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 other ratios of adjacent n-alkanes or pseudo-components, this is the highest value observed in oils. This has led to a concept of "superabundant" 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, gas enrichment and subsequent evaporative fractionation, it is possible that the ratio is commonly secondarily increased (see Thompson, 2004, on enrichment in oils in western Canada). Additionally, 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.

LIGHT END SLOPES

Two slope factors are measurable in petroleum light ends, SF(C2-P5) and SF(C3-nC5). Values are linearly covariant, as illustrated in Figure 13.

C2 - P5

Oil components ethane, propane, pseudo-butane and pseudo-pentane in reservoir fluids form an exponentially decreasing concentration sequence by carbon number, as first observed by Thompson, 2002. Pseudo-butane comprises n-butane and isobutane, pseudo-pentane, n-pentane, isopentane and cyclopentane. Random fragmentation of linear precursors, the predictable result of free radical reaction, ensures the occurrence of this exponential. The occurence of secondary carbenium ionic reaction, generating branched and cyclic isomers, does not affect the outcome, as branched and normal components are summed. Values of the slope of the sequence are expressed as SF(C2-P5).

C3 - nC5

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

Progressively increasing values are indicative of two possible factors, to increasing maturity or to advection, the addition of light gas-liquids to oil. Both exponential series and slope factors obtain in gas/condensates, at a higher series of values than observed in unaltered oils. The maximum value of SF(C3-nC5) attributable to maturation in oils is approximately 1.70. Higher values in gas/condensates reflect large vapor/liquid equilibrium constants, decreasing with increasing carbon number.

Light end slopes are not measurable 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. Calculated values as high as 0.994, for example, are associated with suites which visibly depart from exponential, that is, from linear plots on log-linear axes.

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