Geology glossary

Clear, one-line definitions of the Geology terms used across the OgbonLab textbooks. Each entry links to the interactive sections where the idea is taught.

anticline

A convex-upward fold with the oldest rocks in the core; one of the most common structural traps for hydrocarbons.

carbonate

A sedimentary rock composed mainly of calcite or dolomite, formed in marine settings; reservoirs are governed by depositional facies and diagenesis.

See: Fractured Carbonate, Capstone 2: Carbonate reservoir with facies modeling

carbonate platform

A shallow-marine carbonate depositional environment building thick stacks of reefs and bank carbonates; major exploration target worldwide.

channel

A linear, often sinuous, depositional feature filled with sand or gravel; on seismic appears as cut-and-fill geometries and amplitude anomalies.

See: Channel systems: meandering, braided, distributary, submarine, Capstone 3: Fluvial channel reservoir (multipoint statistics)

charge

In petroleum systems, whether a trap has received hydrocarbons migrating from a mature source rock along an effective pathway.

See: Dynamite: charge, depth, coupling

clastic

A sedimentary rock made of fragments of pre-existing rocks (e.g., sandstone, shale); the most common reservoir and seal lithologies.

See: Capstone 1: Clastic shelf reservoir characterization

compaction

The reduction of rock volume and porosity under overburden stress; alters velocity, density, and reflectivity with depth.

See: The Compaction Correction, Chalk field 4D compaction: Valhall over 40 years

compressional

A tectonic regime of crustal shortening, dominated by reverse and thrust faults plus folding; typical of fold-and-thrust belts and orogens.

deltaic

Pertaining to delta systems where rivers meet a standing body of water, depositing distributary channel and mouth-bar sands.

See: Depositional systems: fluvial, deltaic, shoreface, carbonate, deep-water

detachment folds

Folds formed in a competent layer above a weak detachment horizon, where shortening is taken up by buckling rather than fault offset.

distributary

One of the branching channels that splits off a main river within a delta, depositing channel-fill sands as a reservoir facies.

See: Channel systems: meandering, braided, distributary, submarine

downlap

A reflection-termination pattern where dipping younger strata terminate down-dip against a flatter underlying surface; diagnostic of progradation.

See: Reflection terminations: onlap, downlap, toplap, truncation

drape folds

Gentle folds formed when younger sediments compact and drape over the relief of an underlying fault block or basement high.

extensional

A tectonic regime of crustal stretching, dominated by normal faulting and basin subsidence; typical of rifts and passive margins.

fault block

A region of crust bounded by faults that moves coherently relative to neighboring blocks; can form structural traps when tilted and sealed.

fault-bend folds

Folds produced where a thrust sheet rides over a non-planar fault, bending and folding the hanging-wall to match the footwall ramp geometry.

fault-propagation folds

Folds that develop ahead of the tip of an upward-propagating fault, accommodating slip while the fault is still blind.

fluvial

Pertaining to river systems; fluvial reservoirs are typically channel sandstones encased in overbank shales.

See: Capstone 3: Fluvial channel reservoir (multipoint statistics), Depositional systems: fluvial, deltaic, shoreface, carbonate, deep-water

gas chimney

A vertical zone of disturbed, low-amplitude, dimmed seismic reflections caused by gas leaking upward from a charged reservoir.

limestone

A carbonate sedimentary rock composed mainly of calcite (CaCO₃); can host reservoirs through primary porosity, fractures, or dissolution.

onlap

A reflection-termination pattern where initially horizontal strata terminate against an inclined surface; diagnostic of transgression.

porosity

The fraction of rock volume occupied by pore space; controls fluid storage capacity and, with density, affects acoustic impedance.

See: Porosity, Secondary Porosity

reservoir

A subsurface rock body with sufficient porosity and permeability to store and transmit hydrocarbons or other fluids.

See: Reservoir to Seismic, Unconventional Reservoirs

salt

A ductile, low-density evaporite body (mainly halite) that flows under burial stress, forming diapirs, canopies, and welds that deform surrounding sediments.

salt dome

A diapir of buoyant rock salt that has pierced overlying sediments, forming traps in its flanks, crest, and turtle-back structures.

salt tectonics

The deformation of sedimentary basins driven by the flow of buoyant, ductile rock salt, producing diapirs, withdrawal basins, and complex traps.

See: Salt tectonics: diapirs, walls, canopies, and welds

seal

A low-permeability rock unit, typically shale or evaporite, that prevents hydrocarbons from escaping the underlying reservoir.

sequence stratigraphy

The study of cyclic sedimentary packages bounded by unconformities, using stratal terminations and stacking patterns to infer base-level change.

See: Sequence stratigraphy: sequences, surfaces, and systems tracts

shale

A fine-grained clastic sedimentary rock with low permeability; commonly forms seals and source rocks, and shows strong VTI anisotropy.

source rock

An organic-rich sedimentary rock (often shale) that, when buried and heated within the oil window, generates hydrocarbons.

See: Source Rocks and Organic Carbon

stratigraphic trap

A trap formed by lateral facies changes or pinch-outs of the reservoir against impermeable rocks, without significant structural relief.

See: Subtle Stratigraphic Trap

strike-slip regime

A tectonic stress regime where blocks slide horizontally past each other along near-vertical faults, producing flower structures and pull-apart basins.

structural trap

A trap formed by tectonic deformation, such as an anticline or fault block, where seal geometry alone holds the hydrocarbons.

syncline

A concave-upward fold with the youngest rocks in the core; the structural opposite of an anticline.

toplap

A reflection-termination pattern where dipping reflections terminate upward against an overlying flat surface, indicating bypass or non-deposition.

See: Reflection terminations: onlap, downlap, toplap, truncation

trap

A geometric configuration of reservoir and seal that holds hydrocarbons in place, classified as structural, stratigraphic, or combination.

truncation

A reflection-termination pattern where strata are cut off by an overlying erosional surface; diagnostic of an angular unconformity.

See: Reflection terminations: onlap, downlap, toplap, truncation

turbidite

A sediment deposited by a turbidity current, typically a graded sand-to-mud bed; deep-water turbidite fans are major reservoirs.

See: Deepwater turbidite QI: Girassol-style fluid prediction, Deep-water turbidite fans: anatomy and reservoir architecture