DISTRIBUTION OF LIGHT METHANE HYDROCARBONS IN OILS FROM FIELDS OF THE TOMSK REGION IN CONNECTION WITH CERTAIN QUESTIONS OF THEIR GENESIS

S. I. Khoroshko, N. M. Smolyaninova

(Presented by the scientific-methodological seminar of KhTF)

Individual components of oil, or groups of petroleum components close in chemical nature, are connected with one another by a number of qualitative or 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 was carried out by capillary chromatography according to the procedure described above [1].

Table 1

Relationships among hexanes, % of the sum of isomers

First, 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: n-hexane, 2-methylpentane, 3-methylpentane, 2,3-dimethylbutane, 2,2-dimethylbutane.

In the Sovetskoye oil from well 47, the distribution of hexanes differs from that noted above by only one pair of isomers. Such a distribution of alkanes of composition \(C_6\) corresponds to the composition of their equilibrium mixture at temperatures of about \(1000^\circ K\) [2], which are unrealistic both from the standpoint

the presumed environment of petroleum formation and in the nature of the high-molecular components of oil, which are unstable at these temperatures. The difference between petroleum hexanes and their equilibrium mixtures in the interval of more realistic temperatures of oil formation (up to 500°K) consists mainly in the 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).

Table 2

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

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).

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

The peculiarities of the isomeric composition of the light alkanes can be explained on the assumption that their principal 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—alongside saturated fragments—of olefinic fragments is equally probable (in the case of saturated fatty acids). Under conditions of elevated pressures, isomerization of methane hydrocarbons is hindered. Because of such saturated fragments, which are incapable of isomerization, the resulting mixture of light hydrocarbons is enriched in the n-isomer. Under the same conditions, olefinic fragments may isomerize, while simultaneously becoming saturated...

by hydrogen through disproportionation [3]. 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.

The pathway considered for the formation of light methane hydrocarbons is not the only one. In part, such hydrocarbons may also represent, for example, fragments of hybrid hydrocarbons of mixed structure. In the process of metamorphism, one or several rings in such molecules may open, with the formation of aliphatic fragments of greater or lesser length. However, these processes are apparently not universal and yield a smaller portion of the methane hydrocarbons in petroleum gasolines.

Table 3

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

REFERENCES

1. S. I. Khoroshko, N. M. Smolyaninova. Study of the individual hydrocarbon composition of gasoline fractions of oils from certain fields of the Tomsk Region. This collection.

2. A. I. Bogomolov, V. K. Shimanskii, Geochemistry, I, 115–120, 1966.

3. V. K. Shimanskii, Soviet Geology, 5, 37–44, 1967.