Relationships Between Light Methane Hydrocarbons in Oils from Fields of the Tomsk Region

S. I. Khoroshko, N. M. Smolyaninova

(Presented by the scientific-methodological seminar of the KhTF)

Individual components of oil, or groups of petroleum components close in chemical nature, are connected with one another by a number of qualitative and quantitative relationships common to most oils. On the basis of a study of the individual hydrocarbon composition of the gasoline fractions of oils from the Sovetskoye, Sobolinoye, Poludennoye, Sredne-Nyurolskoye, and Olenye fields of the Tomsk Region, we have attempted to identify these relationships for light alkane hydrocarbons.

The study of the individual component composition of the indicated fractions was carried out by capillary chromatography.

First of all, arranging the hexane isomers in order of decreasing concentration, one cannot fail to note the stability of the series thus obtained (Table 1). In seven oils out of eight, these isomers are arranged as follows:

Table 1

Relationships between hexanes, % of the sum of isomers

n-hexane → 2-methylpentane → 3-methylpentane → 2,3-dimethylbutane → 2,2-dimethylbutane.

In the Sovetskoye oil from well 47, the distribution of hexane differs from that noted above by only one pair of isomers. Such a distribution of alkanes of composition \(C_6\) corresponds to their equilibrium mixture at temperatures of about 1000°K [1], which are unrealistic both from the standpoint of the presumed conditions of oil formation and in view of the nature of the high-molecular components of oil, which are unstable at these temperatures. The difference of oils—

of hexanes from their equilibrium mixtures in the interval of more realistic oil-formation temperatures (up to 500°K) consists chiefly in an increased content of n-hexane, at the expense of 2,2-dimethylbutane.

It has been found that the distribution of methyl derivatives of hydrocarbons containing an even number of C atoms in their molecule differs substantially from the distribution of those with an odd number of carbon atoms (Table 2). Thus, in isostructural hydrocarbons with an even number of carbon atoms in the molecule—for example, for methylpentane—the content of the isoparaffin with the methyl group at the second carbon atom (2-methylpentane) is greater (on average by a factor of 1.5) than the amount of the corresponding hydrocarbon with the methyl group at the third carbon atom (3-methylpentane).

Table 2

Distribution of 2- and 3-methyl-substituted n-paraffins, wt. % of the sum of isomers

For isoalkanes with an odd number of atoms in the molecule—for example, for methylhexanes—this ratio is the reverse: there is more 3-methylhexane than 2-methylhexane. The noted regularity holds for 2- and 3-methylheptanes and, according to preliminary data, for 2- and 3-methyloctanes, and in general extends to all the oils studied, with the exception of Sovetskoye well 47.

The features of the isomeric composition of light alkanes can be explained on the assumption that their main biological precursors are lipid substances with a normal carbon chain.

In this case, the light methane hydrocarbons may be regarded as fragments of lipid hydrocarbon chains. During the cleavage of such chains, the formation of saturated olefinic fragments alongside the paraffinic ones is equally probable (in the case of saturated fatty acids). Under conditions of subsequent [[unclear: continuation on next page]]

Table 3

Composition of equilibrium mixtures of isohexanes and isohexenes at 500°K and the average composition of isohexanes in the oils studied

at elevated pressures, isomerization of methane hydrocarbons is hindered. Owing to such saturated fragments, incapable of isomerization, the resulting mixture of light hydrocarbons is enriched in the n-isomer. Under the same conditions, olefinic fragments can isomerize, becoming simultaneously saturated with hydrogen through disproportionation [2]. Therefore, in such a mixture the ratio between branched hydrocarbons should correspond more closely to the composition of equilibrium mixtures of olefins than of paraffins, which is also confirmed by the data in Table 3.

Thus, the relationships considered between light methane hydrocarbons make it possible to represent possible pathways of their formation in nature and, consequently, to contribute to some extent to solving the problem of the genesis of oil and gas.

REFERENCES

1. A. I. Bogomolov, V. K. Shimanskii. Geochemistry, 1, 115—120, 1966.
2. V. K. Shimanskii. “Soviet Geology,” 5, 37—44, 1967.