GUIDE

LAS file format explained

Log ASCII Standard (CWLS) · structure of a LAS file

In short

LAS (Log ASCII Standard) is a plain-text format for well-log curve data.
A LAS 2.0 file has header sections followed by an ~ASCII data table.
Missing values are written as a null number, commonly −999.25.

Structure of a LAS 2.0 file

~Version: the LAS version (VERS) and whether the data are wrapped (WRAP).
~Well: start, stop and step depth (STRT, STOP, STEP), the null value (NULL), and well details such as WELL, COMP, FLD, API and UWI.
~Curve: one line per curve with its mnemonic, unit and description. The first curve is normally depth.
~Parameter: constants such as elevations or bit size.
~Other: free-text notes.
~ASCII: the data table, one row per depth sample, with one column per curve.

Example

~Version
VERS. 2.0 : LAS version
WRAP. NO  : One line per depth
~Well
STRT.FT 1000 : First depth
STOP.FT 1003 : Last depth
STEP.FT 1    : Depth increment
NULL.  -999.25 : Missing value
WELL.  EXAMPLE 1 : Well
~Curve
DEPT.FT : Depth
GR.API  : Gamma ray
~ASCII
1000 52.1
1001 -999.25
1002 55.4

Null values and wrapped data

Where a curve has no measurement, the file holds the null value from ~Well (commonly −999.25). Some older files are wrapped, with one depth sample spread over several lines; this is indicated by WRAP. YES.

LAS 2.0 and LAS 3.0

LAS 2.0 stores one log table. LAS 3.0 can hold several named data sets in one file, for example a log table and formation tops. Log Digitize reads and writes both; see LAS 3.0 and LAS 2.0.

Opening and creating LAS files

You can open and plot LAS files online, or create a LAS file from a scanned log.