used while the B.O.P. is being changed or a seat for cement when abandoning the well. Some types can be retrieved and reused; others are drilled up after use or abandoned in the hole.

    Btu or British Thermal Unit

    the quantity of heat required to raise the temperature of one pound of water by one degree Fahrenheit. The value of a natural gas is governed by its 'Btu value'.


    abbreviation of the aromatic hydrocarbons, benzene, toluene and xylene.


    the lowest series of formations laid down during the Triassic Period of geological time; it includes sandstones that sometimes contain hydrocarbon


    a hydrocarbon consisting of four carbon atoms and ten hydrogen atoms. Normally a gas, but easily liquified for transport and storage; used in gasolines and for cooking and heating. See also LPG.


    Barrels of water per day

    Calorific value

    the quantity of heat produced by the complete combustion of a fuel. This can be measured dry or saturated with water vapour; and net or gross. The general convention is dry and gross.

    Cap rock

    an impervious layer of rock (e.g. shale or evaporite) which overlies a reservoir rock, thus preventing hydrocarbons from escaping to the surface.


    a solid element which exists in many forms, including diamonds, graphite, coke and charcoal. The combinations of carbon with hydrogen are known as hydrocarbons and can consist of very large molecules (e.g. polypropylenes) or very short ones (e.g. methane).

    Carboniferous Period

    the period of geological time which began roughly 350 million years ago and ended roughly 280 million years ago; the period in which most coal seams originated in North West Europe.

    Carried interest

    when a company pays for all or part of a partner's costs during exploration or development.

    Cash flow profile

    a chart showing expenditure on a project and income from the project over a period of time.


    the steel pipe that is cemented into a well to prevent the wall from caving in and stop unwanted fluids from entering the hole from the surrounding rocks.

    Casing perforation

    the holes that are made in the liner of a finished well in order to allow oil or gas to flow into the production tube; generally produced by a series of shaped charges fitted vertically along a gun-perforator.

    Casing seat

    the lowest point in a well at which casing is set.


    a vessel inserted in a pipeline to remove liquid droplets or solid particles which may be entrained in a gas stream.


    the procedure of pumping cement down through the casing in a well and up into the annular space between the casing and the well bore in order to hold the casing in place

    Cfd (cf/d, cu ft/d)

    cubic feet per day


    a gauged restriction inserted into a fluid flow line in order to restrict the flow rate. Chokes are of various sizes and it is customary to refer to the production of a well in terms of so many barrels through (or on) a (for example) 22/64th-inch choke.

    Christmas tree

    an arrangement of pipes and valves fitted to a production wellhead to control the flow of oil or gas and prevent a possible blowout.

    Cimmerian unconformity

    a period of emergence and subsequent erosion that occurred at the end of the Jurassic period and was followed by further deposition of sediments; see unconformity.


    the process of continuously pumping drilling mud down through the drill string and up the annulus during drilling operations.

    City gate

    this refers to the point where gas passes from a main transmission system to a local distribution system. There is not necessarily a change of ownership

    Closed in

    refers to a shut-in well that is capable of producing.

    Coal Bed Methane (CBM)

    natural gas produced in a coal seam during the formation of the coal. Coal seams form the reservoir rocks and traps for the methane, which is usually of near pipeline quality.

    Coming out of hole

    the withdrawing of the drill string from the well bore.

    Commercial field

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

    Common carriage

    1) the transport of gas through a pipeline system on behalf of a third party; 2) the obligation on transmission or distribution companies to allocate gas transport to customers on a pro-rata basis. Without discriminaton between new and existing clients


    hydraulically operated equipment that compensates for the upward and downward motion (heave) of a floating rig or drill ship during drilling operations.


    the process by which a finished well is either sealed off or prepared for production by fitting a wellhead.


    equipment used to compress gas for re-injection into a well or for pumping through a pipeline.

    Compressor station

    hydraulically operated equipment that compensates for the upward and downward motion (heave) of a floating rig or drill ship during drilling operations.


    a licence area that is leased to a company for a given period for exploration and development under specified terms and conditions.

    Concrete platform

    a production platform made of reinforced concrete rather than steel.


    a hydrocarbon mixture that becomes liquid and separates from natural gas when the gas is produced and is similar to crude oil. It is usually produced from a deep well where the temperature and pressure are high. Gas condenses as it rises up the wellbore and reaches the surface as condensate.

    Connate water

    fossil water' which has always occupied pore spaces in permeable rock since it was first deposited. Unlike edge and bottomwater, which can help production, connate water present in an oil reservoir may adversely affect production, since oil is denied access to the pores and the total volume of oil recoverable per unit volume of rock may be low. Connate water is also called interstitial water.


    a group of unrelated companies that act together in a particular venture.

    Continental shelf

    the edge of a continental mass that lies under the sea in comparatively shallow water (up to a water depth of 200 metres)

    Conversion factors (approximate)

    1 tonne crude oil = 7.5 barrels = 1.9 cubic metres, 1 barrel (US) = 35 imperial gallons = 0.16 cubic metres, 1 cubic metre = 35.31 cubic feet, 1 billion cubic metres of gas = 0.83 million tonnes of oil equivalent, 1 cubic metre of gas = 0.36 therms, 1 tonne fuel oil = 406 therms , 1 therm = 105.5 megajoules (Mj), 1 kilowatt hour = 3.6 megajoules


    the cylindrical section of rock or sediments obtained when a core barrel wuth an annular bit is withdrawn from a well during drilling.


    a person or company joined with others in a particular venture; see consortium.


    computer-processed interpretation.

    Cretaceous period

    the period of geological time that began roughly 130 million years ago and ended roughly 60 million years ago.


    the uppermost section of a derrick.

    Crown block

    the fixed system of pulleys fitted at the top of a derrick for use in raising and lowering the drill string, casing, etc.

    Crude oil

    the oil that is produced from a reservoir, after separation from any associated gas , and processed in a refinery; often referred to as crude.

    Cubic foot

    the standard unit used to measure quantity of gas (at atmospheric pressure); 1 cubic foot = 0.0283 cubic metres.


    earth and rock removed during a drilling operation to make an exploration or production well. Cuttings are invariably contaminated with oil from drilling fluids (oil-based and other muds).


    drilling exponent. Unitless number derived from rate of penetration normalised for the effect of weight on a bit, hole diameter, rotary speed and mudweight. Can be indicative of changes in formation pore pressure gradient or lithology.

    Daisy chain

    the process by which a cargo of oil or oil products is sold many times before being delivered to the customer

    Delineation well

    an exploration well drilled as part of a carefully planned programme with the objective of appraising the value of an oil or gas discovery. Delineation, or step-out wells, are drilled so that the probable outline of the oil or gas field may be delineated.


    steel structure mounted over the borehole to support the drill pipe and other equipment which is lowered and raised during drilling operations

    Derrick floor

    a raised dais at the base of the derrick carrying the drill table; often called the drilling or rig floor.

    Development drilling

    extension of a drilling programme once a strike has been established and proved to be capable of economic production. Development wells, which may be vertical or deviated, are usually drilled from permanent platforms. Some development wells may be production wells, others injection wells.

    Development phase

    the phase in which a proven oil or gas field is brought into production by drilling production wells.

    Deviated well or directional well

    borehole that is intentionally deviated from vertical in order to reach some remote or inaccessible subsurface target.


    drill floor; see BDF.


    a measure of the inclination of rock strata with respect to the horizontal.


    the products of condensation during the fractional distillation process (gaseous fuels, naphtha, gasoline, kerosene and gas oils.


    (fractional distillation) a process based on the difference in boiling points of the liquids in the mixture to be separated. Successive vaporisation and condensation of crude oil in a fractionating column will separate out the lighter products, leaving a residue of fuel oil or bitumen. Distillation is carried out in such a way as to avoid any cracking. It is the basic process that takes place in an oil refinery.

    Diurnal storage

    literally, daily storage. Refers to short-term or peak storage in pipelines or gas holders, as opposed to seasonal storage.


    this name was originally given to a shelter alongside a land-based rig, but offshore installations also have their doghouses -usually a small room on the edge of the rig floor used as office, tool store and for coffee breaks. Doghousing' refers to idle conversation, time-wasting at work.


    a sharp deviation in direction of the wellbore.


    thread sealing compound used in connecting pipe fittings. On the rig it refers to the metalised grease used on drillpipe, collar and casing connections.

    Down dip

    a term used where one well is below another well due to inclination of the strata in which the reservoir lies.

    Down time

    time during which no drilling is possible due to adverse weather conditions, while downhole equipment is being changed, during well logging, etc.


    in a well bore.

    Downhole safety valve

    a valve fitted to the production tube of a well some distance below the sea bed in order to permit flow to be stopped in an emergency.


    those activities which take place between the loading of crude oil at the export terminal and the use of the oil by the end-user. This encompasses the ocean transportation of crude oil, supply and trading, refining and the distribution and marketing of the oil products. See also upstream.


    the differences between the static and the flowing bottom-hole pressures in a well


    the control centre from which the driller operates the drilling machinery.


    the part of the drilling tool that actually cuts through the rock.

    Drill collars

    lengths of extra-heavy steel tubing located immediately above the drill bit in order to maintain pressure on the bit and keep the drill string in tension.

    Drill ship

    a ship fitted with a drilling derrick that is used to drill in waters that are too deep for a jack-up or semi-submersible rig.

    Drill stem test (DST)

    a test whereby the nature and quantity of the formation fluids in a possible oil- or gas-bearing stratum are determined by allowing them to flow to the surface through the drill string under carefully controlled conditions.

    Drill string

    lengths of steel tubing roughly 10 metres long screwed together to form a pipe connecting the drill bit to the drilling rig. The string is rotated to drill the hole and also serves as a conduit for drilling mud. Also called the drill pipe.

    Drilling contractor

    a company that undertakes to provide the equipment and manpower needed to drill a well on behalf of an operator.

    Drilling crew

    the team who actually work on or above the derrick floor during drilling operations. Generally a driller plus four roughnecks.

    Drilling engineer

    the engineer who designs the casing and is responsible for the drilling programme for a well.

    Drilling line

    the wire rope connecting the drawworks, crown block and travelling block that is used to raise or lower drill pipe or casing from or into a well.

    Drilling mud

    a mixture of clays, water and chemicals pumped down the drill string and up the annulus during drilling in order to lubricate the system, carry away rock cuttings, maintain the required pressure at the bit end, provide an aid to formation evaluation, etc.

    Drilling platform

    an offshore platform used to drill exploration and development wells but lacking the processing facilities of a production platform.

    Drilling table

    the table on the derrick floor whereby the drill pipe is rotated; also called the rotary table.

    Dry gasfield

    the production from such a reservoir will yield dry/lean gas and very small quantities of condensate; typically less than 10 barrels per million cubic feet

    Dry hole

    a well drilled without finding oil or gas.

    Dry natural gas

    natural gas consisting principally of methane and devoid of readily condensable constituents (low in associated liquids). Dry gas contains less than 0.1 gal natural gas liquid vapours per 1000 cu ft

    Dynamic positioning

    the method whereby a vessel is kept on station by computer-controlled thruster propellers rather than by anchors.


    logs run on a wireline into the borehole. Originally 'Electric' Log but now refers to all such logs (eg. Neutron, sonic, etc).

    Edge water

    water underlying and/or marginal to an oil or gas reservoir.

    Enhanced oil recovery (EOR)

    a process whereby oil is recovered other than by the natural pressure in the reservoir; see secondary recovery, tertiary recovery.

    Equity crude

    the proportion of crude oil to which a producing company is entitled as a result of its financial contribution to a project.


    the saturated hydrocarbon (alkane) with two carbon atoms in its molecule (C2H6); the second member of the paraffin series -a gas under normal conditions.

    Ethene (ethylene)

    the simplest member of the alkene series, containing two carbon atoms connected by a double (unsaturated) bond (C2H4); an important raw material in the chemical and plastics industries

    Exploration Drilling

    drilled to find and produce oil or gas reserves not classified as proved, to find a new reservoir in a field previously found to be productive of oil or gas in another reservoir or to extend a known reservoir

    Exploration licence

    a licence to explore for oil or gas in a particular area issued to a company by a governing state.

    Exploration phase

    the phase of operations in which a company searches for oil and gas by carrying out detailed geological and geophysical surveys followed up where appropriate by exploratory drilling.

    Exploration rig

    a structure used to carry the equipment needed for exploratory drilling; see jack-up, semi-submersible and drill ship.


    where one company acquires an interest in an exploration or production licence by paying some of the past or future costs of another company which is relinquishing (farming out) part of its interest.


    where a company relinquishes part of its interest in an exploration or production licence to another company in return for part-payment of its costs.


    a fracture in the Earth's crust along which the rocks on one side are displaced relative to those on the other.

    Fault trap

    a trap where a reservoir layer is faulted and brought against an impervious formation.


    the supply of crude oil, natural gas liquids or natural gas to a refinery or petrochemical plant or the supply of some refined fraction of intermediate petrochemical to some other process.


    a geographical area under which an oil or gas reservoir lies; see commercial field, marginal field.

    Field appraisal

    the process of quantifying reserves levels and production potential of a newly discovered petroleum reservoir, usually by drilling a delineation well.


    an object accidentally lost down a well, sometimes referred to as junk.


    an attempt to recover tools or drilling equipment (fish) lost down hole.


    an open flame used to burn off unwanted gas; see flaring.

    Flare stack

    the steel structure on a rig or platform from which gas is flared; see flaring


    burning off of gas produced in association with oil which, for technical or economic reasons, cannot be re-injected or shipped ashore.

    Flotation collar

    the specially designed raft used to float steel platform jackets into position.


    a floating accommodation rig or barge used as quarters for offshore personnel.


    Floating Production System Offshore


    the separation of single fractions from a mixture of hydrocarbon fluids, mainly by distillation

    Fracture pressure

    the amount of pressure which is required to 'crack' or 'split' the rock at a particular depth so that fluid can enter it.

    Fuel gas

    refers to gaseous fuels, capable of being distributed by pipeline, such as natural gas, liquefied petroleum gas, coal gas and refinery gas.

    Fuel oils

    the heavy distillates from the oil refining process; used as fuel for power stations, industry, marine boilers, etc.

    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.

    Heavy fractions, or heavy ends

    the high-molecular-weight, high-boiling-point fractions that emerge from the lower part of a fractionating column during the oil refining process.

    Heavy oil

    crude oil with a high specific gravity and a low API gravity due to the presence of a high proportion of heavy hydrocarbon fractions


    the saturated hydrocarbon (alkane) with seven carbon atoms in its molecule (C7H16); the seventh member of the paraffin series - a liquid under normal conditions.


    the saturated hydrocarbon (alkane) with six hydrocarbon atoms in its molecule (C6H14); the sixth member of the paraffin series - a liquid under normal conditions


    a stratum or layer of strata assumed to have been laid down in a continuous manner. Horizontal well: deviated wells that exceed 80 degrees from the vertical are known as horizontal wells. Horizontal wells are used when the reservoir permeability is low or the oil and gas is being produced from fractures in the rock. The flow from a horizontal well may be over five times the flow from a normal vertical well .

    Hybrid platform

    a gravity platform where the base and storage area is made of reinforced concrete but the upper sections are made of steel.


    compounds containing only the elements hydrogen and carbon. They may exist as solids, liquids or gases. Bitumens such as petroleum are composed principally (but not only) of hydrocarbons.

    Igneous rocks

    rocks that were formed by the solidification of molten magma.

    Injection well

    a well used to inject gas or water into the reservoir rock in order to maintain reservoir pressure in secondary recovery or (in the case of gas) for conservation purposes.

    Interruptible gas

    gas made available under agreements permitting the termination or interruption of delivery by the suppliers, usually for a limited number of days in a specific period. The opposite is 'firm' gas.

    Interstitial water

    water present in the pores of the oil- or gas-bearing zone of a reservoir rock. Another name for connate water


    British Institute of Petroleum.

    Isopach map

    a geological map that gives the thickness of a particular stratum.

    Jack-up rig

    mobile offshore drilling platform with retractable legs on which the platform rests on the sea bed when in use. Used for drilling in relatively shallow water.


    the structure used to support an offshore steel production platform.

    Jacket platform

    a platform constructed entirely of steel. Such platforms are generally kept in position by means of steel piles driven into the sea bed.

    Jet bit

    a modified drill bit utilising a hydraulic jet to increase the drilling rate.


    the nozzles (usually three in number) in the drill bit through which the drilling mud emerges from the drill string during drilling operations.

    Joint venture

    an investment undertaken by a consortium, usually with one member acting as operator.

    Jug hustler

    a member of a seismic crew who operates the geophones


    when equipment is lost down a well and cannot be retrieved economically (or fished), the well is junked, i.e. plugged and abandoned

    Jurassic period

    the period of geological time which began roughly 180 million years ago and ended roughly 130 million years ago.


    a square or hexagonal hollow shaft that engages at one end with the drilling table and at the other with the drill pipe.

    Kelly joint

    the stand of piping that carries the kelly


    a bituminous material occurring in certain oil shales which yield a type of oil when heated.


    a medium-light distillate from the oil refining process intermediate between gas oil and gaseous fuels; used for lighting and heating, and for the manufacture of fuel for jet and turbo-prop aircraft engines.

    Key seating

    the situation that occurs when the drill string wears a groove in the high side of a deviated well bore - this can produce problems by causing the drill string to stick.


    the situation that occurs when the formation pressure in a well exceeds the hydrostatic head of the mud column, thus allowing formation fluid to enter the well bore.


    initiation of planned well deviation

    Killing a well

    overcoming the tendency of a well to flow naturally by filling the well bore with drilling mud of sufficiently high density. To control or stop a kick.


    a formation in the upper strata of the Jurassic Period that contains both source and reservoir rocks.

    Lay Barge

    a specially designed barge used to lay an underwater pipeline; also called a pipe laying barge.

    Leak-off test

    the process of applying pressure to the formation below the casing seat to test the quality of cementing and to determine the fracture pressure in the permeable zone immediately below the casing seat.

    Lean gas or dry gas

    gas with relatively few hydrocarbons other than methane. The calorific value is typically around 1,000Btu per scf, unless there is a significant proportion of non-hydrocarbon gases present.


    an authority to explore for or produce oil or gas in a particular area issued to a company by the governing state; see exploration/production licence.

    Licence round

    a stage in the allocation of licences (both onshore and offshore) in which a country places a number of specified areas in its sector on offer to oil companies at one

    Light crude

    crude oil with a low specific gravity and high API gravity due to the presence of a high proportion of light hydrocarbon fractions.

    Light fractions

    the low molecular weight, low boiling point fractions that emerge from the upper part of a fractionating column during the oil refining process.


    small-diameter casing that extends into the production layer from just above the casing seat and is perforated when production or well testing starts.

    Liquefied natural gas (LNG)

    natural gas that has been liquefied by refrigeration or pressure in order to facilitate storage or transport; generally consists mainly of methane. See also gas liquefaction.

    Liquefied petroleum gas (LPG)

    a mixture of light hydrocarbons derived from oil-bearing strata which is gaseous at normal temperatures but which has been liquefied by refrigeration or pressure in order to facilitate storage or transport; generally consists mainly of propane and butane.

    LNG carrier

    a tanker specially designed to carry LNG, fitted with insulated pressure tanks made of stainless steel or aluminium. The load is refrigerated to 162C.

    Lost circulation

    the loss of substantial volumes of drilling mud into a formation; also called loss of returns.

    Lost time

    time in which no work is possible on an operation due to bad weather, industrial dispute, etc.


    the molten material that makes up the Earth’s crust. When extruded onto the Earth’s surface and cooled it becomes igneous rock.

    Magnetic survey

    a geological survey method in which sedimentary basins are identified and their size determined by measuring the magnetic properties of the underlying igneous rocks

    Making a connection

    the act of screwing a length of drill pipe onto a drill string suspended in a well bore, i.e. lengthening the drill string by one section of pipe during a drilling operation.

    Making a trip

    see round trip.

    Making up a joint

    the act of screwing individual sections of pipe together to form a longer length.


    an accessory to a piping system or other conductors that serves to divide a flow of liquid or gas to any one of several possible destinations.


    an instrument used for the measurement of gaseous pressures or pressure differentials in a system.

    Marginal field

    a field that may or may not produce enough net income to make it worth developing at a given time; should technical or economic conditions change, such a field may subsequently become a commercial field.

    Marine riser

    the pipe that connects an exploration rig, drilling or production platform to a sub-sea wellhead or sub-sea pipeline during drilling or production operations.


    thousands of barrels of oil


    thousand cubic feet of natural gas

    Median line

    the dividing line between two national sectors of an ocean or sea; the locus of points equidistant from the territorial boundaries of the countries concerned.


    strong-smelling compounds of carbon, hydrogen and sulphur found in gas and oil. Sometimes added to natural gas for safety reasons.

    Mesozoic era

    the era of geological time, comprising the Triassic, Jurassic and Cretaceous periods, which began approx 230 million years ago and ended approx 60 million years ago.

    Metamorphic rocks

    rocks changed considerably from their original composition and structure by heat and / or pressure.


    the hydrocarbon with the lightest molecule (CH4); the first member of the paraffin (alkane) series - a gas under normal conditions.

    Metric ton

    equivalent to 1000 kilos, 2204.61 lbs or 0.9842 tons.


    millions of barrels of oil.


    million barrels of oil equivalent.


    one million British Thermal Units


    million cubic feet of natural gas

    MMSCFPD (mmscfpd)

    millions of cubic feet per day of gas (production figure).

    Monkey board

    a high-level platform in the derrick on which the derrick man works.

    Moon pool

    an open hole on a drilling rig through which conductor pipe and other equipment are lowered into the sea, and drilling operations are conducted. On a floating rig the moon pool is located at the centre of gravity where vessel motion is least felt, but its site on a jack-up rig is not critical. Sometimes called moonwell.


    the man responsible for the care and operation of the drilling engines.

    Mouse hole

    a shallow hole in the drilling floor near the drilling table in which the kelly joint and other stands of pipe are temporarily stored while making a connection; sometimes called the rat hole.


    see drilling mud.

    Mud engineer, mud man

    the service man in charge of maintaining the mud systems on a drilling rig to the specifications set out in the drilling programme.

    Mud pits

    large steel storage pits through which the drilling mud is circulated.

    Mud pumps

    pumps used to circulate drilling mud at the desired flow rate and pressure; also called slush pumps

    Mud weight

    the density of the drilling mud, usually recorded in pounds per gallon; the mud weight is changed

    Multiservice vessel (MSV)

    a support vessel designed to provide a variety of construction, fabrication, diving, emergency and other services to offshore installations.


    a range of distillates lighter than kerosene, used as feedstock for motor gasoline production and the chemical industry (e.g. ethylene manufacture).

    Natural gas

    a mixture of light hydrocarbons (mainly methane) found naturally in the Earth’s crust, often in association with oil (when it is known as associated gas).

    Natural gas liquids (NGL)

    liquid hydrocarbons found in association with natural gas.

    Net Production

    production that is owned by a company less royalties and production due others

    Nippling up

    (1) connecting or hooking up a blowout preventer stack. The operation is usually done from the nippling-up platform on a jack-up drilling unit, and the name probably stems from the cone-shaped mating flanges. (2) any act of connecting two components together.

    Obligatory wells

    exploration wells that an operator undertakes to drill in a given area as a condition for being allocated an exploration licence. Also known as commitment wells.


    the saturated hydrocarbon (alkane) with eight carbon atoms in its molecule (C8H18); the eighth member of the paraffin series - a liquid under normal conditions.


    substance such as mercaptan with a characteristic smell, added to odourless natural gas or NGLs when they are used as fuel, in order to aid detection.


    the point in a gas distribution system where gas is taken by supply pipe to a major consumer.


    Office of Gas Supply, the UK gas industry regulator.


    a mixture of liquid hydrocarbons of different molecular weights.

    Oil field

    a geographical area under which an oil reservoir lies.

    Oil gasification

    the manufacture of gas from oil for use as a fuel

    Oil in place (OIP)

    an estimated measure of the total amount of oil contained within a reservoir and, as such, a higher figure than the estimated recoverable reserves.

    Oil pipeline

    a pipeline used to pump crude or refined oil between two places, either offshore or onshore.

    Oil shale

    a compact sedimentary rock impregnated with organic materials (mainly kerogen) which yields oil when heated

    Oil slick

    a layer of oil floating on the surface of the sea; generally caused by some sort of accident or spillage, but which is sometimes caused by natural seepage from the ocean floor.

    Oil trap

    geological structure of any type which traps migrating hydrocarbons, thereby causing an oil field to form.


    a class of hydrocarbons, including ethylene and propylene, of particular importance as feedstock to the chemical industry.


    Organisation of Petroleum Exporting Countries. Formed in 1960, its member countries are Algeria, Ecuador, Gabon, Indonesia, Iran, Iraq, Kuwait, Libya, Nigeria, Qatar, Saudi Arabia, United Arab Emirates and Venezuela.

    Open hole

    the lower section of the borehole that does not have casing.


    the individual or company responsible for the exploration, development, and production of an oil or gas well or licence. The operator is often part of a consortium and acts on behalf of the consortium.


    the right to obtain, within a certain period of time, other rights, e.g. a company that has the right to obtain exploration rights in a particular block is said to have an option thereon.

    Outstep well

    see step out well


    a fishing tool for recovering lost drill pipe or casing.


    a mechanical device containing a rubber packing element which can be expanded against the wellbore or casing to isolate the annular sections. Used in DSTs and squeeze cementing.


    era: the era of geological time (comprising Cambrian, Ordovician, Silurian, Devonian, Carboniferous and Permian Periods) which began approx 600million years ago and ended approx 230 million years ago.


    the simplest homologous series of hydrocarbons, containing no unsaturated carbon-to-carbon bonds; also known as the alkanes.


    a productive horizon in a formation that is sufficiently thick and contains a high enough concentration of oil and/or gas to make production from it commercially viable.

    Peak shaving

    increasing the normal supply of gas from another source during emergency or peak periods.


    the saturated hydrocarbon (alkane) with five carbon atoms in its molecule (C5H12), the fifth member of the paraffin series - a liquid under normal conditions.

    Perforating gun

    see gun-perforator.


    a measure of the capacity of a rock to allow water or other fluids such as oil to pass through it. Dependent on the connectivity of the pore spaces.

    Permian Period

    the period of geological time which began approx 280 million years ago and ended approx 230 million years ago; the period in which the gas-bearing Rotliegendes sandstone formations were laid down in the southern North Sea.


    a chemical derived from petroleum or natural gas (e.g. benzene, ethylene).


    a generic name for hydrocarbons, including crude oil, natural gas liquids, natural gas and their products. The name is derived from the Latin oil, oleum, which occurs naturally in rocks, petra.

    Petroleum engineer

    the engineer who is responsible for the overall strategy of oil and gas drilling and production operations for a particular field.

    Petroleum Revenue Tax (PRT)

    a tax levied by the UK Government on net income from petroleum production in the UK sector after deducting all costs, Royalties and certain other allowances; this tax is deductible in computing Corporation Tax.


    a piece of equipment that is inserted into a pipeline and is carried along by the flow of oil or gas; used to clean or monitor the internal condition of the pipeline or to mark an interface between two different products


    steel piles driven into the seabed to anchor fixed steel structures such as production and drilling platforms

    Pinch-out trap

    a trap where a bed or reservoir rock gets progressively thinner as it nears an impervious area and eventually disappears.


    a tube for the transportation of crude oil or natural gas between two points, either onshore or offshore.

    Pipeline capacity

    the amount of oil or gas required to keep a pipeline full, or the amount that can be passed through a pipeline in a given period of time


    an unroofed sump or tank for holding liquids such as drilling muds

    Plateau level

    the level of peak production reached by an oil or gas field; it is always followed by declining levels of production.


    a fixed or floating offshore structure from which wells are drilled; see drilling platform, production platform.

    Plugged and abandoned (p & a’d)

    the process whereby a well that is no longer needed (usually because it was dry) is filled with concrete and abandoned.

    Pore pressure

    the natural internal pressure in a formation


    a measure of the amount of free space, or voids, in a rock; generally expressed as a percentage of the total volume.

    Possible reserves

    undeveloped oil and/or gas reserves which best judgement indicates might eventually be recoverable from undrilled or untested structures

    Pour point

    the temperature at which a liquid hydrocarbon ceases to flow, or at which it congeals; it is a function of the viscosities and melting points of the various hydrocarbons.


    parts per million.

    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.

    Probable reserves

    undeveloped oil and/or gas reserves considered to be recoverable from penetrated formations but lacking information to be classified as proven reserves.

    Produced water

    water naturally present in the reservoir or injected into the reservoir to enhance production, produced as a co-product when oil is produced.

    Producing horizon

    rock from which oil or gas is produced.

    Production drilling

    wells drilled in order to bring a field into production.

    Production licence

    a licence to produce oil or gas in a particular area issued to a company by the governing state.

    Production phase

    the phase of the life of a field in which oil or gas is produced.

    Production platform

    a platform from which development wells are drilled and which carries all the associated processing plant and other equipment needed to maintain a field in production.

    Production tubing

    the pipe in a production well through which oil or gas flows from the reservoir to the surface; also known as the production string

    Production well

    well used to remove oil or gas from a reservoir.

    Project finance

    a method of financing in which the lender has a claim to receive loan payments only out of the revenues earned by the project he helped finance. In this type of finance, the lender has no recourse to the borrower’s other assets.


    the saturated hydrocarbon (alkane) with three carbon atoms in its molecule (C3H8); the third member of the paraffin series - a gas under normal conditions

    Propene (propylene)

    the member of the alkene series containing three carbon atoms, two of which are connected by a double (unsaturated) bond (C3H6); an important raw material in the chemical and plastics industry.

    Proven field

    an oil and/or gas field whose physical extent and estimated reserves have been proven.

    Proven reserves

    the estimated quantities of hydrocarbons that geological and engineering data demonstrate with reasonable certainty to be recoverable in future years from known oil and gas reservoirs under existing economic and operating conditions.

    Quaternary era

    the most recent era of geological time, which started roughly one million years ago.

    Rat hole

    see mouse hole.

    Rate of penetration (ROP)

    the depth of hole drilled in a specified time; usually given in feet per hour.

    Raw natural gas

    natural gas containing impurities and unwanted substances such as water, nitrogen, carbon dioxide, hydrogen sulphide gas and helium. Before the gas is marketed, these are removed.


    the completion for production of an existing well bore in another formation from that in which the well has been previously completed.

    Recoverable reserves

    that proportion of the oil and/or gas in a reservoir that can be removed using currently available techniques.

    Recovery factor

    the ratio of recoverable oil and/or gas reserves to the estimated oil and/or gas reserves in place in the reservoir.


    a plant used to separate the various components present in crude oil and convert them into usable products or feedstock for other processes.

    Relief well

    a deviated well drilled into a structure for the purpose of relieving pressure in an adjacent well that has has suffered a blowout

    Relinquishment requirement

    an agreement whereby oil companies give up their some or all of their licence rights in concessions after a certain period of time. Often a partial relinquishment is made after the initial work programme has been completed.

    Rental payment

    an annual payment made by an oil company to a State in order to retain rights to a particular concession.

    Reserves-to-production ratio

    for any given well, field or country. The length of time that reserves would last if production continued at its current rate, at the current level of technology.


    a stratum in which oil or gas is present. In a reservoir gas occupies the upper part of the reservoir as it is the lightest, oil the middle section, while water occupies the lower section.

    Reservoir pressure

    the fluid pressure in an oil or gas reservoir.

    Reservoir rock

    the porous rock containing interconnected pores or fissures in which oil or gas may be found.

    Residual fuel oil

    very heavy fuel oils produced from the residue from the fractional distillation process rather than from the distilled fractions.

    Return on assets (ROA)

    the net profit after tax expressed as a percentage of the total money invested in an enterprise.


    the gross income from the sale of products produced.

    Reverse circulation

    pump heavy fluid into the annulus in order to displace lighter fluid from the drillstring. Used in testing and squeeze cementing

    Rich gas

    gas which is predominantly methane but with a relatively high proportion of other hydrocarbons. Many of these other hydrocarbons would normally be separated out as NGLs.


    run into hole.

    Ring fence

    a fiscal procedure whereby taxable profits from oil or gas production cannot be offset against other losses made by the company involved.


    see marine riser.

    Riser pipeline

    a pipe that connects a platform to a sub-sea wellhead or spur line.

    Risk capital

    equity capital raised to finance a development that has technical, economic and other risks attached to it and thus cannot guarantee a return on investment


    rotary kelly bushing. Depth reference. Well depths often quoted as below RKB.


    instrument used to measure the flow rate of a liquid or gas in a pipe.

    Rotary drilling

    drilling in which the entire drill string and bit are rotated, as opposed to turbine drilling.

    Rotary hose

    the flexible hose that transfers the drilling mud from the stand pipe to the top of the drill string via a swivel coupling

    Rotary RPM

    the number of revolutions per minute of the drilling table, i.e. the angular velocity of the drill bit during rotary drilling

    Rotary table

    see drilling table.


    formations laid down during the first part of the Permian Period.


    one of the assistants to a driller; a labourer who works on the actual derrick floor of an offshore rig or platform.

    Round trip

    the process which occurs every time a drill bit has to be replaced, whereby the entire drill string is removed from the hole in roughly 90 ft sections and is then returned with a new bit on the end; also described as making a trip


    a general labourer in an oil drilling and/or production operation.


    an interest in an oil and gas lease that gives the owner of the interest the right to receive a portion of the production from the leased acreage (or of the proceeds of the sale thereof), but generally does not require the owner to pay any portion of the costs of drilling or operating the wells on the leased acreage. Royalties may be either landowner's royalties, that are reserved by the owner of the leased acreage at the time the lease is granted, or overriding royalties, which are usually reserved by an owner of the leasehold in connection with a transfer to a subsequent owner

    Sales gas

    raw gas, after processing to remove LPG, condensate and carbon dioxide. Sale gas consists of methane and ethane.

    Salt dome

    a dome of salt that has broken through overlying formations

    Salt dome trap

    a trap produced by an impervious salt dome.

    Salt pillow

    a mass of salt that has risen up under overlying formations, causing them to bulge upwards, but which has not actually broken through


    the process of obtaining a small quantity of material which is as representative as possible of the total volume.


    a type of rock composed primarily of quartz grains. Often forms an excellent reservoir for oil and gas


    oil or gas saturation is a measure of the proportion of pore space in a reservoir rock that is filled with oil or gas.

    SBM (Single Buoy Mooring)

    also known as single point mooring (SPM), this consists of a single floating chamber moored near an offshore production platform to serve as a connection to a tanker. It has no storage capacity.

    Sealed bid auction

    an auction in which the competing parties submit confidential bids by a specified date; sometimes known as a blind auction.

    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.

    Sedimentary basin

    an area in which thick layers of sedimentary rocks have been laid down over a long period of time.

    Sedimentary rocks

    rocks laid down by the accumulation of sediments at the bottom of the bottom of a sea, lake, river or swamp over millions of years.

    Seismic exploration

    an exploration technique involving the use of seismic methods; see seismic survey.

    Seismic survey

    a technique for determining the detailed structure of the rocks underlying a particular area by passing acoustic shock waves into the strata and detecting and measuring the reflected signals. It is used to locate potential oil or gas bearing structures before drilling.

    Semi-submersible rig

    a floating drilling platform that is supported by underwater pontoons; generally used for exploration purposes only.

    Shale shaker

    a vibrating or rotating screen which removes the coarse drill cuttings from the drilling mud prior to its recirculation


    an indication of oil or gas from an exploratory well.


    the status of a well that has been closed temporarily to become a sealed pressure vessel.

    Shuttle tanker

    an oil tanker that makes regular round trips between a producing field and an onshore terminal or refinery

    Sidetrack drilling

    a remedial operation which results in the creation of a new section of well bore for the purpose of detouring around 'junk', redrilling a lost hole or straightening key seats and crooked holes. Also used for geological reasons when there is a desire to reach another part of the reservoir adjacent to the main well bore.

    Sidewall cores

    cylindrical sections or rock or sediments obtained by shooting hollow cylinders of steel into the wall of the well bore and subsequently recovering them together with their contents.

    Skidding the rig

    moving the rig from the location of a lost or completed hole preparatory to starting a new hole with little or no dismantling of equipment

    Slush pumps

    see mud pumps.


    the procedure of running pipe or casing into the well while the well is exerting pressure on the surface equipment.

    Solution gas

    natural gas which is dissolved in the crude oil within the reservoir

    Solution gas drive

    a primary recovery process in which oil is driven out of a reservoir by the natural pressure exerted by the dissolved gas

    Sour crude

    crude oil with a high sulphur content.

    Sour gas

    natural or associated gas with a high sulphur content.

    Source rock

    the rock in which oil or natural gas originates.


    a floating on-field storage tank into which oil flows from a production platform prior to being transferred to a tanker for shipping ashore.

    Specific gravity

    the ratio of the density of a substance at a particular temperature to the density of water at 4°C.


    an accidental release of oil, or another liquid. All spills exceeding 100 kg must be reported.

    Spot market

    an international market in which oil or oil products are traded for immediate delivery at the current price (the 'spot price').

    Spud (spudding in)

    to start drilling a new well (or restart), using a large diameter bit.

    Spur line

    a small-diameter underwater pipeline connecting a production or drilling platform to a terminal platform, on-field storage facility, or main pipeline to the shore


    an operation whereby cement is injected under pressure between the casing and the well bore at a particular depth


    sub-sea level. Depth reference


    inserting the threaded end of a pipe or rod into the coupling at the end of another pipe or rod.

    Stabbing board

    a retractable platform in the upper part of the derrick on which the derrick man stands in order to align two sections of pipe which are to be joined.

    Stand of pipe

    a length of drill pipe, tubing or other pipe consisting of one or more sections that is stood, racked or hung in a derrick during drilling operations.

    Stand pipe

    the rigid pipe that conducts the drilling mud up the derrick into the rotary hose.

    Standby boat

    a boat (often a converted fishing boat) which maintains permanent station near an offshore structure in order to supply support facilities in an emergency

    Steel jacket

    the steel lattice structure used to support an offshore steel platform

    Step out well

    a well drilled beyond the proven limits of a field to investigate a possible extension to that field.


    the boom used to lower an underwater pipeline onto the seabed from a lay barge.


    successive layers of sediment or sedimentary rock: singular stratum

    Stratigraphic trap

    a trap formed at the time the sediments containing it were actually laid down and where the hydrocarbons are enclosed as a result of a change from porous to non-porous rock, rather than by subsequent tectonic processes


    thin, usually discontinuous bed, within a massive lithology, e.g. 'shale with sandstone stringers'

    Structural trap

    a trap produced by the distortion of once-horizontal strata by some tectonic process(es).

    Sub-sea completion

    the process of installing a sub-sea, or ocean floor well head

    Supply base

    an onshore base from which supply boats operate.

    Supply boat

    a specially designed vessel to ferry supplies, material and equipment to and from offshore structures.

    Support vessel

    a specially designed vessel that provides a range of safety and support facilities on an offshore field.


    inward fluid flow caused by pulling equipment out of a fluid-filled bore hole.

    Sweet crude

    crude oil with a low sulphur content.

    Sweet gas

    natural gas with a low sulphur content. Sweet gas reduces sulphur dioxide emissions into the atmosphere.


    the amount by which the rate of gas to be supplied under a contract at any one time may differ from the daily contracted quantity of the buyer's choice.


    the rotating coupling that connects the rotary hose to the drill string


    a tectonic structure in which strata are folded so as to form a dip or bowl (i.e. concave on top).

    Synthetic gas

    methane-rich gas manufactured from oil or coal that has the same basic characteristics and chemical composition as natural gas. After treatment to remove carbon dioxide, it is suitable as low-calorific town gas.


    a contractual clause obliging a gas buyer to pay the seller for a contracted amount of gas in a fixed period, whether or not he takes physical delivery of it.


    a ship or vehicle used to transport oil, refined products or liquefied gas.

    Tar sands

    mixture of sand, water and heavy hydrocarbons; a potential alternative source of hydrocarbons


    total depth, or planned final depth.

    Tectonic process

    a process whereby rocks or strata are deformed by natural forces within the Earth's crust.


    an onshore installation designed to receive oil and/or gas from a pipeline or from tankers; it is not a refinery.

    Terminal platform

    an offshore platform from which oil or gas is pumped ashore through a pipeline.

    Tertiary era

    the era of geological time which began approx 60 million years ago and ended approx one million years ago

    Tertiary recovery

    recovery of oil or gas from a reservoir over and above that which can be obtained by primary and secondary recovery; it generally involves using sophisticated techniques such as heating the reservoir to reduce the viscosity of the oil.


    100,000 British Thermal Units; a unit used to measure quantities of gas.


    a term used to describe the total amount of raw materials that are processed by a plant, such as an oil refinery, in a given period

    Tight hole

    a well in which the information obtained is restricted and passed only to those who may be authorised to receive information. Often occurs if nearby acreage is unlicensed and the company wants to maintain a competitive advantage


    hydraulically operated grabs used to hold sections of pipe during screwing or unscrewing; sometimes known as pipe tongs.


    the supervisor in charge of the drilling rig and associated equipment.

    Town gas

    gas piped to consumers from a gas plant. It can comprise manufactured gas, as well as natural gas for enrichment.


    the transport of large quantities of gas at high pressures, often through national or regional transmission systems. From the latter, gas is transferred into local distribution centre, for supply to customers, at lower pressures.

    Transmission pipeline

    a network of pipelines distributing natural gas from an onshore station, via compressor stations, to storage centres or distribution points


    a geological structure in which hydrocarbons build up to form an oil or gas field.

    Travelling block

    the moving pulley system used in conjunction with the (fixed) crown block for raising and lowering the drill string, casing, etc.

    Triassic period

    the period of geological time which began approx 230 million years ago and ended roughly 180 million years ago

    Turbine drilling

    drilling in which the drill string remains stationary, the bit being rotated by a down-hole multi-stage turbine powered by the drilling mud


    the down-hole drilling mechanism used in turbine drilling.


    catastrophic failure at the joint of drillpipe and collar, usually caused by the application of excessive torque from the rotary table


    United Kingdom Continental Shelf.


    a tectonic feature in which strata are eroded and subsequently overlain with younger strata (sometimes angularly), which are said to lie unconformably on the older strata; the actual boundary between the two sets of strata.

    Unconformity trap

    a trap in which the escape of hydrocarbons is prevented by impervious rocks unconformably overlying the reservoir


    when owners or oil and/or gas reserves pool their individual interests in return for an interest in the overall unit, which is then operated by a single company on behalf of the group, thus increasing efficiency.


    the opposite of downdip.


    those activities relating to the exploration, production and delivery to an export terminal of crude oil.


    a measure of the resistance that a fluid makes to motion or flow (stickiness); it usually decreases as temperature rises.

    Water drive

    a recovery process in which oil or gas is driven out of a reservoir by the pressure of underlying water.

    Water injection

    a process whereby treated water is pumped into the reservoir rock in order to maintain the reservoir pressure. Sometimes referred to as water flood.


    solid hydrocarbon that is present in some crude oils, especially in paraffinic crudes. Wax deposits in pipelines can cause mechanical problems

    Weather window

    the part of the year when the weather can normally be expected to be suitable for carrying out offshore operations.

    Well bore

    the hole in the rock made by the drill bit.

    Well logging

    a comprehensive record of all data collected during the drilling of a well, enabling a highly detailed picture of the strata to be built up.


    the control equipment fitted to the top of a well casing, incorporating outlets, valves, blowout preventers, etc.

    Wellsite geologist

    oil company's supervising geologist responsible for the drilling operation

    Wet natural gas

    natural gas that contains large amounts of associated liquids.


    a wedge-shaped piece of equipment placed at the bottom of a well, thus forcing the bit and drill pipe to deviate from their original direction when drilling is resumed


    an exploration well drilled without knowledge of the contents of the underlying rock structure.

    Wildcat appraisal well

    an appraisal well drilled with minimum preliminary information about the underlying structure and conditions; it usually follows a wildcat well that reported shows of oil or gas.

    Working Interest

    an interest in an oil and gas lease that gives the owner of the interest the right to drill for and produce oil and gas on the leased acreage and requires the owner to pay a share of the costs of drilling and production operations. The share of production to which a working interest owner is entitled will sometimes be smaller than the share of costs that the working interest owner is required to bear, with the balance of the production accruing to the owners of royalties.


    the process whereby a completed production well is subsequently re-entered and any necessary cleaning, repair and maintenance work done. Often done to restore or increase production.


    waiting on weather.


    formations containing salt that were laid down in the second part of the Permian Period.

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