Glossary of Terms

Key terms and definitions used in well log data and petrophysics.

A

API Number
A unique numeric identifier assigned to each well drilled in the United States, used for regulatory tracking and data management.
ASCII
A plain-text data format used to store well log data in human-readable columns of numbers. Simple but lacks the metadata structure of LAS or DLIS formats.

B

Basin
A large-scale geological depression in the earth's surface where sediments have accumulated over geological time. Sedimentary basins are the primary targets for petroleum exploration.

C

Caliper
A wireline log that measures the diameter of the borehole. Used to identify washouts, breakouts, and assess cement volume requirements.
Cased Hole
A wellbore that has been lined with steel casing and cemented. Cased hole logging is performed through the casing and provides more limited formation information than open hole logging.
Completion Date
The date on which well drilling and initial testing operations were finished.
Curve
A single continuous measurement channel recorded by a logging tool, plotted as a line on a log display. A single logging run may record multiple curves simultaneously.

D

Datum
A reference point or surface from which depth measurements are made, such as Kelly Bushing (KB), Rotary Table (RT), or Mean Sea Level (MSL).
Density Log
A wireline log that measures the bulk density of the formation by emitting gamma rays and detecting their attenuation. Used for porosity calculation and lithology identification.
Depth Matching
The process of aligning log curves from different logging runs to a common depth reference to correct for cable stretch, tool sticking, and other depth errors.
Digitisation
The process of converting print or image wireline logs into digital format (typically LAS) for use in modern petrophysical software. See our digitisation services.
DLIS
Digital Log Interchange Standard -- a binary file format for storing well log data, defined by the API RP66 standard. Supports multiple frames and complex data structures.
Driller
The company responsible for the physical drilling of the well, as distinct from the operator who holds the permit.

F

Formation Evaluation
The process of analysing well log data to determine rock and fluid properties of subsurface formations, including porosity, permeability, and fluid saturation.

G

Gamma Ray (GR)
A wireline log that measures the natural radioactivity of formations. Used primarily to distinguish shale from non-shale lithologies and for correlation between wells.

K

Kelly Bushing (KB)
A device on the drill floor through which the kelly passes. Often used as the depth reference datum for wireline logs, with depths measured from KB elevation.

L

LAS
Log ASCII Standard -- a widely used text-based file format for storing well log data. Versions include LAS 2.0 and LAS 3.0. The most common format for exchanging digital log curves.
Log Header
The information section at the top of a well log containing well name, location, date, depth range, tool details, and other metadata.

M

Mean Sea Level (MSL)
A standard geodetic datum representing the average height of the ocean surface, used as a reference for elevation measurements and subsea depth calculations.
Measured Depth (MD)
The length of the wellbore measured along its actual path from the surface datum to a given point, as opposed to true vertical depth. In deviated wells, MD is always greater than TVD.

N

Neutron Log
A wireline log that measures hydrogen concentration in formations, primarily used for porosity determination. Often displayed alongside the density log for lithology identification.

O

Open Hole
A section of wellbore that has not been cased. Open hole logging provides the most accurate formation measurements as sensors are in direct contact with the formation.
Operator
The company that holds the exploration or production permit and is responsible for well operations.

P

Permit
A government-issued licence granting the right to explore for or produce petroleum in a defined area.
Petrophysics
The study of physical and chemical rock properties and their interactions with fluids, using well log and core data to characterise subsurface formations.

R

Resistivity
A wireline log that measures the electrical resistance of formations. Used to identify fluid type (oil, gas, or water) and calculate water saturation.
Rotary Table (RT)
The rotating mechanism on the drill floor used to turn the drill string. Often used as an alternative depth reference datum to Kelly Bushing.

S

Sonic Log
A wireline log that measures the speed of sound through formations. Used for porosity calculation, seismic calibration, and mechanical properties estimation.
Spontaneous Potential (SP)
A wireline log that measures the natural electrical potential between the borehole fluid and formation waters. Used to identify permeable zones and estimate formation water resistivity.
Splice
The process of combining overlapping log sections from different logging runs into a single continuous curve, typically after depth matching.
Spud Date
The date on which drilling of a well commenced.

T

Total Depth (TD)
The maximum measured depth reached by a well during drilling operations.
True Vertical Depth (TVD)
The vertical distance from the surface datum to a point in the wellbore, accounting for the well's deviation from vertical. Used for pressure and fluid contact calculations.

W

Well Data Summary (WDS)
A standardised document summarising the key metadata and available data for a well, including location, depth, and log curve inventory.
Well Status
The current operational state of a well, such as producing, suspended, plugged and abandoned, or injecting.
Wireline Log
A continuous record of formation properties measured by instruments lowered into a wellbore on an electrical cable. The primary source of subsurface data for formation evaluation.

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