insert, Oil & Gas Glossary

In wireline well logging, an insert is the printed block placed on a log that states the scales of the curves displayed and the depth scale used over the interval, so that anyone reading the log can correctly interpret the wiggle traces against quantitative units. A logging print is, at its core, a strip chart: one or more measured properties, such as natural gamma radiation, resistivity, bulk density, neutron porosity, or sonic travel time, are recorded as continuous curves running down the page against depth. Those curves are meaningless without knowing what value each horizontal position represents and what depth each vertical position represents, and that is exactly the information the insert supplies. A typical insert names each curve, its measurement mnemonic, the units, the left-hand and right-hand scale limits for the track it occupies, and the line style or colour used to draw it, alongside the depth scale, commonly 1:240 or 1:600 in metric WCSB practice, that fixes how many metres of hole correspond to a given length of paper. Because a single logging run can survey a long interval and because scales are sometimes changed partway through a job to better resolve a zone of interest, an insert is conventionally printed at the beginning and at the end of each interval surveyed, and often wherever the scaling changes, so the reader is never left guessing which calibration applies to the curves in front of them. The insert is closely related to, but distinct from, the main log heading: the heading at the top of the log carries the well identification, location, run conditions, fluid properties, and tool string details, while the insert is the recurring scale legend that travels with the curves themselves. In the Western Canadian Sedimentary Basin, where logs from Cardium, Viking, Montney, and Mannville wells are submitted to the AER and archived for decades, a correct and complete insert is essential for the petrophysical evaluation that turns raw curves into porosity, water saturation, and net pay. The concept connects directly to the broader practice of well logging, the use of gamma ray and resistivity curves, and the standardized log heading that accompanies every submitted survey.

Key Takeaways

The scale legend for the curves: An insert is the printed block on a log that states each curve's scale limits, units, and mnemonic together with the depth scale in use. Without it the wiggle traces cannot be read quantitatively, because the reader would not know what value the left and right edges of each track represent or how many metres of hole map to a length of paper. It is the key that makes the curves interpretable.
Printed at interval start and end: By convention an insert appears at the beginning and the end of each surveyed interval, and wherever the scaling is changed mid-run. This repetition guarantees that a reader opening the log at any point can find the applicable calibration nearby, which matters when a long WCSB logging run spans hundreds of metres and the analyst is jumping to a specific zone of interest.
Distinct from the log heading: The heading at the top of a log carries well identification, surface and bottomhole location, run number, mud properties, and tool string, while the insert is the recurring scale legend bound to the curves. Both are required for a complete, archivable log, but they answer different questions: the heading says where and how the log was run, the insert says how to read the traces.
Depth scale fixes vertical interpretation: The insert states the depth scale, commonly 1:240 for detailed correlation or 1:600 for overview in metric WCSB logs. That ratio determines vertical resolution and how thin a bed can be picked. Mismatched or unstated depth scales between runs cause correlation errors when an analyst ties a new well to offset logs across a Cardium or Viking pool.
Foundation for petrophysical analysis: Turning curves into porosity, water saturation, and net pay requires reading exact values off each trace, which is impossible without the insert's scale limits and units. AER-submitted logs that lack clear inserts are difficult to re-evaluate years later, so the insert is both an interpretation aid and a long-term archival necessity for reserve and royalty work.

Curve Scales, Mnemonics, and Track Layout

A logging print is divided into vertical tracks, with a depth column down the middle or left and curve tracks beside it. The insert assigns each curve to its track and states the scale across that track, for example a gamma ray reading from 0 to 150 API units across track one, or a deep resistivity plotted logarithmically from 0.2 to 2,000 ohm-metres across the resistivity track. It also lists the curve mnemonic, such as GR for gamma ray, RT or ILD for deep resistivity, RHOB for bulk density, and NPHI for neutron porosity, so a reader can match each line to its measurement. When two curves share a track, the insert distinguishes them by line style or colour. This compact legend is what lets an analyst quote, for instance, a 12 percent neutron porosity reading directly off the paper.

Why Scales Change Within a Run

Scales are not always constant down a hole. A logging engineer may compress the resistivity scale through a thick, uniform shale to keep the curve on-track, then expand it through a thin, resistive Cardium sand to resolve detail, or switch the depth scale from 1:600 overview to 1:240 detail across a pay zone. Each change is announced by a fresh insert so the reader knows the calibration shifted. This is also why an insert is placed at the end of an interval: it confirms the scaling that applied through the section just logged before the next, possibly differently scaled, section begins. Failing to read the governing insert is a classic source of misread values, where an analyst applies the wrong scale limits to a curve and computes an erroneous saturation.

Fast Facts

The convention of repeating a scale legend at the start and end of every interval predates digital logs entirely; it comes from the era of optical film and photographic log printing, when a curve drawn on a moving strip could not carry embedded metadata the way a modern digital LAS file does. Even today, after the industry standardized the digital LAS format in the late 1980s so that scales live in the file header, paper and PDF prints still carry the printed insert, because a human reading a hardcopy needs the legend in front of the curves, not buried in a data file.

The insert is part of the apparatus of well logging, the wireline measurement of formation properties down a borehole. The curves it scales include the gamma ray, used to distinguish shale from clean reservoir, and the resistivity curves, used to identify hydrocarbon-bearing zones through high resistance. The insert works alongside the log heading, which carries the well and run identification, so that together the two printed elements make a log both identifiable and quantitatively readable for the petrophysical analysis that follows.

Real-World WCSB Scenario: Re-Evaluating a Viking Log Near Provost

A geologist re-examines a 1980s vintage paper log from a Viking well near Provost, Alberta, scanned and submitted to the AER, to assess a bypassed-pay opportunity. The inserts at the top and bottom of the logged interval confirm the resistivity track ran 0.2 to 2,000 ohm-metres logarithmic and the depth scale was 1:240, letting the geologist read a 9 ohm-metre resistive streak that a coarser overview would have hidden.

Reading the exact values off the scaled curves, the analyst computes about 1.8 m of net pay at 18 percent porosity that the original operator overlooked. A re-completion of the zone costs roughly 240,000 CAD and adds incremental Viking oil, a result made possible only because the log's inserts preserved the precise curve and depth calibration across four decades.