Them methlypantene with n-hexane atc. On the contrary, when comparing two very different

strucktures such as linier and cyclic, e.g., n-heptane and methylcyclohexane, there is no

collerelation(Fig.IV.1.10).

This situation can be explained by the fact that low to medium molecular weight hydrocarbons are

not inherited biogenic molecules, but are generated throught thermal degradation and cracking of C-

C bonds of either kerogen or larger bitumen molecules already formed. The succession of chemical

reactions resulting in the various light alkanes,for instances, id alike and the more the structures of

two alkanes resemble each other, the greater is the chance that the reaction paths at are the same.

Therefore paralel reactions and comprable kinetics result in rather constan ration of coselyrelated

molecules.on the contrary, different parts of the kerogen macromolecule, throught a different

succession of reactions, and no correlation can be expected betwen them.

Some Cm isoalkanes are particularly abundant in crude oils. For example 2.6-dimethloctane and 2-

methly-3-ethlyheptane amount to 0.55 and 0.64% of ponca city crude oil. Respectively This

predominance. Compared to other decanes, is probably due to their origin from momoterpenes of

heigher plants(Fig.11.3.12)

In the medium rangeof molecular weight, the ,most remarkable molecules belong to the series of

isoprenoids(Fig.II.3,13) they are isoalkanesfrom C9 to C25 with one methly branch on every fourth

carbon atom,their genectic origin is discussed in part II. They frequently amount to about 1% of a

crude oil. The most abundant are pristane(tetramethylpentadecane C19) and

phytane(tetramethylhexadecane are pristane(tetramethylpentadecane C20).together they average

up to 55% of all acyclic isoprenoids.Pristane is often more abundant than phytane , with an average

ratio of 1.35 the pristane nonmarine origin from the gippsland. Cooper and bowen-Surat

basins.australia(powell and Mckirdy.1975).other important isoprenoids are often C16 and C18.whereas

C17 and C22 are practically absent, due to an unlikely mode of cleavage of the precursor.some

irregular isoprenoids are often C16 and C18.whereas C17 and C22 are practically absent,due to unlikely

mode of present in certain crude oils,such as squalane (C30) and lycphane(C40).

In addition to the series of isoprenoids, gas chromatography of the branched cyclic fraction of

saturated hdyrocarbons shows the existence in some crude oils of homologous series of sioalkanes

in the C10-C30 range. Are present in the praffin wax.they are especialy abundant in certain crude

oils,for exmples the uinta basin oils derived from nonmarine source beds, or indonesia oils derived

from a paralic environment(Tissot et al.,1977)

1.4.4 alkanes9 (fig. IV.I.7)

Unsaturated chains are relative unstable,and thus very unusual in crude oils.However, very small

quantities of n-hexene, n-heptene and octene were identified by putscher(1952) in a pennsylvania

crude oil.on the other hand polycyclic cycloalkanes,such as strerenes or hopenes,have been recently

reported in ancient sediments.they have also been found in certain young crude

oils(Hollerbach,1976).however,this type of compound has not been investigated very actively until

now.

1.4.5 Cycloaknes(Fig.IV.1.11)

Cycloopentane,cyclohexane,and their derivaties of low molecular wight(<C10) are important

contituens of petroleum.in particular, mthylcyclohexane is Fig. IV.1.11 Exmples of cycloalkanes

found in crude oils and coals

1.4 Principal types of hydrocarbons in crude oil

Fig.IV.1.12 . Distribution of saturated hydrocarbones as found by ring analysis.Additional data on

normal and isoalkanes are from separation of normal and iso-cyclic fraction with the help of 5 a

molecular sieves,followed by quantitative GLC analysis.the analysis clearly shows the effect of

biodegradation and thermal alternation(data on Alberta oils from deroo et al,. 1977; data on west

africans oils from claret et al,. 1977)