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I wanted to share a document on LAS file format standards. The first post is on the first version (V1.2) of this standard and modified from data from the CWLS.

Introduction
Log analysts who use personal computers have, in the past, been entering log data into their machines mainly through a hand digitizing procedure because most personal computers are unable to handle data from magnetic tapes. The industry was beginning to address this inefficiency by making log data available on floppy disks in a variety of formats. It was at this point that the Canadian Well Logging Society set up the Floppy Disk Committee to design a standard for log data on floppy disks that would meet the needs of personal computer users.

Various standards for digital well log data already exist. The LIS format (Log Information Standard) is one of the more popular standards. A more complete standard is presently being prepared by the American Petroleum Institute which is known as the DLIS format (Digital Log Interchange Standard). Both of these standards are very useful but because of their completeness, and resulting complexity, they do not address the needs of most personal computer users.

Users of personal computers have serious space limitations and are normally only interested in the optically presented curves. They also want to get this data into their machines quickly and easily. The LAS format addresses these needs and can be compared to a "Reader's Digest" version of the LIS or DLIS formats. If more detailed log information is required, then the LIS or DLIS format should be used. The LAS format is intended to complement the LIS or DLIS formats as each has its own specific purpose.

General Description
The LAS format was designed to be easily understood by the user and at the same time contain enough flags to assist the programmer. The LAS format must always be written in ASCII. If it is written using a compression routine, in binary, or in any other form, an executable program must exist on the floppy disk that will convert the file back to LAS format.

The LAS format consists of files. For example, a repeat section would make up one file and the main pass another. Each of the file names end in ".LAS" so that they can be easily recognized. An individual floppy disk must not contain partial files that continue onto a second floppy disk. Large files that do not fit on one floppy must be split into two or more files.

Each file consists of numerous sections. These sections are not order specific except that the last section must always be the data section. The first section is usually the "VERSION" section containing the version number of the LAS format and identifies whether the data is in wrap mode. 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 the definitions of these mnemonics, in the order that they appear in the data section. The "PARAMETER" section contains informat on parameters or constants (such as mud resistivities) 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 a space.

LAS Format specification for Version 1.2

The most basic specifications of this format are that the file will be written in ASCII and All files in LAS format must end in ".LAS" so that they may be easily recognized. The LAS format is a minimum standard. It is expected that most users and suppliers will exceed this minimum standard and supply sufficient other formats for data loading and storage.

Major Components of the LAS file:
Each LAS file contains numerous sections and each section begins with a tilde (~) mark. The last section in an LAS file must always be the log data section.

The sections that make up an LAS file are as follows:

~V - contains version and wrap mode information
~W - contains well identification
~C - contains curve information
~P - contains parameters or constants
~O - contains other information such as comments
~A - contains ASCII log data

These sections are described in more detail in part four of this appendix.

Flags
Flags are used to assist computers in identifying specific lines in a file.
The following flags are used in the LAS format:

a) "~" (tilde):
The ASCII equivalent of this flag is decimal 126 or hexadecimal 7E. This flag when used will be the first non-space and non-quotation character on a line. It is used to mark the beginning of a section in a file. The first letter directly after the tilde identifies the section. (See part two.) All upper case letters in the space following a tilde mark are reserved for use by the committee. The remainder of the line will be treated as comments.

b) "#" (pound):
The ASCII equivalent of this flag is decimal 35 or hexadecimal 23. This flag when used will be the first non-space and non-quotation character on a line. The Pound sign is used to indicate that the line is a comment line. Comment lines can appear anywhere above the data section.

c) "." and ":" :
In sections other than the data section, dots and colons are used to delimit information within the line. They are usually aligned for ease of reading. Information to the right of the colon is a detailed definition of the mnemonic that is located to the left of the colon. The dot is used to separate two mnemonics. Spaces may occur to the right or left of a dot or colon.

Further Details
The actual format for each of the sections discussed in this part of the paper is best understood by looking at the examples in the boxed areas. The exact spacing is not critical because computer programs will use the dots, colons and spaces to decipher each line.

The different LAS sections

I) ~V (Version Information)

This section is mandatory and usually appears at the very beginning of the file.
It identifies which version of the LAS format is being used and whether wrap mode is used.

This section must contain the following lines:
Refers to whether a wrap around mode was used in the data section. If no wrap mode is used the line will have a maximum length of 256 characters (including the carriage return and line feed). 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).

Additional lines in the version section are optional.

The following is an example of a Version Information Section.


(II) ~W (Well Information)
This section is mandatory.
It identifies the well, its location and indicates the start and stop depths of the file.

This section must contain the following lines with the mnemonics as indicated:

Refers to the first depth in the file. The "nnn.nn" refers to the depth value. The number of decimals used is not restricted. The ".M" refers to meters and can be replaced when other units are used. The start depth can be either greater or less than the stop depth.

Refers to the last depth in the file. The "nnn.nn" refers to the depth value. The number of decimals used is not restricted. The ".M" refers to meters and can be replaced when other units are used.

Refers to the depth increment used. A minus sign must precede the step value if the start depth is greater than the stop depth (ie, from TD to casing has a minus step value). A step value of zero indicates a variable step.

Refers to null values. Two common ones in use are -9999 and -999.25.

Refers to company name.

Refers to the well name.

Refers to the field name.

Refers to the well location.

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


Refers to logging company.

Refers to date logged.

Refers to unique well identifier. For areas outside of Canada this may be replaced by:

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

The following is an example of a Well Information Section:


(III) ~C (Curve Information)

This section is mandatory.
It describes the curve and its units in the order they appear in the data section of the file.
The mnemonics used are not restricted but must be defined on the line in which they appear.
API codes are optional. (See References.)
The curves described in this section must be present in the data set.
The first curve described should normally be depth.

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


(IV) ~P (Parameter Information)

This section is optional. It defines the values of various parameters relating to this well.
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:


(V) ~O (Other Information)

This section is optional. Its intended use is as a remarks or comments section.

(VI) ~A (ASCII LOG DATA)

The data section will always be the last section in a file.
Depths should always appear in the first column.
Each column of data must be separated by at least one space.
A line of less than 256 characters (including a carriage return and line feed) will normally not be wrapped. Wrap mode will be used if the data is longer than 256 characters.
In wrap mode, depth 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.
If wrap mode is used, the decimal points must be aligned for ease of reading.
Exponents are not permitted. The curve section can be used to overcome this limitation by changing the units.

Example files

EXAMPLE #1 - ILLUSTRATING THE LOG ASCII STANDARD IN UNWRAPPED MODE


EXAMPLE #2 - ILLUSTRATING THE MINIMUM REQUIREMENTS OF THE LOG ASCII STANDARD IN UNWRAPPED MODE.


EXAMPLE #3 - ILLUSTRATING THE WRAPPED VERSION OF THE LOG ASCII STANDARD

Very informative. Thanx a lot. It has solved one of my big problem. I wanted to see .LAS files in graphic form. I tried to use schlumberger's freeware for that; but it returned error "it's a UNIX file so cannot be read". Now I know how to deal with special characters of header portion.Thanx

Thanks for your kind words. Happy this helped you!

thanks alot for the specifications..

but I want to know is there any library or a class (a DLL file) in .net (c#/c++/vb) that is able to read the LAS file?

Gents,

I need your assistance with LAS file usage.

I am using GeoGraphix discovery for my work and sometimes I work with old digitized las files. And those las files, when loaded into GeoGraphix, are displayed incorrectly: curves are partly flat, different peaks are appeared. When loading recently obtained las files - GeoGraphix is OK.

But when I use another software - everything looks fine! Maybe I should make some changes in the las header in order to get it displayed correctly?