base-line study (environmental) An assessment of the original, natural state of the environment before development takes place.
bitumen Solid form of naturally occurring petroleum that consists of high molecular weight hydrocarbons. Sometimes known as tar, although that can also be produced from coal during the coking process.
cap rock An impermeable rock layer that seals petroleum in rocks beneath it, to form a trap.
combination traps (petroleum) Traps with both structural and stratigraphic components.
creaming curve A plot showing the varying rate of increase in petroleum reserves as exploration of an oilfield or play progresses.
crude oil Liquid petroleum that occurs naturally in reservoirs within sedimentary rocks.
decommissioning Disposal of an installation after its useful lifetime has expired.
environmental impact assessment (EIA) Ecological study aimed at assessing the likely effects of developing physical resources on the pre-existing environment, as defined in a base-line study, and means of mitigating the effects.
gas hydrates Crystalline hydrocarbons that form by the combination of methane and/or carbon dioxide with water under the low-temperature and high-pressure conditions of the deep ocean floor.
gas sequestration Disposal into long-term storage of the carbon dioxide produced by the use of fossil fuels, either by using petroleum reservoirs or other secure sites from which the gas cannot escape.
gas venting and flaring The deliberate release and burning of natural gas that comes up wells along with oil, because collecting the gas for sale is not economic.
geothermal gradient The rate at which temperature increases with depth in the Earth.
gravity surveys Measurements of variations in the Earth’s gravitational field, which can show areas underlain by rocks of different densities.
heavy oil Oil with high viscosity and density that occurs in oil sand deposits.
kerogen A general term for organic material in petroleum source rocks; the name is derived from the Greek for ‘wax producer’.
magnetic surveys Measurements of the Earth’s magnetic field, which can show variations due to the occurrence of rocks with different levels of magnetization, mainly due to oxides containing iron and titanium.
maturation The process – involving chemical reactions, time and burial temperature – that transforms natural kerogen into crude oil and natural gas.
natural gas Gaseous form of petroleum that consists mainly of methane, with other low molecular weight hydrocarbon gases.
net to gross The ratio or percentage of porous and permeable rock thickness to that of the overall reservoir rock.
oil sand A sedimentary sandstone whose pores are saturated with heavy oil or bitumen that is too viscous to flow easily – sometimes known as a tar sand because of the tarry consistency of the oil.
permeability A measure of the degree to which fluid passes through a porous rock.
petroleum A complex mixture of hydrocarbons and lesser quantities of other organic molecules containing sulphur (S), oxygen (O), nitrogen (N) and some metals. Petroleum is derived from the chemical alteration of kerogen in source rocks.
petroleum charge A concept concerning the formation, migration and accumulation of petroleum in a body of sedimentary rocks. It involves consideration of source rocks, maturation, geological history and migration paths in a sedimentary basin.
petroleum play A model that can be used to direct petroleum exploration. Plays consolidate what is known about petroleum charge, reservoirs, seals and traps, together with geological structure, and enable the chances of petroleum accumulations to be assessed at specific stratigraphic levels within a sedimentary basin.
play fairway map Depiction of areas where geological conditions imply a high likelihood of petroleum occurrences, as deduced from a petroleum play.
porosity The void space in a rock, commonly expressed as a percentage.
possible reserves (petroleum) A category of reserves for which there is a significant (>0 but <50%) probability that they are technically and economically recoverable. A definition used widely in the petroleum industry.
primary migration The initial expulsion of petroleum from its source rock, before its later movement over long distances through a reservoir rock eventually to meet a trap and accumulate.
primary recovery A phase of hydrocarbon production that relies only on the internal pressure within the reservoir rock to expel fluids; contrast with secondary recovery.
probable reserves (petroleum) Reserves of a resource for which there is a better than 50% probability that they are technically and economically recoverable. A definition used widely in the petroleum industry.
proved reserves (petroleum) Reserves of a resource for which there is a better than 90% probability that they will be produced over the lifetime of a deposit. A definition used widely in the petroleum industry.
remote sensing Gathering data about the Earth’s surface – mainly in the form of images and detectable geophysical properties – from a distance, either from satellites in the case of images or an aircraft for geophysical data.
reservoir rocks Sedimentary rocks – usually sandstones or limestones – that contain enough space within them to store hydrocarbons.
seals (petroleum) Impermeable rocks that prevent the flow of fluids out of a reservoir rock. They are also often called cap rocks.
secondary migration The movement of petroleum through permeable pathways into a reservoir rock after it has escaped from the source rock, generally by moving upwards under gravity: it floats on any water in the reservoir rock.
secondary recovery A phase of hydrocarbon production, involving the injection of natural gas into the reservoir above the oil forcing the oil downwards, and/or injecting water below the oil to force it upwards.
seismic velocity The speed at which seismic waves (generally sound waves in exploration) pass through rock.
source rocks Sediments, usually shales, that contain enough organic carbon to generate petroleum under the right burial conditions. Source rocks usually contain at least 5% total organic carbon.
stratigraphic traps (petroleum) Traps that occur where there is a barrier caused by lateral variation in permeability
structural traps (petroleum) A trap where a barrier to petroleum migration formed as a result of tectonic deformation of strata.
threshold (petroleum) The subsurface temperature and depth at which petroleum starts to form from kerogen.
trap A structure (tectonic or sedimentary) that involves a seal to constrain petroleum within a reservoir rock.
two-way travel time (TWT) The time taken for a seismic wave to pass from a sound source to a reflector, and back again to a detector. The vertical or y-axis of a seismic section
wireline logging Measurement of the physical properties of rock using instruments lowered down a borehole.

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