Ask a logging contractor for the logs from a well and you will almost certainly get a LAS file. LAS, the Log ASCII Standard, is a plain-text format published by the Canadian Well Logging Society. Because it is plain text, you can open it in any text editor, and because it is simple, almost every geology and petrophysics package can read it. Most of the time.
This article explains what is inside a LAS file, the differences between the versions, and the problems that make an import fail or, worse, succeed with the wrong numbers.
The Sections of a LAS File
A LAS file is divided into sections. Each starts with a line beginning with a tilde (~) followed by a letter. Lines starting with # are comments.
~V: Version
The first section. It states the LAS version (VERS, for example 2.0) and whether the data is wrapped (WRAP, YES or NO). Software reads this first to decide how to read the rest.
~W: Well information
Facts about the well and the data range:
- STRT, STOP, STEP: the first depth, the last depth, and the depth increment. A STEP of 0 means the depths are not evenly spaced.
- NULL: the value used where there is no data, most often -999.25.
- WELL, COMP, FLD, LOC, UWI or API: the well name, the company, the field, the location and the unique well identifier.
~C: Curve information
One line per curve, in the same order as the columns in the data. Each line gives the mnemonic, the unit and a description, for example GR .GAPI : Gamma Ray. The first curve is the index, usually depth (DEPT) in metres or feet.
~P: Parameters
Values that apply to the whole log, such as the mud weight, the bit size or the bottom hole temperature. Optional, but useful when a petrophysicist needs to correct the logs later.
~O: Other
Free text: logging remarks, depth shifts applied, tool problems. Worth reading before you trust the data.
~A: ASCII log data
The data itself, one row per depth, one column per curve, in the order the ~C section lists them. It must be the last section.
Wrapped and Unwrapped Data
In an unwrapped file (WRAP. NO), each depth is one line. In a wrapped file (WRAP. YES), a depth's values can run over several lines, to keep lines under 80 characters. Wrapping dates from the days of fixed-width terminals. Some software still cannot read wrapped files; if an import fails on a file with many curves, check the WRAP line first.
LAS 1.2, 2.0 and 3.0
| Version | Main features | How common |
|---|---|---|
| LAS 1.2 | The original. Same sections, looser rules for the line format | Older data, still common in archives |
| LAS 2.0 | Stricter line format, the standard most software expects | By far the most common today |
| LAS 3.0 | Several data sets in one file, comma-delimited data, sections such as ~Log_Definition | Used, but much less widely supported |
If a LAS 3.0 file will not load, ask the contractor for a 2.0 export. Most can provide one.
Problems That Break Imports
- Wrong or missing units. A gamma ray curve labelled with no unit, or depths in feet with a header that says metres, imports cleanly and gives wrong answers. Always check the depth unit against the STRT and STOP values.
- Null values that are not the stated NULL. Some files declare -999.25 but use -999 or 0 in places. Those values then plot as real data.
- Uneven depth steps. A STEP that does not match the data, or a STEP of 0 with software that expects even spacing, misplaces every sample.
- Mnemonics that differ between contractors. GR, GRC, GAM, GR_EDTC and many others can all be gamma ray. Map them to your standard names once per project.
- Duplicate or reversed depths. Merged runs sometimes overlap or run upwards. Sort and de-duplicate before loading.
- Missing well identifiers. Without a UWI or API number, the file has to be matched to its well by name, which is where mistakes happen.
LAS and WITSML
LAS is a file format: a snapshot of the logs at the moment it was exported. For real-time work, the same logs are usually delivered through a WITSML server, which your software queries as the well is drilled. Many teams use both: WITSML while drilling, and a final LAS from the contractor after the run, with the memory data and the final depth shifts. Our article on WITSML versioning and integration covers the real-time side.
Look Before You Load
Before importing a LAS file into a project, open it and check five things: the version and WRAP line, the depth unit, the NULL value, the curve list, and the ~O remarks. It takes a minute and saves hours of chasing a correlation that will not line up.
Our free LAS viewer does this in the browser: it reads LAS 1.2 and 2.0 files, including wrapped ones, shows the header and plots every curve. The file is read on your computer and never uploaded.
LAS in GeoMaster
GeoMaster®, registered in the United Kingdom (UK00004424070), imports LAS 2.0 files into a project, alongside surveys, and pulls logs from a WITSML 1.4.1.1 server on demand. GeoEngine, the AI assistant in each project, can prepare a file import for you to confirm. Once loaded, the gamma ray is ready to correlate against a type well in GeoMatch.
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Frequently asked questions
Written by the GeoMaster team at Greenstone Energy Ltd
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