LAS FILES
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 work.

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 also.

LAS FILE HANDLING SOFTWARE
LAS file editing and repair software is available from KC Petrophysics (free and commercial versions). Follow the link to view a detailed features list.

An excellent low-cost LAS File View and Depth Plot program is available at Everett Energy Software. Follow the link to view a detailed features list.

The latest Schlumberger Freeware data viewing software can be downloaded HERE.  Older but quite serviceable 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
    ~Well identification
     ~Curve 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

-Version Information
VERS.     1.20: CWLS log RSCII Standard -VERSION 1.2.
WRAP.      NO: One line per depth step
#

~Well Information Block
#MNEM.  UNIT    Data Type

STRT.M               635:
STOP.M              400:
STEP.M         -0.1250:
NULL.            -999.25:
COMP.     COMPANY: ABC OIL CO
WELL.            WELL: ABC 01-02
FLD.              FIELD:  NA
LOC       LOCATION:  01-02-03-04W5
PROV.    PROUINCE: ALTA
SRUC.    SERUICE COMPANY:  SLB
DRTE.     LOG DATE: 04 DEC 86
UWI.       UNIQUE WELL ID:  10001020304W500
#

~Curue Information Block
#MNEM.UHIT        API CODE#------------------- --- Curue Description
DEPT.M                                  :  1 DEPTH
RHOB.K/M3           7 350 02 00:  2 BULK DEHSITY N

HPHI.V/V               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       NPHI       SFLU      SFLA
635.0000   2256.0000   0.4033   22.0781   22.0781
634.8750   2256.0000   0.4033   22.0781   22.0781

Here's a sample of the ASCII Data section of a Wrapped file:
~A Log data section
910.000000
-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
909.875000
-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
909.750000

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.

~VERSION INFORMATION
VERS.     2.0: CWLS LOG ASCII STANDARD - VERSION 2.0
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 VALUE
NULL.        -999.2500: NULL VALUE
COMP.    ANY OIL COMPANY LTD. : COMPANY
WELL.    ANY ET AL OIL WELL #12 : WELL
FLD .      WILDCAT : FIELD
LOC .      XX DEGREES N,YYY DEGREES W : LOCATION
PROV.     N.W.T. : PROVINCE
SRVC.      ANY LOGGING COMPANY LTD. : SERVICE COMPANY
DATE.      01-FEB-1988 : LOG DATE
UWI .       400XXXXXXXXXXXXX : UNIQUE WELL ID
#

~CURVE INFORMATION
#MNEM.UNIT API CODE CURVE DESCRIPTION
#--------- ------------- ------------------------------
DEPT.M                        : 1 DEPTH
DT .US/M                      : 2 SONIC TRANSIT TIME
RHOB.K/M3                   : 3 BULK DENSITY
NPHI.V/V                       : 4 NEUTRON POROSITY
SFLU.OHMM                 : 5 RXO RESISTIVITY
SFLA.OHMM                 : 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 supported.
   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 format files.