"The E&P Thesaurus is a collection of words and phrases that are descriptive of the concepts and equipment pertaining to petroleum exploration, development and production, exclusive of geographic areas and named geologic terms, which are included in the Geographic Thesaurus and its supplement. Further, the E&P Thesaurus is a controlled vocabulary for this subject area, in which the various concepts have been linked by means of generic or hierarchical relationships. These relationships may be truly generic or may be associations by use, or even convenience. Synonyms are handled as follows: all terms of like meaning are directed to one term for use as the index term for that particular concept. This selection usually follows popular usage and the patterns set by existing terms in the Thesaurus. The chosen term is called a 'valid' index term; the other like terms are called 'invalid' and are not used for indexing.
The E&P Thesaurus terms are interrelated on the concept-oriented (faceted) principle, and all descriptors are narrower terms of one of ten facet headings."