Discovery of Exponential Series

    As described in the Glossary, and in Section 1, the six-carbon compounds, P6, cause a slope break such that P6 > P5, defining a secondary maximum after methane. C6+ compounds are preponderantly liquid under surface conditions. This major reservoir fluid fraction exhibits two exponential series of concentrations, firstly, that of the the pseudo-components, discovered by Lohrenz and Bray in 1964 (Soc. Pet. Eng. Paper 792,); secondly, that of the n-alkanes, the principal contributors to the first, described by Kissin in 1987 (Geochim. et Cosmochim. Acta, 51, 2445). The first series yields Slope Factors principally quoted here as SF(P15-P25), the range which exhibits the greatest resistance to, and least perturbation by, alteration. In unaltered oils other pseudo-component series such as SF(P6+) or SF(P10+), yield values identical to SF(P15-P25), but low carbon number components are highly prone to alteration.

    P6+ normal-alkane Slopes Factors are designated as, for example, SF(nC15-nC25). There is a consistent relationship between n-alkane and pseudo-component Slope Factors, illustrated in Figure 14.

    SF(P15-P25): PRECISION

    Slope Factor determinations, each based on multiple measurements of all components in a given carbon number range, eleven in the case of SF(P15-P25), can lead to accurate values in individual cases and to extreme precision in multiple evaluations. This is evidenced in Figure 15 which illustrates three of five analyses of samples taken along the 43 mile length of the East Texas pool, originally comprising nine billion barrels. The remarkable homogeneity of the charge is further illustrated in Table 1, where five values of SF(P15-P25) measured in the East Texas field are shown to exhibit a coefficient of variation of 0.1%.

    Because of this precision, provided that neither poor analytical work nor alteration have degraded the concentration series, values of SF(P15-P25) may often be relied upon to one part in a thousand.


    SF(P15-P25) is a direct indicator of maturity, not an indicator of the isomerization of minor compounds possessing kinetics of unknown relationships to those of the principal reaction, but an indicator of the conversion of the precursors themselves. Its utility as a maturity indicator may be assessed by examination of its relationship to API gravity.

    In Figure 16 API gravity versus SF(P15-P25) is illustrated for three suites of oils. In the unaltered oils from Magnolia field (Gulf of Mexico, data of Weissenburger, 2004, Spec. Pub. Geol. Soc. 237) the correlation is extremely high, r = 0.99. In the Rainbow area, where limited alteration is known to have occurred, the correlation decreases. Evaluating a large sample of oils from western Canada the relationship is degraded by gas advection, evaporative fractionation, and biodegradation.