Spectographic chemical analysis of organic material in source rocks as well as oil have shown that the relative quantities of alkanes (example: CH3 - CH2 - CH2 - CH3) for odd- end even-numbered C-atoms are markers for type of organic matter and maturity. The composition is analysed by a gas chromatograph equipped with a flame ionization detector. The interesting range is that of C15 to C31. This has led to several geochemical markers. These are only a small subset of the many geochemical markers that are in use. We refer to Peters, et al (2005) for more.
The index was introduced by Bray & Evans in 1961. It focuses on the C23 to C30 range. The index can be indicative for the type of source material, hence environment of deposition, but it has also been found that the odd-predominance tends to decrease with greater maturity.
The OEP has been introduced by Scalan & Smith (1970). This uses only the C21 to C25 range, with a heavy weight on C23.
The TAR index is due to Bourbonniere and Meyers, (1996). As the formula shows, it uses only odd-numbered Carbon conponents, where the C27 to C31 are indicative for a terrigenous source, versus the lighter C15 to C19 are more common for aquatic sources.