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  • Magma

    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.

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