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  • 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).

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