Glossary of commonly used terms

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

Abandon
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’.

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

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).

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

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

Blowdown
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.

Blow-out
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.

bopd
Barrels of oil per day.

Borehole
The hole as drilled by the drill bit.

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

Capex
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.

Cementation
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.

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

Clastic
Rocks composed of broken pieces of older rocks.

Coccolithic
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.

Completion
The installation of permanent wellhead and downhole equipment for the production of oil and gas.

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

Condensate
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 categorized 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..

DECC
The Department of Environment and Climate Change.

Derrick
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.

Discovery
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.

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

Downstream
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 cuttings
Rock chips from the subsurface formations recovered by filtering the drilling mud.

Drilling 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.

Dry hole
Any exploratory or development well that does not find commercial quantities of hydrocarbons.

E&A
Abbreviation for exploration and appraisal.

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

Effective Porosity (PHIE)
The interconnected pore volume or void space in a rock that contributes to fluid flow or permeability in a reservoir. Effective porosity excludes isolated pores and pore volume occupied by water adsorbed on clay minerals or other grains.

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.

Farm-in
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 Oil & Gas Authority) before it can be implemented.

Fishing
Retrieving objects from the borehole, such as a broken drillstring, or tools.

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.

Fluorescence
The natural property of minerals and fluids to emit light, characteristic in certain situations, of the presence of hydrocarbons.

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.

Formation pressure
The pressure at the bottom of a well, in the reservoir, when it is shut in at the wellhead.

Formation water
Water, typically saline, underlying gas and oil in the formation.

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.

Free water
Water that is mobile, available to flow, and not bound to surfaces of grains or minerals in rock.

Gamma-ray log
An electric log, which measures natural background radioactivity emitted mainly by potassium, uranium and thorium isotopes used as a sedimentary lithology discriminator.

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.

Gas-to-Liquids (GTL)
The conversion of natural gas to a liquid form so that it can be transported easily, usually using a refrigeration process. Typically, the liquid is converted back to natural gas prior to consumption.

Generative potential (S2)
The amount of hydrocarbons that can be generated from a unit volume of source rock established via the S2 peak from rock-evaluation pyrolysis, normally expressed in milligrammes of hydrocarbon per gramme of rock (or kilogramme per tonne). The S2 figure is achieved when 100% of the organic matter is transformed into hydrocarbons.

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

Hydrogen index (HI)
The amount of hydrogen relative to the amount of organic carbon in a sample, normally expressed in milligrammes of hydrogen per gramme of TOC.  The higher the amount of hydrogen the more oil prone the source rock when subjected to time, temperature and pressure; an initial HI over 450 normally indicates an oil prone source rock.

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.

Hydrocarbon
A compound containing only the elements hydrogen and carbon. It may exist as a solid, a liquid or a gas. The term is mainly used in a catch-all sense for oil, gas and condensate.

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.

Kerogen
Organic material in a sedimentary rocks that is insoluble in the usual organic solvents, being composed of a variety of organic materials, including algae, pollen, wood, vitrinite, and amorphous material. Over time and subjected to temperature and pressure, kerogen converts to hydrocarbons.

Jacket
The lower section, or ‘legs’, of an offshore platform.

Lead
A potential accumulation that is not yet sufficiently well-defined to represent a viable drilling target.

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

Liquefied natural gas (LNG)
Oilfield or naturally occurring gas, largely methane, liquefied for transportation.

Liquefied petroleum gas (LPG)
Light hydrocarbon material, gaseous at atmospheric temperature and pressure, held in the liquid state by pressure to facilitate storage, transport and handling. Commercial liquefied gas consists essentially of either propane or butane, or mixtures thereof.

Limestone
A sedimentary rock predominantly composed of calcite (a crystalline mineral form of calcium carbonate) of organic, chemical or detrital origin. Minor amounts of dolomite, chert and clay are common in limestones. Chalk is a form of fine-grained limestone.

Lithology
The macroscopic nature of the mineral content, grain size, texture and color of rocks.

Mbbl
Thousands of barrels.

MD
Measured depth.

Mean (expected value)
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.

MICP
Mercury injection capillary pressure, a measure of rock porosity and permeability, from rock cores or cuttings, and a calibration of porosity logs.

Micrite
Sedimentary rock formed of very fine-grained calcareous particles, ranging in diameter from 0.06 to 2mm; often referred to as lime mudstone.

Midstream
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.

MilliDarcy (mD)
A standard unit of measure of permeability. One Darcy describes the permeability of a porous medium through which the passage of one cubic centimeter of fluid having one centipoise of viscosity flowing in one second under a pressure differential of one atmosphere where the porous medium has a cross-sectional area of one square centimeter and a length of one centimeter. A milliDarcy (mD) is one thousandth of a Darcy and is a commonly used unit for reservoir rocks.

Missed Pay
Pay that has been identified by more sophisticated analysis of the electric logs and was previously overlooked.

MMbbl
Millions of barrels.

MMboe
Million barrels of oil equivalent.

MMcfd
Millions of cubic feet per day.

MMybp
Millions of years before present.

Mudstones
An extremely fine-grained sedimentary rock consisting of a mixture of clay and silt-sized particles.

Natural Gas
Gas, occurring naturally, and often found in association with crude petroleum.

Natural gas liquids (NGLs)
The portions of gas from a reservoir that are liquified at the surface in separators, field facilities, or gas processing plants. NGL from gas processing plants is also called liquified petroleum gas (LPG).

Net
Ownership interest share, generally in this case UKOG’s share, for example net reserves and net resources.

Net Present Value (NPV)
The difference between discounted cash inflows and discounted cash outflows, commonly used to analyse the economics of a project or asset investment.

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

OGA
The Oil and Gas Authority.

Oil
A mixture of liquid hydrocarbons of different molecular weights.

Oil down to
The deepest level where oil saturation is measured at the base of a porous reservoir where it directly overlies rock of very low porosity and permeability where no reliable oil water contact can be established.

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.

Oil in place (OIP), oil initially in place (OIIP), or stock tank oil initially in place (STOIIP)
The quantity of oil or petroleum that is estimated to exist originally in naturally occurring accumulations before any extraction or production.

Oil-mature
A term applied to source rocks, which have received sufficient temperature and pressure over geological time to generate oil.

Oil saturation (So, water saturation, Sw)
The amount of the pore space within a reservoir containing oil; the remainder of the pore space contains water, water saturation, such that So + Sw = 1. Both are expressed in volume/volume, percent or saturation units.

Oil water contact
A bounding surface in a reservoir, above which predominantly oil occurs and below which predominantly water occurs.

Oolite
Oolite is a sedimentary rock formed from ooids, being spherical grains comprised of concentric layers of calcium carbonate and of diameter 0.25-2 mm. Ooids are usually formed in warm, supersaturated, shallow, highly agitated marine water intertidal environments such as the present day Bahama Banks.

Organic rich
A rock rich in organic matter, which, if subjected to sufficient heat and pressure over geological time, will generate oil or gas.  Typical source rocks, usually shale or limestone, contain above an initial 2% organic matter by weight.

P10
A 10% probability that a stated volume will be equalled or exceeded.

P50
A 50% probability that a stated volume will be equalled or exceeded.

P90
A 90% probability that a stated volume will be equalled or exceeded.

Payzone
Rock in which oil and gas are found in exploitable quantities.

Permeability
The capability of a porous rock or sediment to permit the flow of fluids through its pore spaces.

Petrophysics
The study of physical and chemical rock properties and their interactions with fluids utilising electric logs, physical rock and fluid measurements.

Play
A set of known or postulated oil and or gas accumulations sharing similar geologic, geographic, and temporal properties, such as source rock, migration pathways, timing, trapping mechanism, and hydrocarbon type.

Porosity
The percentage of void space in a rock formation, where the void may contain, for example, water or petroleum.

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.

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

Prospective resources
Prospective resources are estimated volumes associated with undiscovered accumulations. These represent quantities of petroleum which are estimated, as of a given date, to be potentially recoverable from oil and gas deposits identified on the basis of indirect evidence but which have not yet been drilled. This class represents a higher risk than contingent resources since the risk of discovery is also added. For prospective resources to become classified as contingent resources, hydrocarbons must be discovered, the accumulations must be further evaluated and an estimate of quantities that would be recoverable under appropriate development projects prepared.
 
Proven field
An oil and/or gas field whose physical extent and estimated reserves have been effectively determined.

Proven reserves (P1)
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).

Pyrolysis
Pyrolysis is the decomposition of organic matter by heating in the absence of oxygen. Organic geochemists use pyrolysis to measure TOC, generative potential (S2), richness (HI) and maturity of potential source rocks. In a pyrolysis analysis, the organic content is pyrolysed in the absence of oxygen, then combusted. The amount of hydrocarbons and carbon dioxide released is measured.

Operator
The company that has legal authority to drill wells in a licence and undertake production of hydrocarbons in a field. The operator is often part of a consortium and takes the leading role on behalf of this consortium.

Opex
Operating expenditure.

Permeability
The property of a formation, which quantifies the flow of a fluid through the pore spaces and into the wellbore.

Petroleum
A generic name for hydrocarbons, including crude oil, natural gas liquids, natural gas and their products.

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

Permeability
A measure of the ability of a rock to transmit fluid through pore spaces.

Platform
An offshore structure that is permanently fixed to the seabed.

Porosity
A ratio between the volume of the pore space in reservoir rock and the total bulk volume of the rock. The pore space determines the amount of capacity available for storage of fluids.

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.

Produced water
The water extracted from the subsurface with oil and gas. It may include water from the reservoir, water that has been injected into the formation, and any chemicals added during the production/treatment process. Produced water is also called ‘brine’ (and may contain high mineral or salt content) or ‘formation water’.

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

Recovery factor
Those quantities of petroleum, as a proportion of OIP, anticipated to be commercially recoverable by application of development projects to known accumulations from a given date forward under defined conditions.

Reserves
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 categorized 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”.

Reservoir
A subsurface rock formation containing an individual natural accumulation of moveable petroleum.

Riser (drilling)
A pipe between an offshore seabed BOP and a floating drilling rig.

Riser (production)
The section of pipework that joins an offshore seabed wellhead to the Christmas tree.

RockEval
The most widely used pyrolysis technique.

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

Roustabout
Drill crew members who handle the loading and unloading of equipment and assist in general operations around the rig.

Royalty
A percentage interest in the value of production from a lease that is retained and paid to the mineral rights owner (sometimes a previous owner).

Sandstone
A clastic sedimentary rock whose grains are predominantly sand-sized. The term is commonly used to imply consolidated sand or a rock made of predominantly quartz sand.

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.

Seismic
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.

Separation
The process of separating liquid and gas hydrocarbons and water. This is typically accomplished in a pressure vessel at the surface, but newer technologies allow separation to occur in the wellbore under certain conditions.

Shale
A laminated and fissile very fine-grained sedimentary rock, consisting of compacted silt and clay-size mineral particles. Can contain high proportions of organic material, which if subjected to heat and pressure over geological time can generate petroleum (a petroleum source rock).

Shutdown
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).

Sidetrack
Re-entry of a well from the well's surface location with drilling equipment for the purpose of deviating from the existing well bore to achieve production or well data from an alternative zone or bottom hole location, or to remedy an engineering problem encountered in the existing well bore.

Silicite
Fine grained rocks composed primarily of layered silica.

Source rock
A rock rich in organic matter, which, if subjected to sufficient heat and pressure over geological time, will generate oil or gas.  Typical source rocks, usually shale or limestone, contain above an initial 1% organic matter by weight.

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

Structural spill point
The structurally lowest point in a hydrocarbon trap that can retain hydrocarbons.

Suspended well
A well that has been capped off temporarily.

Sweet spot
The area within a shale source rock unit showing highest TOC and generative potential, normally associated with basin centred deposition.

tcf
Trillion cubic feet.

TD
Total depth.

Thermally mature
A term applied to source rocks, which have received sufficient temperature and pressure over geological time to generate hydrocarbons.

Thermal maturity (Ro)
A term applied to source rocks which have received sufficient temperature and pressure over geological time to generate hydrocarbons.

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).

TOC
Total organic carbon: the weight percent amount of organic carbon within rock, which is a commonly used measure of hydrocarbon source rock richness.

Toolpusher
Second-in-command of a drilling crew under the drilling superintendent. Responsible for the day-to-day running of the rig and for ensuring that all the necessary equipment is available.

Topside
The superstructure of a platform.

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

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

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

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

Vitrinite reflectance (Ro)
A measure of the percentage of incident light reflected from the surface of vitrinite particles in a sedimentary rock.  It is referred to as % Ro and is a measure of the thermal maturity of a rock.  Top of the oil window is dependent on source rock type, but is widely recognized to be at an Ro equivalent of between 0.5-0.7%.

VSP
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.

Waterflooding
The injection of water into an oil reservoir to ‘push’ additional oil out of the reservoir rock and into the wellbores of producing wells.

Wellhead
The equipment at the surface of a well used to control the pressure; the point at which the hydrocarbons and water exit the ground.

Well log
A record of geological formations penetrated, typically during drilling, including technical details of the operation.

Well stimulation
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.

Wet gas
Natural gas containing significant amounts of liquifiable hydrocarbons.

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

Workover
Operations on a producing well to restore or increase production. A workover may be performed to stimulate the well, remove sand or wax from the wellbore, to mechanically repair the well, or for other reasons.

WTI
West Texas Intermediate, a type of crude oil commonly used as a price benchmark in the USA.
 
XRD
X-ray diffraction; scattering of x-rays by the atoms of a rock or crystal that gives information on the structure, composition and identity of the rock or crystal.