Rock-Eval is a useful new tool to study sedimentary organic matter by pyrolysis and determination of hydrocarbons and CO2. Certain interpretive and operational aspects of the Rock-Eval technique, however, need to be carefully considered. Rock-Eval S2 peak or Hydrogen Index (HI = S2 normalized to organic carbon) is obtained by the flame ionization detector (FID) that responds to carbon-hydrogen bonds, carbon electrons, and carbon mass; thus, the FID response is very nearly the same to benzene, hexane, and six molecules of methane, but the atomic H/C of these molecules varies by a factor of 4. A further problem with the assumption that HI is proportional to H/C is that Rock-Eval does not measure either H2 or H2O, both of which are important pyrolysis products. Despite this, Rock-Eval HI is commonly correlated empirically with atomic H/C. Similarly the Rock-Eval oxygen index (OI) measures CO2 but not H2O or CO, which are important pyrolysis products. The OI is commonly correlated empirically with atomic O/C. The fact that these two correlations exist probably is due partly to regularities in the pyrolysis mechanisms of kerogen and partly to a predominance of methane from type III kerogen, which accentuates the low HI. These factors are the reason that the HI versus H/C and OI versus O/C plots do not go through the origin but intercept the H/C axis at 0.45 and O/C axis at 0.04. Incorrect classification of kerogen types or interpretation of diagenetic history can result from these OI and HI variations from actual O/C and H/C measurements. Examples of kerogen incorrectly classified as type I and confused evolution paths are documented.
Rock-Eval should be most successful on core samples containing organic matter of relatively uniform composition. Analysis of recent samples, outcrop samples, or a single sample should not be used for unqualified interpretation of kerogen type or evolution path, but it may give useful organic richness and maturity information.