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LAS Version 2.0: A Digital Standard for Logs

Update January 2014

BY

Canadian Well Logging Society (www.cwls.org)
LAS Committee:
C. Struyk, KC Petrophysics Inc
J. Karst, Schlumberger Canada Ltd.

1.0 Abstract:

The LAS 2.0 log data standard was introduced in 1992 and continues to be
popular. This paper updates the LAS 2.0 documentation and makes a minor change
to the LAS 2.0 specifications to better reflect the technological advances made
since its introduction.

The changes and clarifications are as follows:

- Line length is unrestricted in unwrapped mode (change)
- The depth value divided by the step value must be a whole number 
  (clarification)
- Rounding of depth values is not acceptable. (clarification)
- The delimiters in a non-comment line are the first dot in the line, the first
  space after that dot and the last colon in the line. (clarification)
- Most LAS 2.0 files have a depth based index however a time based index is 
  permitted (clarification).

2.0 Introduction:

This paper updates the LAS 2.0 documentation (Log ASCII Standard version 2.0).
The updating was necessary to clarify some items not specifically stated in the
earlier documentation and to better reflect the technological advances made
since its introduction.

The LAS standard was introduced by the Canadian Well Logging Society in 1989 to
standardize the organization of digital log curve information for personal
computer users. It did this very successfully and the standard became popular
worldwide. Version 1.2 was the first version and followed in September 1992 by
version 2.0 to address some inconsistencies. A more versatile version LAS 3.0
was released in 1999 however at present, LAS 2.0 remains the dominant product.
LAS 3.0 clarifies several of the poorly defined specifications of LAS 2.0 and
provides expanded data storage capabilities, but has seen limited
implementation.

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3.0 LAS 2.0 Overview:

- An LAS file is a structured ASCII file containing log curve data and header 
  information. The header information is located at the beginning of the file
  and followed by curve data.
- The standard was designed to simplify the exchange of digital log data between
  users.
- The LAS format is intended for optically presented log curves although other
  curves may also be included.
- The ASCII character set is limited to ASCII 13 (carriage return), ASCII 10
  (line feed), and ASCII 32 to ASCII 126 inclusive. All other ASCII characters
  are not allowed and it is suggested that software readers convert them to a 
  space (removing them may cause issues if the character was intended to 
  represent a space such as the tab character). Line termination will consist of
  ASCII 13 ASCII 10 (CR LF) except for the last line.
- Each LAS 2.0 file contains only one continuous interval in the data section. 
  For example, a repeat section would make up one file and the main pass 
  another.
- LAS files end in ".LAS" so that they can be easily recognized.
- Each LAS file consists of sections. Sections begin with a header line defined 
  as beginning with the ~ tilde character when it occurs as the first non-space 
  character on a line. The character immediately following the tilde character 
  defines the section with the remainder of the line being ignored. The 
  characters "V", "W", "C", "P", "O", and "A" are reserved in the LAS 2.0 
  standard. The sections defined by the LAS 2.0 standard are limited to one 
  occurrence per file. Customer defined sections are permitted but must be 
  located after the first section (~V) and before the last section (~A).
- The sections defined for the LAS 2.0 standard are as follows:

  - "~V" (also known as "~VERSION INFORMATION SECTION") is a required section;
    has formatting requirements; must be the first section; identifies the 
    version number and whether data is in "wrapped" or "un-wrapped" mode.
  - "~W" (also known as "WELL INFORMATION SECTION") is a required section; has 
    formatting requirements; is preferably the second section; contains 
    information on the well name, location, and start and stop values of the 
    data in this file.
  - "~C" (also known as ~CURVE INFORMATION SECTION") is a required section; has 
    formatting requirements; contains curve mnemonics and their definitions in 
    the order that they appear in the data section.
  - "~P" (also known as ~PARAMETER INFORMATION SECTION") is an optional section;
    has formatting requirements; contains information on parameters or constants
    relevant to the wellbore such as mud resistivity, wireline engineer, truck 
    number etc.
  - "~O" (also known as "~OTHER") is an optional section; has no formatting 
    requirements; contains other information or comments.
  - "~A" (also known as ~ASCII LOG DATA") is a required section; has formatting 
    requirements; is the last section in the file and also referred to as the 
    data section. The index of the data columns is either Depth or Time. The 
    index values always appear in the first column and each column of data must 
    be separated by at least one space (ASCII 32). All values in the ASCII log 
    data section must be floating point or integer (long) values. Other formats
    such as Text or Exponential values are not supported.
  - Two flags are used in LAS files: 1) "#" signifies a comment line when used 
    as the first non-space character on a line and 2) "~" signifies the 
    beginning of a section when used as the first non-space character on a line.

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- The sections "VERSION", "WELL", "CURVE" and "PARAMETER" use line delimiters. 
  The delimiters are: 1) first dot in a line 2) first space after the first dot 
  in a line and 3) the last colon in a line.
- Example LAS files can be found at the end of this paper.

4.0 Software:

Software exists for LAS data and can be found on the CWLS website
(www.cwls.org).

The Certify program was designed to verify that files meet the LAS standard and
will identify any errors encountered. The checks are based on structure, not
content. That is, it will not flag an empty well name field, but will recognize
that required sections are missing or if a line is not structured correctly. In
case of disagreement between this program and the printed LAS standard document,
the document will be deemed to be correct. The Windows based LAS CERTIFY program
was written by J. Karst of Schlumberger.

An LAS utility was written by C. Struyk. The utility includes the following
processes:

1) reverse depth direction
2) convert LAS 1.2 to 2.0 and LAS 3.0 to LAS 2.0
3) resample data
4) change depth from metres to feet or feet to metres
5) fix start depth and step issues
6) unwrap LAS files
7) wrap LAS files
8) Scan and fix some common errors in LAS files
9) Merge LAS files
10) Convert text files to LAS files
11) Check LAS files for errors

The above programs are not part of the LAS standard. The authors of these
programs do not reserve any rights and do not warrant the programs for any
specific purpose.

5.0 Details:

This portion of the paper provides a detailed look at all of the components of
an LAS 2.0 file. Flags and delimiters are discussed first, followed by a
discussion of the ‘sections’ as defined by the LAS 2.0 format. This portion of
the paper is best understood by looking at the examples in the boxed areas and
the examples provided at the end of this paper.

5.1 Flags

Certain characters are used to assist software in identifying specific lines
within a file. The following flags are defined in the LAS 2.0 format:

"~" (tilde): The ASCII equivalent of this flag is decimal 126. This character is
recognized as a flag when it occurs as the first non-space character on a line.
This flag is used to mark the beginning of a section within an LAS file. The
first letter directly after the tilde identifies the section. The upper case
letters "V", "W", "P", "C", "O", and "A" in the space following a tilde

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mark are reserved for use by the committee. The remainder of the line will be
treated as comments.

"#" (pound): The ASCII equivalent of this flag is decimal 35. This character is 
recognized as a flag when it occurs as the first non-space character on a line.
This flag is used to indicate that the line is a comment line. Comment lines can
appear anywhere above the ~A section.

5.2 Line Delimiters

Three line delimiters are used in the "~V", "~W", "~C", and "~P" sections of LAS
files. The line delimiters are as follows:

a) the first dot in a line,
b) the first space after the first dot in a line
c) the last colon in a line

All non-comment lines in these sections must contain all three of the above
delimiters.

An example line is as follows:

Where:

MNEM = mnemonic. This mnemonic can be of any length but must not contain any
internal spaces, dots, or colons. Spaces are permitted in front of the mnemonic
and between the end of the mnemonic and the dot.

UNITS = units of the mnemonic (if applicable). The units, if used, must be
located directly after the dot. There must be no spaces between the units and
the dot. The units can be of any length but must not contain any colons or
internal spaces.

DATA = value of, or data relating to the mnemonic. This value or input can be of
any length and can contain spaces, dots or colons as appropriate. It must be
preceded by at least one space to demarcate it from the units and must be to the
left of the last colon in the line.

DESCRIPTION = description or definition of the mnemonic. It is always located to
the right of the last colon. The length of the line is no longer limited.

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5.3 ~V (Version Information)

- This section is mandatory and must appear as the first section in the file.
- Only one "~V" section can occur in an LAS 2.0 file.
- It identifies the version of the LAS format and whether wrap mode is used.
- This section must contain the following lines:

VERS. 2.0 : CWLS LOG ASCII STANDARD - VERSION 2.0

Refers to which version of LAS used.

and

WRAP. YES : Multiple lines per depth step

or

WRAP. NO : One line per depth step

Refers to whether a wrap around mode was used in the data section. If the wrap
mode is “NO”, there is no limit to the line length. If wrap mode is used, the
depth value will be on its own line and all lines of data will be no longer than
80 characters (including carriage return and line feed).

- There is no longer a line length limited on LAS files. The original LAS format
  was limited to a line length of 256 characters because of early computer 
  limitations. Modern computer equipment does not have an issue with line 
  lengths and therefore the line length limitation has been withdrawn. The 
  updated LAS 2.0 standard retains the "WRAP YES" definition as an option and 
  for backwards compatibility.
- Additional lines in the version section are optional.
- The following is an example of a Version Information Section.

~Version Information Section

VERS. 2.0 : CWLS log ASCII Standard -VERSION 2.0
WRAP. NO : One line per depth step

5.4 ~W (Well Information)

- This section is mandatory.
- Only one "~W" section can occur in an LAS 2.0 file.
- It identifies the well, its unique location identifier and indicates the start
  and stop depths (or times) of the file.
- This section must contain the following lines with the mnemonics as indicated:

STRT.M nnn.nn : START DEPTH

Refers to the first depth (or time) in the file. The "nnn.nn" refers to the
depth (or time) value. The value must be identical in value to the first depth
(time) in the ~ASCII section although its format may vary (123.45 is equivalent
to 123.45000).

The number of decimals used is not restricted. If the index is depth, the units
must be M (meters), F (feet) or FT (feet). Units must match on the lines
relating to STRT, STOP, STEP and the index (first) channel in the ~C section. If
time, the units can be any unit that results in a floating point number
representation of time. (dd/mm/yy or hh:mm:ss formats are not supported). The
logical depth order (shallow to deep or deep to shallow) is optional.

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Successive time index values must increase if the index is "Time". The start
depth (or time) when divided by the step depth (or time) must be a whole number.

STOP.M nnn.n : STOP DEPTH

Same comments as for STRT except this value represents the LAST data line in the
~ASCII log data section. The stop depth when divided by the step depth must be a
whole number.

STEP.M nnn.nn : STEP

Same comments as for STRT, except this value represents the actual difference
between every successive index value in the ~ASCII log data section. The sign (+
or -) represents the logical difference between each successive index value. (+
for increasing index values). The step must be identical in value between every
index value throughout the file. If the step increment is not exactly consistent
between every index sample, then the step must have a value of 0.

NULL. nnnn.nn : NULL VALUE

Refers to null values. Commonly used null values are -9999, -999.25 and
-9999.25.

COMP. aaaaaaaaaaaaaaaaaaaaa : COMPANY

Refers to company name.

WELL. aaaaaaaaaaaaaaaaaaaaa : WELL

Refers to the well name.

FLD. aaaaaaaaaaaaaaaaaaaaa : FIELD

Refers to the field name.

LOC. aaaaaaaaaaaaaaaaaaaaa : LOCATION

Refers to the well location.

PROV. aaaaaaaaaaaaaaaaaaaaa : PROVINCE

Refers to the province. For areas outside Canada this line may be replaced by:

CNTY. aaaaaaaaaaaaaaaaaaaaa : COUNTY
STAT. aaaaaaaaaaaaaaaaaaaaa : STATE
CTRY. aaaaaaaaaaaaaaaaaaaaa : COUNTRY

SRVC. aaaaaaaaaaaaaaaaaaaaa : SERVICE COMPANY

Refers to logging company.

DATE. aaaaaaaaaaaaaaaaaaaaa : DATE

Refers to date logged. The preferred data is of the form yyyy mm dd

UWI . aaaaaaaaaaaaaaaaaaaaa : UNIQUE WELL ID

Refers to unique well identifier. Within Canada, the most common UWI consists of
a 16 character string. Please exclude all dashes, slashes and spaces from such
UWIs.

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For areas outside of Canada this may be replaced by:

API . aaaaaaaaaaaaaaaaaaaaa : API NUMBER

- Additional lines in the well information section are optional. There is no 
  limit on the number of additional lines.

LIC. nnnnnn : LICENCE NUMBER

Refers to a regulatory licence number. Required by ERCB in Alberta

- The following is an example of a Well Information Section in LAS version 2.0:

_____________________________________________________________________
~Well Information Section
#MNEM.UNIT VALUE/NAME DESCRIPTION
#-------- -------------- ---------------------
STRT.M 635.0000 :START DEPTH
STOP.M 400.0000 :STOP DEPTH
STEP.M -0.125 :STEP
NULL. -999.25 :NULL VALUE
COMP. ANY OIL COMPANY INC. :COMPANY
WELL. ANY ET AL 12-34-12-34 :WELL
FLD . WILDCAT :FIELD
LOC . 12-34-12-34W5M :LOCATION
PROV. ALBERTA :PROVINCE
SRVC. ANY LOGGING COMPANY INC. :SERVICE COMPANY
LIC . 12345 :ERCB LICENCE NUMBER
DATE. 13-DEC-86 :LOG DATE
UWI . 100123401234W500 :UNIQUE WELL ID
_____________________________________________________________________

5.5 ~C (Curve Information)

- This section is mandatory.
- Only one "~C" section can occur in an LAS 2.0 file.
- It describes the curves and its units in the order they appear in the ~ASCII 
  log data section of the file.
- The mnemonics used are not restricted but must be defined on the line in which
  they appear.
- API curve codes are optional. (May be required by some regulatory agencies)
- The channels described in this section must be present in the data set.
- The first channel described is the index of all other channels, and is either 
  Depth or Time. The only valid mnemonics for the index channel are DEPT, 
  DEPTH or TIME.

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The following is an example of a Curve Information Section with API codes.

_____________________________________________________________________________
~Curve Information Section
#MNEM.UNIT API CODE Curve Description
#------------------ ---------------- -----------------
DEPT.M : 1 DEPTH
RHOB .K/M3 45 350 02 00 : 2 BULK DENSITY
NPH .VOL/VO 42 890 00 00 : 3 NEUTRON POROSITY - SANDSTONE
MSFL .OHMM 20 270 01 00 : 4 Rxo RESISTIVITY
SFLA .OHMM 07 222 01 00 : 5 SHALLOW RESISTIVITY
ILM .OHMM 07 120 44 00 : 6 MEDIUM RESISTIVITY
ILD .OHMM 07 120 46 00 : 7 DEEP RESISTIVITY
SP .MV 07 010 01 00 : 8 SPONTANEOUS POTENTIAL
GR .GAPI 45 310 01 00 : 9 GAMMA RAY
CALI .MM 45 280 01 00 : 10 CALIPER
_____________________________________________________________________________

5.6 ~P (Parameter Information)

- This section is optional. It defines the input values of various parameters 
  relating to this well. These input values can consist of numbers or words.
- Only one "~P" section can occur in an LAS 2.0 file.
- The mnemonics used are not restricted but must be defined on the line on which
  they appear.
- There is no limit on the number of lines that can be used.
- The following is an example of a Parameter Information Section.

________________________________________________________________________________
~Parameter Information Section
#MNEM.UNIT Value Description
#----------------- ------------ ----------------------
MUD . GEL CHEM : Mud type
BHT .DEGC 114.0000 : Bottom Hole Temperature
BS .MM 222.0000 : Bit Size
CSGL .M 345.7 : Casing Depth
FD .K/M3 999.9999 : Fluid Density
MDEN .K/M3 2650.0000 : Logging Matrix Density
MATR . SAND : Neutron Matrix
FNUM . 1.0000 : Tortuosity Const. Archie's(a)
FEXP . 2.000 : Cementation Exp Archie's (m)
DFD .K/M3 1200.0000 : Mud Weight
DFV .S 50.0000 : Mud Viscosity
DFL .C3 8.0000 : Mud Fluid Loss
DFPH . 10.00 : Mud pH
RMFS .OHMM 2.8200 : Mud Filtrate Resistivity
EKB .M 566.9700 : Elevation Kelly Bushing
EGL .M 563.6799 : Elevation Ground Level
________________________________________________________________________________

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5.7 ~O (Other Information)

- This section is optional. It is intended as a remarks or comments section.
- Only one "~O" section can occur in an LAS 2.0 file.
- This section has no delimiter requirements.
- The following is an example of an "Other Information Section"

------------------------------------------------------------------------------------------------------------
~Other Information Section
The log digits for this well were hand digitized from poor half scale log prints. This was the best information available at the time. Every attempt should be made to track down the original films.
.Dec. 12,1990 John Doe, Petrophysics
------------------------------------------------------------------------------------------------------------

5.8 ~A (ASCII Log Data)

- The data section will always be the last section in a file.
- Only one "~A" section can occur in an LAS 2.0 file.
- Embedded blank lines anywhere in the section are forbidden
- Each column of data must be separated by at least one space. Consistency of 
  format on every line, while not required, is expected by many LAS readers. 
  Right Justification of each column of data and the same width of all data 
  fields is highly recommended.
- Line length in the data section of unwrapped files are no longer restricted
- In wrap mode, the index channel will be on its own line
- In wrap mode, a line of data will be no longer than 80 characters. This 
  includes a carriage return and line feed.

6.0 References

C. Struyk, R. Bishop, D. Fortune, E. Foster, D. Gordon, T. d'Haene, D. Joyce, 
S. Kenny, H. Kowalchuk and M. Stadnyk, 1989; LAS, A Floppy Disk Standard For Log
Data, Canadian Well Logging Society, 12th Formation Evaluation Symposium , Paper
J ; The Log Analyst, V30,No.5 P 395-396; Geobyte 1989.

CWLS Floppy Disk Committee; 1992; LAS 2.0, A Floppy Disk Standard For Log Data;
www.cwls.org.

CWLS LAS Committee; 2009, LAS Version 2.0 Updated: July 2009, A digital Standard
for Logs; www.cwls.org
