PARAFFINICITY,
AROMATICITY
AND
COMPOSITIONAL RATIOS

  • PARAFFINICITY AND AROMATICITY

    Aromaticity and paraffinicity are useful concepts in the evaluation of light hydrocarbon compositional data. All LHC ratios developed here employ relative weight concentrations, proportional to gas chromatographic areas.

    Paraffinicity, as defined here, employs n-alkane (paraffin) versus naphthene ratios. Increase in paraffinicity in sediment extracts is associated with increasing yield and the progressive dominance of n-alkanes. In oils, it is associated with increasing levels of maturity. Decrease in paraffinicity in oils is diagnostic of alteration, particularly of biodegradation.

    Aromaticity is defined on the basis of the relative concentrations of aromatic hydrocarbons and n-alkanes of matching carbon number, thus: benzene / n-hexane, toluene / n-heptane, extended in later studies, to m- + p-xylene / n-octane. These ratios are of primary value in the recognition of evaporative fractionation (Thompson, 1987, 1988), described in Section 10.

  • INDEXING PROGRESSIVE LHC COMPOSITIONAL CHANGE DURING MATURATION

    A set of inter-compound ratios was adpoted (Thompson, (1983), Organic Geochemistry, p.306) to track and investigate compositional change. Ratios were developed under three headings: paraffinicity, aromaticity and branching, illustrating, respectively, the ratio of paraffins to naphthenes, of aromatics to paraffins and of branched to unbranched alkanes, both paraffinic and naphthenic. Two such ratios were introduced earlier (Thompson, (1979), Organic Geochemistry, p.661) as Paraffin Index 1, subsequently termed the Isoheptane Value, and Paraffin Index 2, subsequently termed the Heptane Value.

  • DEFINITIONS OF LHC COMPOSITIONAL RATIOS


    Designation Indicated Property Definition
    A Aromaticity Benzene / n-Hexane
    B Aromaticity Toluene / n-Heptane
    X Aromaticity m+p-xylene / n-Octane
    C Paraffinicity n-Hexane + n-Heptane / Cyclohexane + Methylcyclohexane
    I Paraffinicity, Isoheptane Value 2-Methylhexane + 3-Methylhexane / 1,cis-3-, + 1,trans-3-, +1,trans-2-Dimethylcyclopentane
    F Paraffinicity n-Heptane / Methylcyclohexane
    H Paraffinicity, Heptane Value ((100 x n-Heptane) / (Cyclohexane thru n-Heptane + Methylcyclohexane))%
    U Naphthene Branching Cyclohexane / Methylcyclopentane
    R Paraffin Branching n-Heptane / 2-Methylhexane
    S Paraffin Branching n-Hexane / 2,2-Dimethylbutane

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