Subsurface intervals are indicated in the oil and gas well database with "stratigraphic codes." These codes are an extension of the code adopted by the American Association of Petroleum Geologists (AAPG) Committee on Stratigraphic Codes in the 1960's. The code consists of a three digit prefix followed by an up to five character mnemonic for the formation. The first (leftmost) digit indicates the era:
The second (middle) digit indicates the system, for example, in the Paleozoic:
The third (rightmost) digit indicates the stage (upper, middle, lower, Chesterian) with the numbers increasing with age. That is, lower units get higher numbers. Thus, for example:
This arrangement allows formation data to be sorted into stratigraphic order (younger units sort above, or on top of, older units). This gross sorting is inadequate for many applications involving stratigraphy down to the member or bed level, however. To implement those distinctions, the KGS implemented a unique sort sequence code assigned to each stratigraphic code. The sort sequence code consists of the first 3 digits of the AAPG stratigraphic code as defined followed by a 4-digit sequence code. Using this 7-digit code allows sorting stratigraphic units at a much finer level of detail without altering the original AAPG code.
The up to five character mnemonic code for a formation is derived using rules established by the AAPG Committee on Stratigraphic Codes. In general, the formation is coded by striking letters until the code is four or fewer characters. The fifth character, if needed, is reserved for modifiers like "U" (upper), "1" (first), "S" (stray), etc., that often occur in drillers' informal names for subsurface intervals.
Copies of the current stratigraphic code are available as an Adobe Acrobat pdf file for viewing or printing or as a comma and quote delimited text file for use in database or spreadsheet applications.