Geology Dictionary

Average seismic velocity

Effective vertical seismic speed from a specified datum to a target, calculated as vertical distance divided by one-way travel time under the stated ray-path assumptions. For a vertical two-way reflection time t, the equivalent average is 2z/t; it is not generally the arithmetic mean of the interval velocities.

Worked example: two layers, travel time and depth

Also called: Average velocity (seismic)

Depth conversion

Transformation of seismic interpretations from travel-time coordinates into estimated depth using a velocity model and consistent reference datums. The result depends on the velocity information and imaging assumptions; changing the axis label alone does not establish a depth model.

Worked example: two layers, travel time and depth

Also called: Seismic time-to-depth conversion

Global Boundary Stratotype Section and Point GSSP

An internationally ratified reference point in a specified stratigraphic section that fixes a boundary on the geological time scale. Correlation markers help identify its time-equivalent level in other successions.

Interpretation note A local fossil occurrence or log pick is evidence for correlation, not a new GSSP. Check the ratified reference and supporting markers.

Worked example: a fixed boundary and a revised age

Also called: Golden spike

Global Standard Stratigraphic Age GSSA

An internationally agreed numerical age used to define a boundary on the geological time scale, particularly in parts of the Precambrian. Its defining reference is a number rather than a designated point in a stratigraphic section.

Interpretation note Distinguish a defining GSSA from the estimated numerical age of a GSSP. Consult the current ICS chart for the standard assigned to a particular boundary.

Worked example: a fixed boundary and a revised age

Graded bedding

A systematic change in grain size through the thickness of a sedimentary bed. Describe the direction and scale of the change before interpreting the depositional process.

Worked example: grading within one bed

Also called: Sedimentary grading

Normal grading

An upward decrease in grain size within a bed, with coarser particles below finer particles. Upward refers to stratigraphic younging, which need not coincide with present vertical.

Interpretation note Determine stratigraphic way-up separately from present vertical: a normal graded bed fines toward its original top even after overturning. Confirm the depositional interpretation with other indicators before assigning younging.

Worked example: which way is stratigraphic up?

Also called: Normally graded bedding

Inverse grading

An upward increase in grain size within a bed. The observation describes a texture; it does not by itself identify a unique transport mechanism or depositional environment.

Worked example: grading within one bed

Also called: Reverse grading, Inversely graded bedding

Aa lava

Basaltic lava characterized by a rough, broken, clinker-covered surface.

Also called: A'a, ‘A‘ā

Absolute permeability

Rock permeability measured with a single fluid phase occupying its connected pores.

Interpretation note In the single-phase Darcy-flow context, this corresponds to intrinsic permeability of the pore network. Hydraulic conductivity additionally incorporates fluid properties: K = kρg/μ. This comparison assumes unchanged pore geometry and a fully saturated medium.

Worked example: permeability versus hydraulic conductivity

Also called: Intrinsic permeability

Accommodation space

Space available for sediment accumulation, influenced by subsidence and changes in sea level.

Also called: Accommodation

Acoustic impedance

Product of rock bulk density and compressional-wave velocity; contrasts between layers influence seismic reflections.

Interpretation note For a P wave, Z = bulk density × P-wave velocity. Use compatible units: kg/m³ × m/s gives kg/(m²·s). Equal impedance can occur in different lithologies; impedance is not a unique rock identifier. For example, 2000 kg/m³ × 3000 m/s and 2500 kg/m³ × 2400 m/s both give 6 million kg/(m²·s), despite different densities and speeds.

Worked example: different properties, equal impedance

Also called: Seismic impedance

Agglutinate

A pyroclastic deposit whose hot, partly molten fragments welded together after landing.

Aggradation

Upward accumulation of sediment with relatively little overall advance or retreat of the depositional system.

Amphibole

A group of double-chain silicate minerals, commonly hydrous, found in many igneous and metamorphic rocks.

Amphibolite

A metamorphic rock dominated by amphibole and typically plagioclase, commonly derived from mafic parent rocks.

Amplitude variation with offset AVO

Change in seismic reflection amplitude with source-receiver separation, analyzed for information about lithology and fluids but subject to nonunique interpretation.

Also called: Amplitude versus offset

Andesite

A volcanic rock of intermediate composition, commonly associated with volcanic arcs.

Angular unconformity

Erosional or nondepositional surface separating younger beds from older beds tilted at a different angle.

Anhydrite

Anhydrous calcium-sulfate mineral and associated evaporite rock that may form sealing intervals.

Anisotropy

Variation of a material property with measurement direction, such as different seismic velocities parallel and perpendicular to bedding.

Interpretation note Name the property and scale: hydraulic anisotropy concerns conductivity, not necessarily seismic velocity or electrical resistivity. Isotropic beds can form an anisotropic layered unit.

Worked example: conductivity along and across layers

Anticline

Fold with older beds in its core, commonly forming an upward arch that can create a structural hydrocarbon trap.

Interpretation note Separate fold shape from stratigraphic age: antiform and synform describe closure, whereas anticline and syncline require age evidence. Record the profile orientation before measuring limb angles.

Worked example: fold profile and bed age

Antiform

Upward-convex fold shape, without specifying the ages of its layers.

Interpretation note Separate fold shape from stratigraphic age: antiform and synform describe closure, whereas anticline and syncline require age evidence. Record the profile orientation before measuring limb angles.

Worked example: fold profile and bed age

Aphanitic texture

An igneous texture in which most crystals are too small to distinguish without magnification.

Apparent dip

Inclination of a geological plane seen in a vertical section not aligned with its maximum-dip direction.

Interpretation note For a nonvertical plane, tan(apparent dip) = tan(true dip) × cos(theta), where theta is the horizontal angle from the down-dip direction to the section heading. A negative result means the surface rises along that heading. Two perpendicular signed section slopes of the same planar surface can recover its dip: combine their tangents as orthogonal components, rather than averaging the angles.

Worked example: recover dip from perpendicular sections

Aquifer

Water-bearing permeable rock or sediment capable of transmitting water; an adjoining aquifer may support reservoir pressure.

Worked example: aquifers separated by a leaky layer

Arenaceous

Containing sand-sized material or having a sandy texture; does not by itself specify quartz content.

Also called: Sandy

Arete

A narrow ridge sharpened by erosion in adjacent glacial valleys or cirques.

Also called: Arête

Argillaceous

Containing fine muddy material, commonly clay minerals; used as a compositional or textural rock descriptor.

Also called: Clay-rich

Artesian condition

Confined groundwater pressure that raises water above the aquifer top in a well; surface flow is not required.

Interpretation note Compare static head with both the aquifer roof and the discharge outlet using the same elevation datum. A level above the roof can remain below land. Head above an outlet indicates discharge potential, but does not determine flow rate.

Worked example: artesian head and surface flow

Also called: Artesian pressure

Asthenosphere

Relatively weak, deformable upper mantle beneath the lithosphere; predominantly solid despite possible small melt fractions.

Authigenic

Describes minerals formed in place within a rock, rather than transported into it as sediment grains.

Average linear groundwater velocity

Mean advective groundwater speed estimated as v = q/ne, where q is Darcy flux and ne is effective flow porosity. This averages pore paths and does not include dispersion or solute retardation.

Interpretation note For uniform steady flow, t = L ne/q. State the flow-porosity assumption and test its sensitivity; this time is not a first-arrival or contaminant-breakthrough prediction.

Worked example: flow porosity and travel-time sensitivity

Also called: Seepage velocity, Average pore-water velocity

Background gas BGG

Baseline mud-gas level under reasonably steady drilling conditions, used to compare departures from the prevailing trend.

Bafflestone

Boundstone in which organisms slowed flow and trapped sediment.

Basalt

A generally dark, iron- and magnesium-rich volcanic rock with relatively low silica content.

Basement

Older rock foundation beneath a sedimentary succession, commonly crystalline igneous or metamorphic rock.

Batholith

A large composite intrusive body with more than 100 square kilometres of surface exposure.

Bed

A distinct sedimentary layer; when formally named, the smallest ranked sedimentary lithostratigraphic unit.

Bedding

Arrangement of sediment or rock into layers separated by identifiable boundaries.

Bindstone

Boundstone in which encrusting or binding organisms stabilized sediment.

Biostratigraphy

Use of fossils to date and correlate sedimentary intervals and help interpret depositional environments.

Interpretation note Check assemblage consistency and possible reworking before assigning a host-bed age from a fossil occurrence. State whether a reported range is local or compiled across sections; correlation requires an explicit zonal criterion. Reworking is a depositional history; drilling cavings are sample contamination. Do not interpret a mixed fossil assemblage until these possibilities have been considered.

Worked example: reworking versus drilling cavings

Biotite

A dark mica group containing iron and magnesium, with a sheet structure and excellent basal cleavage.

Bioturbation

Disturbance and mixing of sediment by organisms, often expressed as burrows or disrupted bedding.

Birefringence

The optical property whereby light passing through an anisotropic crystal separates into components travelling at different speeds.

Complementary optical identification observations

Also called: Double refraction

Body fossil

Preserved evidence of an organism's physical body, rather than solely its activity.

Interpretation note A body fossil may retain connected parts or consist of isolated elements; record this observation separately from identification.

Fossil preservation and reworking: worked interpretation

Boudinage

Extension that segments a relatively competent layer into separated, sausage-like bodies.

Boulder

Gravel particle larger than 256 mm on the Wentworth scale.

Boundstone

Carbonate material bound together by organisms during deposition.

Breccia

Rock composed of angular fragments bound together; the fragment origin and surrounding material determine its geological interpretation.

Bright spot

Locally strong seismic amplitude that may indicate hydrocarbons but can also result from lithology or thin-bed interference.

Calcimetry

Estimation of carbonate content from a sample's reaction under a defined analytical method. Report the method and whether the result represents calcite, dolomite or total carbonate.

Calcite

Calcium-carbonate mineral that is a major constituent of limestone and a common cement in sedimentary rocks.

Interpretation note Calcite and aragonite share the ideal formula CaCO3 but differ in crystal structure. Calcite identified in a fossil is not by itself proof that the organism originally secreted calcite.

Worked identification: composition versus crystal structure

Caldera

A broad, basin-shaped volcanic depression substantially wider than the individual vents within it.

Capillary entry pressure

Threshold pressure difference at which a non-wetting fluid first invades an accessible, initially wetting-fluid-filled pore connection under a specified displacement path. Geometry, interfacial tension and wetting conditions control the threshold.

Interpretation note Check whether a measured throat size is radius or diameter. In the ideal tube model, a 10 μm diameter means r = 5 μm, giving 12 kPa for γ = 0.030 N/m and water-side θ = 0°. Treating that diameter as radius incorrectly gives 6 kPa. Preserve phase order and angle convention when transferring software inputs.

Worked example: radius versus diameter

Also called: Entry capillary pressure

Capillary pressure

Pressure difference across a fluid interface. Here Pc is non-wetting-phase pressure minus wetting-phase pressure; always state the phase order because sign conventions differ.

Capillary entry: worked pore-throat example

Also called: Pc

Carbonate mudstone

Mud-supported carbonate rock with fewer than 10% grains in the Dunham classification; distinct from clastic mudstone.

Also called: Lime mudstone

Carbonization

Fossilization in which decomposition removes volatile components and leaves a carbon-rich residue.

Also called: Carbonisation

Catagenesis

Burial-related thermal alteration of organic matter and sediments beyond early diagenesis, including important stages of petroleum generation.

Cavings

Fragments detached from the borehole wall rather than newly drilled at the bit; they can contaminate cuttings samples.

Interpretation note Material from an interval penetrated earlier need not be geologically older. In an upright, unfaulted succession, younger fossils can cave from above into a sample collected while drilling older beds. Folded or faulted sections require their own age-order model.

Worked example: reworking versus drilling cavings

Chalk

Fine-grained marine limestone largely composed of microscopic calcareous skeletal material.

Checkshot survey

Borehole survey measuring travel time from a surface seismic source to receivers at known depths to calibrate time-depth relationships.

Interpretation note Checkshot first arrivals provide source-to-receiver times, not automatically surface-reflection two-way times. Survey geometry, well deviation and seismic reference datum corrections must be reconciled before using the resulting time–depth relationship for a well tie.

Worked example: two layers, travel time and depth

Also called: Check-shot survey

Chemostratigraphy

Correlation and subdivision of rock successions using variations in chemical composition or isotopic signatures.

Interpretation note An isotope excursion requires a common reference scale and checks for alteration and sample comparability before it is used for correlation.

Stable-isotope comparisons and interpretation limits

Chert CHT

Hard, very fine-crystalline silica-rich rock, often found as nodules or layers within sedimentary successions.

Chevron fold

Fold with straight limbs meeting at a sharp hinge.

Worked folding geometry and slip evidence

Chlorite

Platy green silicate mineral that can occur as pore-lining coatings in sandstone and influence reservoir quality.

Interpretation note Chlorite can belong to an earlier low-grade assemblage or form during later replacement. Report its texture and associated minerals before interpreting metamorphic history.

Worked guide: mineral evidence, isograds and overprints

Chronostratigraphic correlation

Correlation intended to connect intervals of equivalent geological age. It differs from matching similar rock types, which may occur at different times in different locations.

Chronostratigraphy

Classification and correlation of rock bodies according to their relative or absolute ages.

Cirque

A bowl-shaped erosional hollow at the head of a glaciated valley.

Clast imbrication

An overlapping, preferentially inclined arrangement of sedimentary clasts. In the common upstream-dipping fabric, the broad clast surfaces dip against the inferred flow direction; long-axis orientation must be recorded separately.

Interpretation note Average dip azimuths as circular directions, not ordinary numbers. Keep measured dip directions, inferred flow directions and undirected long-axis orientations distinct; a circular mean is not a bedding-restoration calculation.

Worked example: averaging bearings across north

Also called: Pebble imbrication, Imbricated clasts

Clay-bound water CBW

Water associated with clay surfaces or interlayers that is generally immobile and commonly excluded from log-derived effective porosity.

Clay-size sediment

Particles smaller than 1/256 mm (about 3.9 micrometres) on the Wentworth scale; size alone does not establish mineral composition.

Also called: Clay size

Closed isotopic system

A mineral or rock domain that has neither gained nor lost the relevant parent or daughter isotopes over the interval being dated, apart from internal radioactive decay. Closure is specific to an isotope system and geological history.

Interpretation note A lower retained daughter/parent ratio can reflect daughter loss rather than a younger formation event. The worked loss example holds parent constant; it is not a universal correction for disturbed samples.

Worked example: daughter loss and apparent age

Coal

Organic-rich sedimentary rock formed as buried plant material changes during coalification. It may contain or generate natural gas.

Cobble

Gravel particle in the 64–256 mm Wentworth size class.

Common midpoint CMP

Surface midpoint shared by multiple source-receiver pairs whose seismic traces are grouped for processing.

Compaction

Reduction of sediment volume as grains pack more closely or deform, commonly decreasing pore space during burial.

Interpretation note A pore-pressure decline can increase effective stress even without additional burial. Predicting deformation also requires compressibility, stress history and drainage times; the stress change alone does not give a settlement.

Effective stress and compaction: worked example

Composite well log

Combined presentation of selected geological observations and measurements on a common well and depth reference. Data sources and any alignment adjustments should remain identifiable.

Compression fossil

Flattened organic remains preserving some chemically altered material, commonly as a thin film.

Compressional wave P-wave

Elastic body wave in which particle motion is parallel to the direction of propagation.

Also called: P wave

Conchoidal fracture

A mineral or glass breakage texture with curved, shell-like surfaces rather than repeated planar cleavage. Quartz commonly shows this texture, but the texture alone does not establish mineral identity.

Mineral identification: cleavage, fracture and parting

Also called: Shell-like fracture

Condensed section

Thin sedimentary interval representing a long time span because deposition was unusually slow.

Confined aquifer

Aquifer beneath a confining layer, with water pressure sufficient to raise a well water level above its top.

Interpretation note Compare static head with both the aquifer roof and the discharge outlet using the same elevation datum. A level above the roof can remain below land. Head above an outlet indicates discharge potential, but does not determine flow rate.

Worked example: artesian head and surface flow

Conformable succession

Strata deposited in sequence without a recognized intervening depositional break.

Also called: Conformable

Conglomerate CGL

Sedimentary rock containing abundant rounded gravel-sized fragments within a finer matrix or cement.

Interpretation note Describe clast orientation separately from roundness and sorting. An imbricated fabric can constrain flow interpretation, but a conglomerate name alone does not identify the transport process.

From grain shape to clast fabric

Connate water

Water retained within sediment pores from the time the sediment formed; distinct from all present-day formation water.

Connection gas CG

Gas entering or being drawn into the well around a pipe connection, commonly recognized later at surface after circulation lag.

Continental crust

Crust forming continents and their submerged margins; generally thicker and less dense than oceanic crust.

Continental rift

Region where continental lithosphere stretches and thins, commonly accompanied by normal faulting and volcanism.

Convergent plate boundary

Boundary where plates move toward one another, producing subduction or continental collision.

Convolution

Mathematical operation combining an input signal with a response function; used to model a seismic trace from reflectivity and a wavelet.

Correlation marker

Recognizable feature used to connect intervals between logs or wells. A similar curve feature is evidence for a correlation, not proof that two intervals are identical in age or properties.

Cross-bedding

Sets of inclined internal layers within a bed, commonly produced as ripples or dunes migrate.

Interpretation note Terminology varies: dune-scale cross-stratification is commonly called cross-bedding and ripple-scale structures cross-lamination. Identify the set boundary that truncates older laminae before inferring stratigraphic way-up; internal lamina dip alone is insufficient.

Worked example: cross-bed truncation and way-up

Cross-cutting relationship

The age relationship in which a geological feature is younger than the rock or structure it cuts.

Crust

Earth’s outermost compositional layer, encompassing distinct continental and oceanic types.

Crystal form

A crystallographically defined arrangement of crystal faces; actual specimens may develop only some of those faces.

Cuttings

Fragments generated as the bit breaks rock and carried to surface in the drilling fluid.

Cuttings description

Written observations of recovered rock fragments, including lithology, texture and shows, tied to a stated lag-corrected interval.

Cuttings morphology

Observed size and shape of recovered drilled fragments. Bit action, transport and wall failure affect morphology, so shape alone is not a unique indicator of formation type.

Cuttings sample interval

Measured-depth range assigned to a collected cuttings sample. It must be distinguished from collection time and from the bit depth when the material reached surface.

Cuttings texture

Arrangement and appearance of grains, matrix and other fabric visible in recovered fragments. Drilling damage and small fragment size can limit how faithfully it represents the intact rock.

Dacite

A silica-rich volcanic rock intermediate in composition between andesite and rhyolite.

Darcy flux

Volumetric discharge divided by the bulk cross-sectional area normal to flow: q = Q/A. For one-dimensional saturated Darcy flow, q = −K dh/dx. Its units are length per time, but it is not the velocity within connected pores.

Interpretation note Use the bulk area for q = Q/A; dividing by flow porosity gives an average advective speed, not a local pore-channel speed.

Worked example: flow porosity and travel-time sensitivity

Also called: Specific discharge, Darcy velocity

Daughter isotope

An isotope produced by radioactive decay of a parent isotope.

Interpretation note Measured daughter atoms can include both initial and radiogenic components. Treating all daughter atoms as decay products can bias an age.

Radiometric dating: half-life and age interpretation

Debris flow

A flowing mixture of water and sediment that contains substantial coarse particles and can travel rapidly downslope.

Deconvolution

Signal processing that estimates or removes a filtering effect to improve seismic wavelet shape or suppress predictable reverberations.

Delta

Sedimentary accumulation where a sediment-bearing river enters standing water and loses transport capacity.

Deposition

Accumulation of transported sediment when it is laid down.

Depositional channel

Elongated depression that carries water and sediment and can become filled with a distinctive sediment body.

Depositional environment

Physical, chemical and biological setting in which sediment accumulated; interpreted from multiple rock and fossil observations.

Also called: DE, Sedimentary environment

Depth migration

Seismic imaging that places reflections in depth using a velocity model, particularly where velocity varies laterally.

Detrital

Describes rock or mineral particles derived from breakdown of pre-existing rocks and subsequently transported or deposited.

Also called: Clastic

Diagenesis

Physical, chemical and biological changes to sediment after deposition, generally below metamorphic conditions. These changes can alter reservoir quality.

Diamicton

An unconsolidated sediment with a very broad mixture of grain sizes; the name alone does not establish glacial origin.

Diapir

Mobile material, commonly salt or shale, that intrudes surrounding rock and can deform overlying beds.

Dim spot

Locally weakened seismic reflection amplitude, potentially caused by reduced impedance contrast associated with fluids or lithology.

Diorite

A coarse-grained intermediate intrusive rock commonly dominated by plagioclase with dark silicate minerals.

Dip direction

Compass bearing of the steepest downward line in an inclined geological plane. It is perpendicular to strike and identifies which way the plane descends; it is an azimuth, not the dip angle.

Worked example: strike and apparent dip

Also called: Dip azimuth

Disconformity

Break in the geological record between broadly parallel sedimentary beds, reflecting erosion or nondeposition.

Divergent plate boundary

Boundary where plates move apart, commonly creating new oceanic lithosphere at a spreading centre.

Dolomite mineral

Calcium-magnesium carbonate mineral, CaMg(CO₃)₂, that is the principal constituent of dolostone.

Also called: Dolomite

Dolomitization

Replacement of calcite by dolomite during diagenesis, potentially changing pore structure and reservoir properties.

Dolostone

Carbonate rock composed mainly of dolomite mineral. The rock name distinguishes it from the mineral itself.

Also called: Dolomite rock

Downlap

Termination of inclined strata against an underlying surface with a shallower apparent dip.

Interpretation note Read the termination relative to the lower bounding surface and original depositional geometry. In the standard usage, onlap ends updip and downlap ends downdip; present apparent dip alone may be misleading after deformation.

Worked interpretation: reflection terminations and uncertainty

Drilling-fluid contamination of samples

Presence of introduced fluid, additives or recirculated material in a geological sample. It can distort mineral, fluorescence, gas or chemical observations and should be documented.

Drumlin

A streamlined hill shaped beneath moving ice, generally elongated parallel to former ice flow.

Dyke

A sheet-like igneous intrusion that cuts across the pre-existing structure of surrounding rocks.

Also called: Dike

Earthflow

A flow-like downslope movement dominated by fine-grained earth material, commonly involving abundant water.

Effective permeability

Permeability available to one fluid phase when other immiscible phases also occupy the rock.

Effective porosity

Interconnected pore fraction available for fluid flow; petrophysical conventions commonly exclude clay-bound water, so the interpretation basis should be stated.

Interpretation note State the measurement convention: dry-core connected porosity excludes isolated pores; a common log convention subtracts clay-bound water from total porosity. These are not interchangeable corrections. Retain sample preparation, method and bulk-volume basis with the result.

Worked example: porosity measurement conventions

Effective stress

Stress acting on the solid framework of a porous material. For a saturated soil under the Terzaghi convention, effective normal stress equals total normal stress minus pore-water pressure; compression is positive.

Interpretation note When a falling water table reduces the weight of overlying sediment, total stress also changes. At a point that remains saturated, use Δσ′v = Δσv − Δu rather than treating the pressure decline alone as the effective-stress increase.

Worked example: falling water table and changing load

Also called: Intergranular stress

Electrical anisotropy

Directional variation of electrical resistivity, commonly expressed as a difference between horizontal and vertical formation resistivity.

Interpretation note For continuous horizontal isotropic layers in ideal DC experiments, resistivity parallel to bedding is the thickness-weighted harmonic mean; normal resistivity is the thickness-weighted arithmetic mean. Name the axes and scale. These equivalent material values are not automatically apparent resistivities measured by a surface array or logging tool, and do not determine hydraulic conductivity.

Worked example: electrical versus hydraulic anisotropy

End moraine

A ridge of sediment accumulated at a former glacier margin.

Eolian

Describes sediment transport and deposition by wind.

Also called: Aeolian

Erosion

Removal and transport of rock or sediment by agents such as water, wind, ice or gravity.

Esker

A winding ridge of sand and gravel deposited by glacial meltwater, commonly in channels within or beneath ice.

Estuary

Partly enclosed coastal setting where river water mixes with seawater.

Evaporite

Mineral deposit or rock produced by precipitation as saline water concentrates, commonly containing halite, gypsum or anhydrite.

Extrusive rock

Igneous rock produced by volcanic material at the surface, often fine-grained, glassy or fragmental.

Also called: Volcanic rock

Facies

A rock body or sediment distinguished by a characteristic combination of composition, texture, structures or fossils.

Fault

Fracture or fracture zone across which rock bodies have moved relative to one another.

Field guide: fault core, damage zone and slickenlines

Fayalite

The iron end member of the olivine solid-solution series, with composition Fe2SiO4.

Feldspar

Group of common silicate minerals present in many sandstones and crystalline rocks; alteration can produce clay minerals.

Interpretation note In ideal plagioclase, replacing Na+ with Ca2+ is balanced by replacing Si4+ with Al3+: both paired contributions total +5. An30 denotes 30 mole percent anorthite component, not 30 percent calcium by mass.

Worked example: plagioclase charge balance

Felsic

Describing silica-rich igneous compositions typically dominated by feldspar and quartz, with comparatively few iron–magnesium minerals.

Fence diagram

Perspective display of intersecting geological cross sections used to show three-dimensional relationships among rock units.

Firn

Compacted granular snow surviving beyond a melt season and progressing toward glacier ice through further densification.

Also called: Névé, Neve

Floatstone

Coarse carbonate rock supported by matrix, with more than 10% grains larger than 2 mm.

Flowline sample

Sample of returning drilling fluid collected before solids removal, potentially containing drilled cuttings. Record its collection point because screening changes the solids represented.

Fluid saturation

Fraction of a defined pore volume occupied by a particular fluid.

Fluorescence

Visible light emitted under ultraviolet illumination; hydrocarbon interpretation must consider mineral and drilling-fluid fluorescence.

Fluvial

Describes sediment transport, deposition or landforms associated with rivers and streams.

Fold

Bend in originally straighter rock layers produced by deformation.

Worked folding geometry and slip evidence

Fold axial plane

Plane connecting the hinge lines of successive folded layers.

Also called: Axial plane

Fold hinge

Line of strongest curvature along a folded surface.

Interpretation note A hinge line, an intersection lineation and a stretching lineation are not interchangeable. Compare orientations and fabric relationships before using any lineation as a fold-axis proxy.

Worked example: separate lineation mechanisms

Also called: Hinge line

Fold limb

Part of a fold between neighbouring hinge zones.

Interpretation note Determine stratigraphic way-up separately from present vertical: a normal graded bed fines toward its original top even after overturning. Confirm the depositional interpretation with other indicators before assigning younging.

Worked example: which way is stratigraphic up?

Foliation

A penetrative planar fabric in rock, commonly produced by aligned minerals or compositional segregation during deformation and metamorphism.

Interpretation note Foliation is planar; lineation is linear. A lineation may lie within a foliation, arise from two intersecting foliations, or record a separate fabric element.

Worked example: separate lineation mechanisms

Footwall

Rock block below an inclined fault surface.

Formation

Primary lithostratigraphic unit: a distinguishable, mappable body of rock defined by lithological characteristics.

Interpretation note A lithological boundary is not automatically a time surface. Track the rock pick and its age interpretation separately.

Worked example: rock boundary versus time surface

Formation dip

Inclination of a geological surface from horizontal, specified together with the direction of maximum downward slope.

Interpretation note True dip is the maximum inclination below horizontal. It is measured perpendicular to strike; an oblique section generally shows a smaller angle.

Worked example: strike and apparent dip

Also called: Dip, True dip

Forsterite

The magnesium end member of the olivine solid-solution series, with composition Mg2SiO4.

Forward modelling

Calculation of the measurements a sensor should produce for a specified subsurface model, tool configuration and environment.

Also called: Forward modeling

Fossil

Preserved remains or evidence of past organisms, such as shells, casts, tracks or burrows.

Interpretation note Record preservation and host sediment separately; an older fossil can be redeposited into a younger bed.

Fossil preservation and reworking: worked interpretation

Fossil cast

A positive replica formed when sediment or minerals fill a fossil mould.

Interpretation note A cast is infill, not surviving shell material. Record the fill and surrounding rock separately; do not equate a filled fossil shape with an open pore.

Worked example: describe a mould, cast or steinkern

Fossil disarticulation

Separation of once-connected skeletal elements, such as bones or shell valves. It describes preservation and does not by itself establish transport distance.

Fossil preservation and reworking: worked interpretation

Also called: Disarticulated remains

Fossil mould

A three-dimensional impression preserving an organism's shape after original material has disappeared.

Interpretation note Specify the surface represented: an external mould records the outside of a shell. In a broken cutting, record the visible impression before inferring a complete organism.

Worked example: describe a mould, cast or steinkern

Also called: Fossil mold

Fossil replacement

Substitution of new minerals for original biological material as that material dissolves.

Also called: Replacement fossilization

Fracture porosity

Pore volume contained in natural fractures; even small fracture volumes can strongly affect permeability.

Framestone

Boundstone supported by a rigid framework built by organisms.

Gabbro

A coarse-grained mafic intrusive rock principally containing plagioclase and pyroxene, sometimes with olivine.

Gas chromatography GC

Separation and measurement of individual gas components, allowing composition to be evaluated alongside total-gas readings.

Gas component ratio

Ratio calculated from measured gas components, commonly used to compare changes in gas character. Sampling, detector response and drilling-fluid contamination can affect its interpretation.

Gas show

Increase in detected mud gas relative to the surrounding trend, evaluated at lagged depth and against drilling conditions.

Geological cement

Minerals precipitated within pore spaces that bind sediment grains; distinct from engineered cement placed during well construction.

Also called: Mineral cement, Rock cement

Geological cross section

Vertical view through the subsurface showing interpreted rock units, faults and other structures.

Also called: Cross section

Geophysical inversion

Mathematical estimation of a subsurface model consistent with measured data and stated assumptions; a fit does not guarantee a unique solution.

Also called: Inversion, Inverse problem

Geostatistics

Statistical methods that account for spatial relationships when estimating or simulating subsurface properties from incomplete sampling.

Glacial horn

A sharp mountain peak formed where several cirques erode toward one another.

Also called: Horn (glacial)

Glacier

A persistent mass of land ice that moves under its own weight.

Glaciolacustrine

Relating to deposition or processes in lakes influenced by glacial meltwater.

Glaciomarine

Relating to marine sedimentation or processes influenced directly by glaciers or floating glacial ice.

Glauconite

Green, iron-bearing silicate mineral commonly associated with slowly accumulating marine sediments.

Gneiss

A metamorphic rock characterized by compositional banding, commonly separating lighter and darker mineral-rich domains.

Graben

Relatively lowered rock block bounded by normal faults dipping toward the block.

Grain roundness

Degree to which a grain’s corners and edges are smoothed; distinct from its overall shape or sphericity.

Interpretation note Assess edge and corner curvature separately from overall grain proportions. A two-dimensional view can obscure three-dimensional form; breakage during drilling and preparation may introduce fresh angular surfaces.

Grain size and texture: worked examples

Also called: Roundness

Grain size

Dimensions of sediment particles, described using a stated size scale; size classes should not be inferred solely from rock colour.

Interpretation note State the size scale and measurement method. A sieve aperture, microscope section and laser-derived equivalent diameter need not give identical distributions. Phi increases as diameter decreases.

Grain size and texture: worked examples

Grain sphericity

Degree to which the three-dimensional form of a particle approaches a sphere. Sphericity concerns overall proportions, whereas roundness concerns smoothing of corners and edges; a rounded elongated grain can have low sphericity.

Interpretation note A circular silhouette in one view does not establish a spherical three-dimensional grain. Record orthogonal dimensions or additional views; keep any axis ratio distinct from a named sphericity index and from corner roundness.

Worked example: a circular view can hide flattening

Also called: Sphericity

Grainstone

Grain-supported carbonate rock lacking depositional lime mud; pore space may later contain cement.

Granite

A coarse-grained felsic intrusive rock containing quartz and feldspars, commonly with mica or amphibole.

Granule

Gravel particle in the 2–4 mm Wentworth size class.

Gravel

Sediment coarser than 2 mm, including granules, pebbles, cobbles and boulders.

Groundmass

The finer-grained or glassy material surrounding larger crystals in an igneous rock.

Groundwater

Water occupying connected pores and fractures below the ground in the saturated zone.

Groundwater drawdown

Decrease in groundwater level or hydraulic head caused by pumping.

Worked hydraulic response and head interpretation

Also called: Aquifer drawdown

Groundwater recharge

Addition of water to an aquifer, including infiltrated precipitation that reaches the saturated zone.

Also called: Recharge

Group

Formal lithostratigraphic unit containing two or more formations with shared lithological characteristics.

Growth fault

Fault active during sediment deposition, commonly producing a thicker sedimentary section on its downthrown side.

Gypsum

Hydrated calcium-sulfate evaporite mineral that can convert to anhydrite through dehydration.

Half-life

The time required for half the atoms of a radioactive isotope to decay.

Interpretation note Each equal half-life removes half of the remaining parent population, not half of the original population. After three half-lives, one eighth remains. With 75% of the original parent remaining in a closed system, elapsed time is log₂(1/0.75) ≈ 0.415 half-lives; linear interpolation between 100% and 50% gives the wrong time.

Worked example: a fraction of a half-life

Halide mineral

A mineral built from halogen anions and cations; halite and sylvite are common examples.

Halite

Sodium-chloride evaporite mineral, also called rock salt, capable of flowing into diapirs and forming effective seals.

Hanging valley

A tributary valley left above the floor of a more deeply eroded main glacial valley.

Hanging wall

Rock block above an inclined fault surface.

Also called: Hangingwall

Heterogeneity

Variation of rock properties from one location to another within a formation or reservoir.

Interpretation note Heterogeneity describes changes between locations; anisotropy describes changes with direction. A layered unit can have both, depending on the observation scale.

Worked example: conductivity along and across layers

Hiatus

Interval of geological time missing from a sedimentary record because deposition stopped or material was eroded.

Interpretation note A preserved-thickness/time average spanning a hiatus includes the gap in its denominator. Recovering a rate for depositional intervals alone requires independent constraints on their duration and on erosion and compaction.

Worked example: hiatus and accumulation-rate denominators

Horst

Relatively elevated rock block between normal faults that dip outward from it.

Hydraulic conductivity

Capacity of a porous material to transmit a particular fluid under a hydraulic gradient; depends on both material and fluid properties. K depends on the medium and fluid: K = kρg/μ, where k is intrinsic permeability, ρ density and μ dynamic viscosity. Temperature can change K without changing pore geometry.

Interpretation note Conductivity has velocity units but is not pore-water travel speed. For fixed pore geometry and density, doubling dynamic viscosity halves K. State the fluid, temperature and test direction; the worked example uses a scalar conductivity.

Worked example: permeability versus hydraulic conductivity

Hydraulic gradient

Change in hydraulic head per unit distance. In coordinate x, dh/dx is signed; flow is toward lower head. A positive head-loss magnitude divided by flow-path length is often reported as i. The gradient is dimensionless.

Interpretation note Convert water-level depths to a common elevation datum before calculating a gradient. Two wells constrain a directional difference, not a unique horizontal flow direction; verify hydraulic connection and comparable observation times.

Worked example: depth to water versus head

Hydraulic head

Mechanical energy per unit weight of water, expressed as a length relative to a datum. With negligible velocity head, h = z + p/(ρg): elevation head plus pressure head. Compare measurements using compatible datums and fluid-density assumptions.

Interpretation note For a nonflowing observation well, subtract vertical depth to water from the surveyed measuring-point elevation. A deeper water-level reading can still represent higher head when casing elevations differ.

Worked example: depth to water versus head

Hydrocarbon migration

Movement of hydrocarbons from their source through rocks toward reservoirs or other destinations.

Also called: Petroleum migration

Hydrocarbon show

Observed indication of hydrocarbons in samples or measurements; interpretation requires checking origin, contamination and supporting evidence.

Also called: Show

Igneous rock

Rock formed by solidification of magma or lava.

Illite

Potassium-bearing clay mineral group common in shales and formed through alteration of other silicates.

Impression fossil

A shallow imprint of an organism that retains its outline or surface detail without original organic material.

Index mineral

A mineral used, with rock composition and associations, to constrain metamorphic conditions.

Interpretation note Compare mineral associations in compositionally suitable rocks and check whether the mineral is relict, newly grown or detrital before using it as a grade indicator.

Worked guide: mineral evidence, isograds and overprints

Infiltration

Entry of water from the ground surface into soil or rock.

Interval velocity

Seismic-wave velocity within a particular rock interval, rather than an average through the entire overlying section.

Interpretation note In a horizontal, vertically traversed interval, thickness is interval velocity multiplied by the interval one-way travel time. Use half the interval two-way time, not half the cumulative time to its base. Average and RMS velocities answer different questions. When deriving velocity from two boundary times, evaluate their difference and correlated timing errors; a shared time shift cancels.

Worked example: two layers, travel time and depth

Intrusive rock

Igneous rock solidified below the surface, commonly developing visible crystals during relatively slow cooling.

Also called: Plutonic rock

Irreducible water saturation Swirr

Lowest water fraction retained in a rock sample under a specified displacement pressure or centrifuge condition.

Also called: irreducible water

Isochore

Contour joining equal true vertical thickness; an isochore map differs from a true-stratigraphic-thickness isopach map.

Interpretation note State the thickness convention in the map legend. Subtracting top and base depths at one XY location gives vertical thickness, not automatically normal thickness. In two locally planar examples with TST 10 m, TVT is 10 m at 0° dip but 20 m at 60° dip; the larger vertical interval alone does not demonstrate stratigraphic thickening.

Worked example: isopach versus isochore

Isoclinal fold

Fold with essentially parallel limbs.

Isolated porosity

Pore space disconnected from the larger interconnected pore network.

Isopach

Line connecting equal true stratigraphic thickness; an isopach map shows how unit thickness varies spatially.

Interpretation note State the thickness convention in the map legend. Subtracting top and base depths at one XY location gives vertical thickness, not automatically normal thickness. In two locally planar examples with TST 10 m, TVT is 10 m at 0° dip but 20 m at 60° dip; the larger vertical interval alone does not demonstrate stratigraphic thickening.

Worked example: isopach versus isochore

Isostatic rebound

Upward crustal adjustment following removal of a load, such as an ice sheet or eroded rock mass.

Iterative forward modelling

Repeatedly adjusting an assumed model and recalculating its expected measurement response to improve agreement with observed data. Geological plausibility must be considered alongside the numerical match.

Also called: Iterative forward modeling

Joint

Fracture in rock with no appreciable displacement parallel to its surface; differs from a fault.

Interpretation note Distinguish geological joints from pipe joints, faults and breaks introduced during drilling or handling. Describe orientation and opening separately.

Worked example: joint sets and aperture

Also called: Rock joint

Kaolinite

Clay mineral commonly produced by alteration of feldspar or mica; unlike smectite, it has little swelling tendency.

Karst

Terrain shaped by dissolution of soluble rock, commonly with caves, sinkholes and underground drainage.

Kerogen

Solid organic matter in sedimentary rocks that is insoluble in ordinary organic solvents and can generate hydrocarbons during maturation.

Kettle

A depression produced when a buried or partly buried block of glacial ice melts.

Also called: Kettle hole

Klippe

Erosional remnant of a thrust sheet isolated above its surrounding footwall rocks.

Kriging

Spatial estimation method that weights nearby data using a model of their spatial continuity.

Laccolith

A lens-like intrusion that spreads between layers and arches the overlying strata upward.

Lacustrine

Describes sediments or depositional environments associated with lakes.

Lag depth

Estimated source depth assigned to returned cuttings or gas after correcting for transport delay; mixing and cavings can blur that assignment.

Lag time

Transport delay between a downhole event or sample origin and its arrival at the surface observation point; specify the path considered.

Also called: Cuttings transport lag

Lagerstatte

A fossil deposit notable for exceptional preservation, an unusually concentrated fossil assemblage, or both.

Also called: Lagerstätte, Lagerstätten

Lamination

Fine-scale layering within a sedimentary bed, reflecting changes during deposition or subsequent modification.

Interpretation note Measure layer thickness normal to its boundaries. A common convention separates laminae below 1 cm from thicker beds; record the convention and actual measurement. Apparent width on an oblique core face is not true thickness.

Worked example: apparent versus true layer thickness

Landform

A natural feature of Earth’s surface, such as a valley, ridge or plain.

Lava

Magma that has reached the surface; the term also describes the resulting flow deposit.

Limestone LS

Carbonate sedimentary rock dominated by calcite; fossils, lime mud and cement may contribute to its fabric.

Listric fault

Curved fault whose dip decreases with depth.

Lithification

Transformation of loose sediment into coherent rock through processes such as compaction and mineral cement growth.

Lithofacies

Rock subdivision distinguished by physical features such as composition, texture and sedimentary structures.

Worked example: migrating facies and a missing interval

Lithologic correlation

Connection between intervals based on similar rock characteristics or associated log responses. Similar lithology alone does not establish time equivalence.

Lithology

Description of rock character, including mineral composition, texture, grain size and colour.

Lithology fraction

Portion of a mixed sample attributed to a particular rock type. Estimated sample proportions need not equal the proportions of beds drilled because transport and recovery can be selective.

Lithosphere

Mechanically strong outer shell comprising the crust and uppermost mantle, divided into tectonic plates.

Lithostratigraphic unit

Rock body recognized by lithological properties and stratigraphic relationships; its boundaries need not represent a single age.

Interpretation note A lithological boundary is not automatically a time surface. Track the rock pick and its age interpretation separately.

Worked example: rock boundary versus time surface

Lithostratigraphy

Subdivision and correlation of strata based on rock characteristics rather than directly on age.

Interpretation note A lithological boundary is not automatically a time surface. Track the rock pick and its age interpretation separately.

Worked example: rock boundary versus time surface

Log correlation

Matching recognizable measurement patterns between intervals or wells to support geological interpretation.

Lustre

The appearance of light reflected from a mineral surface, described with terms such as metallic, vitreous or earthy.

Also called: Luster

Mafic

Describing igneous rocks or minerals relatively rich in magnesium and iron compared with felsic compositions.

Magma

Molten rock beneath the surface, commonly accompanied by suspended crystals and dissolved gases.

Magnetostratigraphy

Correlation and ordering of rock units using their recorded magnetic polarity patterns.

Interpretation note Polarity correlation is one part of magnetostratigraphy; remanent intensity and direction can also carry stratigraphic information. Heating or alteration may produce a later magnetic record. Check the age of magnetization before interpreting it as the age of deposition.

Worked example: polarity correlation and age control

Mantle

Silicate-rich layer between Earth’s crust and core; mostly solid but able to deform over geological time.

Marble

A metamorphic carbonate rock produced by recrystallization of limestone or dolostone.

Marker bed

Distinctive layer useful for recognizing or correlating a stratigraphic position between observations.

Marl

Sedimentary rock containing substantial proportions of both clay and calcium carbonate.

Mass wasting

Downslope movement of rock or soil driven primarily by gravity.

Also called: Mass movement

Matrix

Finer material occupying space between larger grains in a rock; petrophysical usage may also mean the solid mineral framework.

Maximum flooding surface MFS

Stratigraphic surface marking the greatest landward extent of marine flooding within a depositional sequence.

Interpretation note This designation needs sequence-scale stacking and correlation context. An individual flooding contact or a high gamma-ray value does not by itself establish maximum flooding.

Worked example: flooding surfaces and parasequence evidence

Member

Named lithostratigraphic subdivision of a formation, recognized where finer subdivision is useful.

Metamorphic facies

A set of mineral assemblages associated with a characteristic range of metamorphic pressure and temperature.

Interpretation note Facies interpretation uses mineral assemblages and bulk composition; the same pressure–temperature conditions can produce different minerals in different protoliths.

Worked guide: mineral evidence, isograds and overprints

Metamorphic grade

The relative intensity of metamorphism, commonly assessed from temperature-sensitive mineral assemblages and textures.

Interpretation note Relative grade does not uniquely specify pressure or reconstruct a complete thermal history. Separate earlier preserved minerals from later replacement products.

Worked guide: mineral evidence, isograds and overprints

Metamorphism

Solid-state modification of rock minerals or texture in response to changed temperature, pressure, deformation or reactive fluids.

Mica

Family of sheet-silicate minerals characterized by thin plates and strong basal cleavage.

Micrite

Very fine carbonate mud or its lithified equivalent, forming the matrix of many limestones.

Microporosity

Pore space of very small characteristic dimensions, often below one micrometre, which commonly retains water strongly.

Mid-ocean ridge

Submarine mountain system at a spreading boundary where new oceanic crust forms.

Migmatite

Rock containing both metamorphic and igneous components, commonly recording partial melting and segregation of melt.

Interpretation note The accounting figure explains phase separation, not migmatite texture or a mapped melt percentage. Interpret the rock using field relationships and petrography, not the invented concentrations.

Worked example: separate melt from its solid residue

Mineral

A naturally formed solid with an ordered atomic structure and a characteristic chemical composition, allowing defined compositional variation.

Mineral cleavage

Preferential breakage along crystallographic planes of weakness within a mineral; distinct from sedimentary bedding or rock cleavage.

Interpretation note Several parallel surfaces represent one cleavage direction. Record the number of distinct directions and their angles; growth faces alone do not prove cleavage.

Mineral identification: cleavage, fracture and parting

Also called: Cleavage (mineral)

Mineral fracture

Breakage of a mineral along surfaces other than its cleavage planes. Describe the surface character, such as uneven, splintery or conchoidal; a mineral with cleavage may also fracture in other directions.

Mineral identification: cleavage, fracture and parting

Also called: Fracture (mineral)

Mineral habit

The characteristic external growth shape or aggregate form of a mineral, which can vary with growth conditions.

Interpretation note Habit describes growth; cleavage describes breakage. A cubic outline can be a growth form rather than a cleavage fragment.

Mineral identification: cleavage, fracture and parting

Also called: Crystal habit

Mineral hardness

Resistance to scratching by another material; a diagnostic property that does not directly measure toughness.

Interpretation note Confirm an actual groove after removing loose powder. Scratch resistance, resistance to breakage and cleavage quality describe different properties. During a streak test, distinguish powder deposited on porcelain from a groove cut into it; an apparent white trace may be plate material rather than a white mineral streak. Scratch tests compare resistance to scratching; a chip breaking during a test is not a hardness measurement. Record the reference material and confirm a groove rather than loose powder. A bracket between reference minerals does not justify an exact midpoint hardness.

Mineral identification: cleavage, fracture and parting

Also called: Hardness (mineral)

Mineral parting

Planar breakage controlled by particular structural weaknesses, for example those associated with deformation or twinning, rather than the consistently available cleavage planes of the mineral lattice. Parting may occur in some specimens of a mineral and be absent in others.

Mineral identification: cleavage, fracture and parting

Also called: Parting (mineral)

Model nonuniqueness

Condition in which different geological or property models can explain the available measurements. A close fit therefore does not establish that one interpretation is the only possible answer.

Model-data misfit

Quantified difference between predicted and observed measurements under a specified comparison method. A small misfit can coexist with uncertain parameters or an incorrect geological explanation.

Mohs hardness scale

A relative scratch-resistance scale using ten reference minerals, from talc at 1 to diamond at 10.

Interpretation note Scratch tests compare resistance to scratching; a chip breaking during a test is not a hardness measurement. Record the reference material and confirm a groove rather than loose powder. A bracket between reference minerals does not justify an exact midpoint hardness.

Worked example: scratch resistance versus breakage

Also called: Mohs scale

Moldic porosity

Secondary pore space left when a grain or skeletal component dissolves while its original shape remains preserved as a cavity.

Interpretation note Moldic describes pore origin and shape, not connectivity. A dissolved grain can leave an isolated cavity or one linked to other pores. Describe visible moulds separately from measured connected porosity and permeability.

Worked example: visible moulds versus connected porosity

Also called: Mouldic porosity

Mud logging

Surface monitoring and geological recording of drilled cuttings, mud gas and relevant drilling measurements.

Also called: Mudlogging

Mud-gas extraction

Separation of gas from returning drilling fluid for measurement. Extraction efficiency and sampling conditions affect the recorded gas signal, which is not a direct reservoir-fluid concentration.

Mud-in sample

Drilling-fluid sample collected before the fluid is pumped into the well. Comparing it with returns can help distinguish introduced materials from changes acquired downhole.

Mud-out sample

Drilling-fluid sample collected after the returning fluid passes through shale-shaker screening. It differs from an unscreened flowline sample and from the incoming suction-pit fluid.

Mudstone MDST

Fine-grained sedimentary rock composed mainly of mud-sized material; in clastic usage, it lacks the strong fissility of shale.

Interpretation note Here mudstone is used in the clastic sense. Visible lamination can occur without strong fissility; do not confuse a carbonate mudstone classification with this breakage description.

Worked comparison: splitting versus layering

Muscovite

A generally pale potassium-rich mica with a sheet structure that permits splitting into thin flakes.

Native element mineral

A mineral composed principally of one chemical element, such as native copper or gold.

Nonconformity

Unconformity where sedimentary rocks overlie an eroded surface of igneous or metamorphic rocks.

Normal fault

Fault along which the block above the fault surface moves downward relative to the block below it.

Normal incidence

A wave approaching an interface along its normal, at zero angle to that normal. For a horizontal interface this path is vertical; normal incidence does not mean zero angle to the interface itself.

Seismic impedance and reflection: worked example

Also called: Perpendicular incidence

Normal moveout NMO

Change in reflection arrival time with source-receiver separation for a horizontal reflector.

Obsidian

A natural volcanic glass formed when lava solidifies with little crystallization.

Oceanic crust

Relatively thin, predominantly mafic crust beneath ocean basins.

Oil show

Observed evidence of oil in recovered material, such as staining or a supported fluorescence/solvent response; not proof of commercial production.

Oil staining

Visible discoloration or coating interpreted as possible hydrocarbon residue on a sample. It must be considered with fluorescence, solvent response and potential contamination.

Olivine

A magnesium–iron silicate mineral group with isolated tetrahedra, common in mantle rocks and many basalts.

Onlap

Termination of younger, gently inclined beds against an older surface that slopes more steeply.

Interpretation note Read the termination relative to the lower bounding surface and original depositional geometry. In the standard usage, onlap ends updip and downlap ends downdip; present apparent dip alone may be misleading after deformation.

Worked interpretation: reflection terminations and uncertainty

Outcrop

Rock exposed at the ground surface, providing direct evidence of lithology and geological structure.

Outwash

Sediment transported and deposited by meltwater beyond a glacier margin.

Packstone

Carbonate rock whose grains support the fabric while carbonate mud occupies some intergranular space.

Pahoehoe lava

Basaltic lava with a comparatively smooth, lobate or rope-like cooled surface.

Also called: Pāhoehoe

Paleocurrent

A past sediment-transporting current whose direction can be reconstructed from preserved sedimentary evidence, including clast fabrics and cross-stratification.

Interpretation note Keep the measured feature, restored orientation and interpreted flow direction distinct. Local transport evidence does not by itself map a whole drainage basin.

From grain shape to clast fabric

Also called: Palaeocurrent, Ancient current

Palynology

Study of organic-walled microscopic remains, including pollen, spores and certain algal cysts, for correlation and environmental interpretation.

Worked example: fossil ranges and sample uncertainty

Parasequence

Relatively conformable package of beds bounded by marine flooding surfaces.

Interpretation note Look for genetically related facies between bounding surfaces, not simply a repeating log motif. Shallowing-upward coastal packages are common examples; thickness alone does not define the unit.

Worked example: flooding surfaces and parasequence evidence

Parent isotope

A radioactive isotope whose decay is used, with its products and decay rate, in an isotopic dating system.

Interpretation note The measured parent abundance is the amount remaining today; it is not generally the starting amount. A decay model and daughter correction are needed to infer elapsed time.

Radiometric dating: half-life and age interpretation

Pebble

Gravel particle in the 4–64 mm Wentworth size class.

Peridotite

An ultramafic rock rich in olivine, commonly with pyroxene; a major constituent of the upper mantle.

Permeability K

Measure of how readily a connected pore system transmits fluid under a pressure gradient.

Interpretation note Notation varies between disciplines. Intrinsic permeability is commonly written k and has area units (m² or darcies); hydraulic conductivity is commonly K and has length/time units. Read the stated units and fluid conditions before comparing results.

Worked example: permeability versus hydraulic conductivity

Permineralization

Fossil preservation by precipitation of minerals within pores of original biological material, often retaining microscopic internal detail.

Also called: Permineralisation

Petroleum system

Linked source, migration, reservoir, seal and trap elements whose timing permits hydrocarbon generation, accumulation and preservation.

Phaneritic texture

An igneous texture in which individual mineral grains are visible without magnification.

Phenocryst

A conspicuously larger crystal within the finer groundmass of a porphyritic igneous rock.

Interpretation note Describe grain size relative to the surrounding groundmass separately from crystal enclosure. A phenocryst need not contain smaller crystals, and a poikilitic host need not be a phenocryst. Size contrast alone does not identify a unique cooling history.

Worked comparison: grain size versus enclosure

Phi scale

Logarithmic particle-size coordinate defined by phi = −log2(d / 1 mm), where d is grain diameter. Each increase of one phi unit halves the diameter; positive phi values indicate finer grains than 1 mm.

Interpretation note A mean calculated in phi converts back to a geometric mean diameter, not an arithmetic mean diameter. For equally weighted 0.125 and 0.5 mm observations, mean phi is 2 (0.25 mm); the arithmetic diameter mean is 0.3125 mm. State weighting and statistical method.

Worked example: averaging phi versus diameter

Also called: Krumbein phi scale

Phosphate mineral

A mineral containing phosphate groups; apatite is a widespread geological example.

Phyllite

A fine-grained foliated metamorphic rock whose aligned mica grains produce a characteristic silky sheen.

Pinch-out

Lateral thinning of a rock unit to its termination. A reservoir pinch-out needs an effective seal and other conditions to form a trap.

Also called: Pinchout

Plagioclase

The sodium–calcium feldspar series, widespread in igneous and metamorphic rocks.

Interpretation note In ideal plagioclase, replacing Na+ with Ca2+ is balanced by replacing Si4+ with Al3+: both paired contributions total +5. An30 denotes 30 mole percent anorthite component, not 30 percent calcium by mass.

Worked example: plagioclase charge balance

Also called: Plagioclase feldspar

Plunge

Downward angle of a geological line, such as a fold axis, measured from horizontal.

Interpretation note Record trend/plunge in degrees and state the order. A line contained in a plane cannot plunge more steeply than that plane dips. Unoriented cuttings do not establish geographic trend and plunge.

Worked example: trend, plunge and rake

Pluton

A body of igneous rock formed by crystallization of magma beneath the surface.

Point bar

Sediment deposited on the inside of a meandering channel bend as the channel migrates.

Poisson's ratio

Elastic ratio of transverse contraction to axial extension under uniaxial loading, expressed with the usual sign convention.

Polymorphism

The occurrence of the same chemical composition in different crystal structures, such as kyanite, andalusite and sillimanite.

Interpretation note A chemical formula alone does not distinguish polymorphs. Record current mineral identification separately from external shape and any proposed precursor; inherited form does not demonstrate present crystal structure.

Worked identification: composition versus crystal structure

Pore-water pressure

Pressure of water within connected pore spaces. Under hydrostatic conditions it increases with water-column height above the point; groundwater flow can produce a different pressure distribution.

Effective stress and compaction: worked example

Also called: Porewater pressure

Porosity PHI

Fraction of bulk rock volume occupied by pores; it describes storage, not by itself flow capacity.

Interpretation note Use bulk volume, including solids and pores, as the denominator. In a hypothetical 100 cm³ sample with 75 cm³ solids and 25 cm³ pores, porosity is 25%, whereas void ratio is 25/75 = 1/3. Neither ratio measures fluid mobility; state the pore-volume convention and sample condition.

Worked example: total versus effective porosity

Porphyritic texture

An igneous texture with larger crystals set in a distinctly finer groundmass, recording contrasting stages of crystallization.

Interpretation note Size contrast is an observation, not proof of a two-stage cooling history. Both changing cooling conditions and crystallization during a single cooling stage can yield porphyritic texture. The reference to contrasting crystallization stages must not be read as requiring two separate cooling episodes.

Worked comparison: grain size versus enclosure

Also called: Porphyritic

Porphyroblast

A metamorphic crystal that grew substantially larger than the surrounding grains.

Interpretation note A porphyroblast is large because it grew during metamorphism; a porphyroclast is large because it survived deformation and grain-size reduction of the surrounding matrix. Test growth relationships and internal fabrics in thin section.

Thin-section guide: porphyroclasts and recrystallization

Potassium feldspar

Potassium-rich feldspar minerals common in felsic rocks, often appearing pink, white or pale grey.

Also called: K-feldspar, K-spar

Preconsolidation stress

Effective-stress threshold associated with the loading history and yield of a sediment framework. In aquifer-system models, exceeding it commonly produces substantially greater, largely irreversible compaction than stress changes below it.

Effective stress and compaction: worked example

Also called: Preconsolidation pressure

Primary migration

Expulsion of newly generated hydrocarbons from source rock into surrounding pathways.

Also called: Expulsion

Primary porosity

Pore space inherited from deposition and retained during lithification.

Progradation

Outward or basinward advance of a depositional body as sediment accumulates.

Worked example: migrating facies and a missing interval

Protolith

The original rock from which a metamorphic rock developed.

Also called: Parent rock (metamorphic)

Pyroxene

A group of silicate minerals with single-chain structures, commonly present in mafic igneous rocks.

Interpretation note Two cleavage directions near a right angle can support a pyroxene identification in a suitable section; they do not identify an end-member composition.

Worked example: pyroxene composition

Quartz

Silicon-dioxide mineral that commonly forms the principal framework grains of sandstone.

Quartzite

A hard, quartz-rich metamorphic rock commonly formed by recrystallization of quartz sandstone.

Radiogenic isotope

An isotope formed by radioactive decay. In geochronology, the radiogenic daughter component must be distinguished from daughter atoms already present when the dated system began retaining them.

Interpretation note A lower retained daughter/parent ratio can reflect daughter loss rather than a younger formation event. The worked loss example holds parent constant; it is not a universal correction for disturbed samples.

Worked example: daughter loss and apparent age

Radiometric dating

Estimating geological ages from radioactive parent isotopes, their decay products and known decay behaviour.

Interpretation note An age refers to the event recorded by the selected mineral and isotope system, potentially crystallization, cooling or later disturbance. A detrital grain age does not directly date sediment deposition. In a closed single-step model with a fixed 10-million-year half-life, radiogenic daughter/parent ratios of 6.6, 7.0 and 7.4 give elapsed times of 29.260, 30.000 and 30.704 million years. Symmetric ratio bounds need not give symmetric age bounds; these stipulated limits are not a laboratory confidence interval. Pooling parent and daughter amounts from components of different ages can yield an apparent age that is neither their arithmetic mean nor evidence of a distinct geological event.

Worked example: pooled amounts versus mean ages

Also called: Isotopic dating

Recumbent fold

Fold whose axial plane is approximately horizontal.

Reef

Biologically influenced carbonate buildup that stands above surrounding sediment and may form a reservoir where porosity is preserved.

Reflection coefficient

Ratio of reflected to incident wave amplitude at an interface, dependent on the properties on either side.

Interpretation note For normal-incidence pressure amplitudes from layer 1 to layer 2, R = (Z2 − Z1)/(Z2 + Z1). A positive value indicates increasing impedance. R is an amplitude ratio, not a reflected-energy fraction; in the ideal lossless case that fraction is R². In the same lossless normal-incidence pressure convention, transmitted amplitude ratio is 1 + R; transmitted energy fraction also requires the incident-to-transmitted impedance ratio.

Worked example: transmitted amplitude and energy balance

Reflectivity series

A sequence of interface reflection coefficients located at their travel times in a layered seismic model. Convolving this sequence with a chosen wavelet produces a simplified synthetic trace; the coefficients alone are not the recorded waveform.

Interpretation note A coefficient at a deeper interface is not automatically the returned primary amplitude: transmission through overlying boundaries modifies the wave on both its downward and upward paths. State whether the synthetic includes these factors, multiples and propagation losses.

Worked example: two transmissions and one deeper reflection

Also called: Reflectivity function

Reflector

Interface across which physical properties change sufficiently to reflect seismic or other wave energy.

Regression

Basinward migration of a shoreline, commonly placing shallower-water deposits above deeper-water deposits.

Relative dating

Establishing whether geological events or materials are older or younger than others without assigning numerical ages.

Relative permeability

Ratio of a phase's effective permeability at a specified saturation to a reference absolute permeability.

Relief

Difference in elevation across an area of the land surface.

Complementary optical identification observations

Reservoir

Subsurface rock body capable of storing fluids and, under appropriate conditions, transmitting them.

Retrogradation

Landward retreat of successive depositional systems when creation of accommodation exceeds sediment supply.

Reverse fault

Fault along which the hanging wall moves upward relative to the footwall, commonly during compression.

Reworked fossil

A fossil eroded from an earlier deposit and incorporated into later sediment. Its original age can be older than the bed that now contains it.

Interpretation note Reworking is a depositional history; drilling cavings are sample contamination. Do not interpret a mixed fossil assemblage until these possibilities have been considered. Fossils introduced through burrows or other pathways are a separate possibility; mixed ages alone do not demonstrate erosion and redeposition.

Worked example: reworking versus drilling cavings

Also called: Redeposited fossil

Rhyolite

A silica-rich volcanic rock broadly corresponding in composition to granite, commonly fine-grained or glassy.

Right-hand rule RHR

Convention for recording a plane by choosing the strike bearing that puts its down-dip side to the observer’s right. In the Canadian convention used here, dip azimuth equals strike azimuth plus 90°, reduced modulo 360°.

Worked example: strike and apparent dip

Also called: Right dip rule

Rock cycle

Interconnected processes that transform rocks among igneous, sedimentary and metamorphic forms over geological time.

Rock flour

Very fine mineral and rock particles produced by glacial abrasion, often clouding meltwater.

Rockfall

Detachment and downward movement of rock from a steep face by falling, bouncing or rolling.

Root-mean-square velocity RMS velocity

Effective seismic velocity based on a time-weighted mean of squared interval velocities, used in layered-earth moveout analysis.

Rotational slide

A landslide moving on a curved failure surface, commonly causing blocks to rotate backward.

Also called: Slump

Rudstone

Coarse carbonate rock supported by its large grains, with more than 10% grains larger than 2 mm.

Sabkha

Arid coastal flat above normal high tide where evaporation commonly produces evaporite minerals.

Salt dome

Domal structure produced as buoyant salt rises and deforms surrounding sedimentary layers.

Sample cleaning method

Recorded treatment used to remove drilling fluid or contamination before describing a sample. Cleaning can affect observed fines, coatings and shows and belongs with the sample record.

Sample drying method

Recorded process used to dry a geological sample before examination or storage. Sample preparation is part of provenance when observations from different collections are compared.

Sample provenance

Record connecting a sample to its well, depth interval, collection and preparation history. Without provenance, even a detailed description can be assigned to the wrong geological interval.

Sand

Sediment particles from 1/16 to 2 mm on the Wentworth scale; the name specifies size, not mineral composition.

Sandstone SS

Sedimentary rock made chiefly of sand-sized mineral grains or rock fragments. Its cement and pore structure influence reservoir behaviour.

Schist

A metamorphic rock with conspicuous foliation defined by visible, aligned platy or elongate minerals.

Schistosity

Foliation defined by the preferred orientation of mineral grains large enough to distinguish individually.

Seal

Rock body that restricts hydrocarbon escape from a reservoir; sealing capacity depends on rock, fluid and pressure conditions.

Also called: Caprock

Secondary migration

Hydrocarbon movement after leaving the source rock, potentially toward a trap or accumulation.

Secondary porosity

Pore space created after deposition by processes such as dissolution or fracturing.

Seismic attribute

Measured or calculated property of seismic data, such as amplitude, frequency, phase or dip, used to aid interpretation.

Seismic datum

Reference surface to which seismic observations are corrected to reduce effects of elevation and near-surface conditions.

Interpretation note Depth and time must refer to consistent reference surfaces. State the seismic reference datum and the well depth origin before comparison; metres below datum, elevation and measured depth along a deviated borehole are not interchangeable.

Worked example: two layers, travel time and depth

Seismic stack

Combined seismic traces, aligned and added to strengthen coherent reflections and reduce random noise.

Seismic tomography

Estimation of subsurface velocity or other properties from many intersecting wave paths or reflection observations.

Also called: Tomography

Seismic trace

Recorded variation of a seismic signal with time for one recording channel.

Seismic tuning

Interference between reflections from closely spaced boundaries that changes the apparent amplitude and shape of a thin-bed response.

Interpretation note Compare a calculated summed wavelet response with interface times before equating trace extrema with bed boundaries. Sample spacing alone does not establish boundary separability.

Worked example: two interfaces and a summed seismic trace

Also called: Tuning effect

Sequence boundary

Surface separating depositional sequences, commonly an erosional unconformity and its correlative conformable surface.

Sequence stratigraphy

Interpretation of sedimentary successions using depositional packages and bounding surfaces linked to changes in accommodation and sediment supply.

Shale SH

Fine-grained sedimentary rock that commonly splits along closely spaced layers; clay minerals and silt are typical constituents.

Interpretation note Record fissility independently of colour and lamination. Plate-like cuttings alone do not identify a cleavage origin, clay species or geographic orientation.

Worked comparison: splitting versus layering

Shale density

Measured density of a selected shale sample or fraction. Preparation, contamination and measurement method affect the value; it is not interchangeable with an uncorrected downhole density curve.

Shear wave S-wave

Elastic body wave with particle motion perpendicular to propagation; it does not propagate through fluids as a shear disturbance.

Also called: S wave

Siderite

Iron-carbonate mineral occurring as grains or cement in some shales, sandstones and carbonate rocks.

Silica tetrahedron

The fundamental silicate structural unit: one silicon atom surrounded by four oxygen atoms.

Also called: Silicon–oxygen tetrahedron

Silicate mineral

A mineral whose crystal structure contains silicon–oxygen tetrahedra, joined in arrangements such as chains, sheets or frameworks.

Sill

A sheet-like igneous intrusion emplaced approximately parallel to the layering of its host rock.

Silt

Sediment particles from 1/256 to 1/16 mm on the Wentworth scale, between clay and sand.

Siltstone SILT

Sedimentary rock dominated by particles finer than sand but coarser than clay.

Sinkhole

Closed surface depression formed by dissolution or collapse into a subsurface void.

Slate

A fine-grained metamorphic rock with closely spaced slaty cleavage, commonly formed from mudrock.

Smectite

Group of clay minerals with expandable layers that can swell when exposed to water.

Soil creep

Very slow downslope displacement of soil or near-surface debris.

Solid solution

Compositional variation within a crystalline phase caused by substitution of compatible ions in its structure.

Feldspar intergrowths: composition, twins and texture

Solvent cut

Observation of material dissolving or diffusing into solvent during sample-show assessment; interpreted together with fluorescence and contamination checks.

Also called: Cut test

Sorting

Spread of grain sizes within sediment: a narrow spread is well sorted, while a broad mixture is poorly sorted.

Interpretation note Sorting describes the spread of sizes, not whether the average grain is fine or coarse. A visual estimate should not be reported as a calculated standard deviation; state the statistical method and weighting when measured. Number-weighted image counts and mass-weighted sieve fractions are not interchangeable; record the weighting and any excluded fines.

Worked example: grain counts versus mass fractions

Source rock

Organic-bearing rock capable of generating hydrocarbons under suitable thermal conditions.

Specific retention

Volume of water retained against gravity drainage divided by the bulk volume of the porous material. Under the corresponding drainage convention, porosity equals specific yield plus specific retention. Retention depends on drainage conditions and time.

Interpretation note Water remaining at a stated drainage endpoint is still part of the pore volume. In the guide example, 0.12 m³ retained in 1.00 m³ of material gives Sr = 0.12. Record test duration and conditions before comparing results.

Worked example: specific yield and retained water

Specific storage

Water volume released from saturated storage per unit bulk aquifer volume per unit decrease in hydraulic head, for specified mechanical conditions. Specific storage has inverse-length units and accounts for changes in pore volume and water density.

Interpretation note For fully saturated layers sharing the same head decline, sum Ss × layer thickness to obtain column storativity; the equivalent Ss is thickness-weighted. If declines differ by layer, calculate each storage-volume change separately. This accounting does not determine the transient heads or a sustainable pumping rate.

Worked example: thickness-weighted confined storage

Specific yield

Volume of water a saturated material releases by gravity drainage divided by its bulk volume. It is dimensionless, not a pumping rate. Drainage is not instantaneous; measurement duration and water-table position affect the result.

Interpretation note Specific yield represents gravity drainage; it should not be substituted for confined elastic storativity. Drainage timescale and delayed yield affect interpretation. Normalize the collected drainage volume by the whole sample volume, not just its initial pore-water volume. In the guide example, 0.18 m³ collected from 1.00 m³ of material gives Sy = 0.18, although 60% of the initial water drains. A finite-duration drainage test gives a time-specific result: in a fixed 1 m³ sample initially holding 0.30 m³ water, cumulative drainage of 0.10 m³ at 1 hour and 0.18 m³ at 24 hours gives measured yield ratios 0.10 and 0.18. The second result does not establish ultimate yield; record duration and drainage boundary conditions.

Worked example: specific yield and retained water

Spill point

Lowest structural point at which hydrocarbons can remain contained in a trap before additional hydrocarbons escape.

Spring

Natural discharge of groundwater at the land surface.

Steinkern

A hardened infilling of a shell's interior that preserves its internal shape after the shell is removed.

Interpretation note The surviving infill represents the shell interior, not its outer ornament. Describe which surface is exposed before using shape for identification.

Worked example: describe a mould, cast or steinkern

Also called: Internal cast

Stock

An exposed intrusive body smaller in area than a batholith, conventionally less than 100 square kilometres.

Stoneley wave

Low-frequency interface wave traveling along a fluid-filled borehole wall, useful in evaluating permeability and fractures.

Storativity

Water volume released from storage per unit aquifer plan area per unit decline in hydraulic head. It is dimensionless. In a uniform confined aquifer S = Ss b; unconfined storage also includes gravity drainage.

Interpretation note For spatially variable head decline, sum S × area × local decline across representative zones. With constant S, use the area-weighted mean decline, not the maximum observed decline; pumped volume can also include boundary inflow.

Worked example: spatial drawdown and storage release

Also called: Storage coefficient

Stratigraphic correlation

Establishing correspondence between separated rock intervals by stated lithological, fossil, age or other criteria.

Stratigraphic horizon

Recognizable position or interface within a rock succession, defined by a stated stratigraphic criterion.

Also called: Horizon

Stratigraphic trap

Hydrocarbon trap formed by changes in rock character or depositional geometry, such as a reservoir pinch-out beneath a seal.

Stratigraphy

Study of rock successions, their properties, spatial relationships and sequence through geological time.

Streak

The colour of a mineral in powdered form, commonly examined using unglazed porcelain.

Interpretation note Distinguish mineral powder from a groove or debris produced by scratching the porcelain itself. A failed plate test leaves streak unassessed, not necessarily white.

Worked example: streak powder or scratched plate?

Also called: Mineral streak

Streak plate

An unglazed porcelain surface used to examine mineral powder colour during identification.

Interpretation note A typical porcelain plate is about Mohs 6.5, so harder minerals may scratch the plate instead of yielding a useful powder streak. Treat the plate as a test material with limitations, not a universal colour detector.

Worked example: streak powder or scratched plate?

Strike

Horizontal direction of a line on a geological plane, normally stated relative to a north reference.

Interpretation note Strike alone has two opposite bearings. Record the dip side or state the orientation convention; a horizontal plane has no unique strike.

Worked example: strike and apparent dip

Strike-slip fault

Fault with displacement predominantly parallel to its strike, so opposing blocks move sideways past one another.

Structural closure

Vertical relief between a trap's crest and its lowest closed structural contour.

Also called: Closure

Structural dome

Rounded or elliptical anticline with beds dipping away from its crest in multiple directions.

Also called: Dome

Structural trap

Hydrocarbon trap created chiefly by deformation, such as folding or faulting, of reservoir and sealing rocks.