Calibration of hydrogen indices with microscopy: A review, reanalysis and new results using the fluorescence scale

A reanalysis of both published and unpublished datasets shows that good and useful statistical relationships often exist between whole-rock hydrogen indices and the nature of the kerogen assemblage determined with light microscopy. The best regression results are reviewed, presented and compared, and the potential reasons for occasional poor results discussed. The most important parameters for predicting the hydrogen index (HI) of immature distal marine and lacustrine sediments are the percentage of plankton- or bacterially-derived amorphous organic matter (AOM, especially when normalised to a palynomorph-free basis) and the visual intensity of autofluorescence of the AOM matrix under blue light excitation. Correct discrimination of AOM and consistent visual assessment of fluorescence intensity are thus essential. The best statistical relationships were obtained using the newly defined fluorescence scale index, which integrates directly the abundance and preservation of the AOM; on average, this parameter explains 70–80% of the observed variation in whole-rock HI and enables HI prediction to within 100 mgHC/gTOC. This study emphasises the considerable preservation-dependent variation in the hydrocarbon generating potential of AOM.