This article was
prepared especially for "Crain's Petrophysical Handbook" by
E. R. (Ross) Crain, P.Eng. in 2005, based on CWLS LAS File
Documentation , 1988.
This webpage version is the copyrighted intellectual
property of the author.
Do not copy or distribute in any form without explicit
Log data has been recorded on magnetic tape and
on discs for many years. Originally, the tape
formats were unique binary formats (LIS, DLIS,
BIT, TAP, etc) that could only be read by the
service companies that created them. Some
petrophysical software suppliers provided tape
reading capabilities, but the many format
variations defeated many attempts to load files.
To overcome this problem, the Canadian
Well Logging Society (CWLS) struck a committee in 1985 and 1986,
chaired by Case Struyk, to develop an ASCII formatted file that
could be viewed easily in a word processor and read by commercially
available analysis software. The result became known as the
Log ASCII Standard (LAS) file and is now a de facto worldwide
standard. The CWLS receives no royalty and little credit for this
Over time, the LAS standard has been
updated to reflect changes in computer technology. The first
published version was labeled 1.20, and Version 2.00 is the most
commonly used version, although version 3.00 has been published
LAS FILE HANDLING SOFTWARE
LAS file editing and repair software is available from
(free and commercial versions). Follow the link to view a detailed
An excellent low-cost LAS File View and Depth Plot program is
Energy Software. Follow the
link to view a detailed features list.
The latest Schlumberger Freeware data viewing software can be
Older but quite servicable versions are on the
Downloads page of this Handbook.
LAS FILE FORMATS
Each LAS file
consists of sections. Each section is labeled by the tilde (~)
character followed by the standard name of that section. The
sections for Version 1.20 are:
~Version and wrap mode information
~Parameters or constants
~Other information such as comments
~ASCII log data
~V and ~A must be the first and last
sections respectively. Other sections can be in any order. The list
above is the most readable for humans.
The "VERSION" section containing the
version number of the LAS format and identifies whether the data is
in wrap mode or not. Wrap mode is a method for handling lines of
data that exceed 80 characters (including delimiting spaces). The
"WELL INFORMATION" section contains information on the well name,
location, and the start and stop depths of the data in this file.
The "CURVE INFORMATION" section contains the curve mnemonics, units
used and definitions of mnemonics, in the order that they appear in
the data section. The "PARAMETER" section contains information on
parameters and constants and is optional. The "OTHER" section is
also optional and contains any other information or comments. The
last section is always the "ASCII LOG DATA" section. Depth values
should appear in the first column and each column of data must be
separated by at least one space. This data is usually formatted into
columns with 3 to 8 significant digits.
Tbe "#" character is used to indicate
comment lines that are not read by the computer but are helpful for
human readers. The first space after the period character is
also a delimiter. All other spaces are ignored.
LAS FILE EXAMPLE -- VERSION 1.200
VERS. 1.20: CWLS log RSCII Standard -VERSION
WRAP. NO: One line per depth step
~Well Information Block
#MNEM. UNIT Data Type
COMP. COMPANY: ABC OIL CO
WELL: ABC 01-02
PROV. PROUINCE: ALTA
SRUC. SERUICE COMPANY: SLB
DRTE. LOG DATE: 04 DEC 86
UWI. UNIQUE WELL ID:
~Curue Information Block
CODE#------------------- --- Curue Description
: 1 DEPTH
7 350 02 00: 2 BULK DEHSITY N
7 890 00 00: 3 NEUTRON POROSITY -SANDSTONE
SFLU .OHMM 7
220 01 00: 4 Rxo RESISTIUITY
SFLA.OHMM 7 222 01
00: 5 SHALLOW RESISTIUITY
~A Depth RHOB
635.0000 2256.0000 0.4033
634.8750 2256.0000 0.4033
Here's a sample id the ASCII Data
section of a Wrapped file:
~A Log data section
-999.2500 2692.7075 0.3140 19.4086 19.4086 13.1709 12.2681
-1.5010 96.5306 204.7177 30.5822 -999.2500 -999.2500 3.2515
-999.2500 4.7177 3025.0264 3025.0264 -1.5010 93.1378 0.1641
0.0101 0.1641 0.3140 0.1641 11.1397 0.3304 0.9529
0.0000 0.1564 0.0000 11.1397 0.0000 0.0000 0.0000
-999.2500 2712.6460 0.2886 23.3987 23.3987 13.6129 12.4744
-1.4720 90.2803 203.1093 18.7566 -999.2500 -999.2500 3.7058
-999.2500 3.1093 3004.6050 3004.6050 -1.4720 86.9078 0.1456
-0.0015 0.1456 0.2886 0.1456 14.1428 0.2646 1.0000
0.0000 0.1456 0.0000 14.1428 0.0000 0.0000 0.0000
LAS FILE EXAMPLE -- VERSION 2.0
LAS Version 2.0 was introduced in 1992, and was widely adopted all
over the World. In Version 2.0, the punctuation delimiters are in a
different arrangement. Data entries in the ~Well Info section now
are in front of the colon, instead of after the colon, giving all
sections the same format, as shown below.
VERS. 2.0: CWLS LOG ASCII STANDARD - VERSION
WRAP. NO: ONE LINE PER DEPTH STEP
~WELL INFORMATION BLOCK
#MNEM.UNIT DATA DESCRIPTION OF MNEMONIC
#--------- ------------- ------------------------
STRT.M 1670.0000: START DEPTH
STOP.M 1660.0000: STOP DEPTH
STEP.M -0.1250: STEP
NULL. -999.2500: NULL
COMP. ANY OIL COMPANY LTD. : COMPANY
WELL. ANY ET AL OIL WELL #12 : WELL
FLD . WILDCAT : FIELD
LOC . XX DEGREES N,YYY DEGREES W :
PROV. N.W.T. : PROVINCE
SRVC. ANY LOGGING COMPANY LTD. :
DATE. 01-FEB-1988 : LOG DATE
UWI . 400XXXXXXXXXXXXX : UNIQUE
#MNEM.UNIT API CODE CURVE DESCRIPTION
#--------- ------------- ------------------------------
: 1 DEPTH
: 2 SONIC TRANSIT TIME
: 3 BULK DENSITY
: 4 NEUTRON POROSITY
: 5 RXO RESISTIVITY
: 6 SHALLOW RESISTIVITY
Parameters and Other data sections are
set up in the same format as the Well data section. Abbreviations
are not standard in these sections.
LAS FILE FEATURES -- VERSION 3.0
LAS Version 3.0 was proposed in the year
2000. Here is a short list of the new features in Ver 3.0:
1. New Data types. (Core, Drilling etc).
2. "Structure" rules separated from "Content" rules.
3. New delimiters and structures.
4. Comma Tab and Space delimited data.
5. Handles 1D, 2D and 3D arrays
6. Supports Multiple Runs.
7. Parameter Zoning.
8. Floating point, string, integer, Date and Time formats
9. Addition of User defined data is easy
10. WRAP YES has been dropped.
11. ~Other section has been dropped.
This new format is considerably more
complicated than Version 2.0. As a result it has not been widely
adopted and the vast majority of work is still done with Version 2.0