SECTION 2,
LIGHT END SLOPES
  • METHANE AND ETHANE

    The modal methane/ethane ratio (C1/C2) in a suite of 535 PVT analyses of oils is 3.5 - 4.0. Comparing this range of values with the ratios between other adjacent n-alkanes or pseudo-components, they are the highest observed in oils. This has led other geochemists to a concept of a "superabundance" of methane, even in oils with relatively low C1/C2 ratios. However, as suggested by the theme developed here, that is, the widespread occurrence of gas advection with gas enrichment, it is possible that the ratio is generally secondarily increased (see Thompson, 2004, on enrichment in oils in western Canada). C1/C2 ratios in asphaltene pyrolysates are relatively low at oil-like maturities, having values such as 2.31 and 1.97, as in the examples illustrated in Figure 12, suggesting that such ratios are in fact representative of generation processes.


  • LIGHT END SLOPES

    In addition to the exponential series present in petroleum P6+ components, two others are developed in light ends. Two Slope Factors are measurable, SF(C2-P5) and SF(C3-nC5). Values are linearly covariant, as illustrated in Figure 13. The two series were discovered by Thompson (2002 and 2006).

    Light end exponential series are not evident in all oils because of alteration, due variously to evaporative fractionation, migration depletion, or biodegradation. With minor alteration, progressively lower correlation coefficients are obtained when an exponential is fitted to the data. Substantial alteration obliterates the original order.

    C3 - nC5

    Normal-alkanes form exponentially decreasing concentration sequences by carbon number, as first observed for n-C10+ by Kissin, 1987. The same holds for the lightest n-alkanes: propane, n-butane and n-pentane. Slopes are expressed as SF(C3-nC5).

    Progressively increasing values are indicative of two possible factors, increasing maturity or to advection, the addition of light gas-liquids to oil. The maximum value of SF(C3-nC5) attributable to maturation in oils is approximately 1.70. Greater values in gas/condensates reflect vapor/liquid equilibrium rather than generative reactions.

    C2 - P5

    Concentrations of ethane, propane, pseudo-butane and pseudo-pentane in reservoir fluids also present exponentially decreasing sequences by carbon number, as first observed by Thompson, 2006. Pseudo-butane comprises n-butane and isobutane; pseudo-pentane comprises n-pentane, isopentane and cyclopentane. Random fragmentation of linear precursors, the predictable result of free radical reaction, ensures the occurrence of this exponential. The occurrence of secondary carbenium ionic reaction, generating branched and cyclic isomers, does not affect the outcome, as branched and normal components are summed in pseudo-components. Values of the slope of the sequence are expressed as SF(C2-P5).


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