Composition of Petroleum H/C:

Petroleum is mixture of Hydrocarbons, Hydrogen and Carbon are an essential components.

The Hydrocarbon on the basis of their structure can occur in nature as:

·         But the principle series occur in the petroleum are, Alkane(Paraffin) and Cycloalkane   (Nephthane).

Aliphatic Compounds:
The compounds that have no benzene or similar ring in its structure,and have the following two principle compounds within petroleum:
1. Alkanes (Paraffin)
2. Cycloalkanes (Nephthenes)
1. Alkanes (Paraffins):

·         (Methane Series) are saturated Hydrocarbons, having general formula: CnH2n+2.

·         Compound starts from methane and through homologue series witheach heavier molecule than the one above it by adding CH2.

The series:

Methne     CH4

Ethane      C2H6

Propane    C3H8

Butane      C4H10

Pentane    C5H12

Hexane     C6H14

Heptanes   C7H16

Octane      C8H18

Nonane     C9H20

Decane     C10H22

And so on………..

·         At STP; the first four are gaseous in nature.

·         First liquid is n-pentane (C5H12)

·         First solid is n-hexadecane (C16H34

·         From 16th onward are high molecular weight hydrocarbon called as
“Solid Paraffin Waxes”.

2.Cycloalkane(Nephthene):

·         (Cycloparaffin) are cyclic chain saturated Hydrocarbons, having general

                                                             Formula: CnH2n

·         Ring cannot posses fever than three carbon.

·         At STP; cyclopropane (C3H6) and cyclobutane (C4H8) are gaseous

·         Cyclopentane (C5H10) and cyclohexane (C6H12) are in liquid in
most of the crude oil.

Note: by combining above two main series within petroleum, we can divide Aliphatic compounds into three types based on ratio between Alkanes and Cycloalkanes as:

Paraffin based Aliphatic Compounds: ratio of paraffin greater than cycloparaffin (when fractioned leaves wax as residuum)

Cycloparaffin based Aliphatic Compounds: ratio of cycloparaffin greater than paraffin (when fractioned leaves asphalt as residuum)

Mixed Aliphatic Compounds: if ratio between paraffin and cycloparaffin are equal.

Besides, Alkane and Cycloalkane we have important petrochemical compounds: those which are formed during fractional distillation of crude oil (they are manufactured). They never exceeds in crude oil more than trace amount. They are unstable and highly reactive.

They are very common in living matter (many natural oils, vitamins A, pigments, rubber, sterols, etc.). These compounds are:

Alkenes (Olefins)

These are isomers of cycloparaffins (Cycloalkanes) are unsaturated Hydrocarbons, having general formula: CnH2n

First three members are ethene (C2H4), propene (C3H6), butene (C4H8). Methene is not possible due to single carbon.

Alkynes

Unsaturated hydrocarbons, having general formula CnH2n-2

Aromatic Compounds

The compounds that have benzene or similar ring in its structure, and have formula CnH2n-6

They may range in crude oil 1-10%.

The density of a compound depends on Aromacity: the ratio between aromatic carbon to total carbon (A=Aromatic C/Total C).

High aromacity means low in density and vice versa.

Aromacity low in lubricating oil while high in gasoline, toulene.

Geochemical Fossils

Some crude oil do not contain paraffins and asphalts, instead contains branching saturated hydrocarbons called as geochemical fossils, produced by action of certain organism.

These are helpful in locating the source on the basis of structure. These come from animals and plants.

High molecular weight having formula CnH2n-x ratio between C and

H < 2.  ’

Types of Geochemical Fossils

Long Chain Geochemical Fossils: derived from waxes of higher plants

Complex Ring Alcohols: based on cyclohexane, and are derived from plants and animals

Triterpentinoid Alcohols:

are aromatic compounds based on Terpene: organic compound having basic structure (C10H16) or multiple of (C10H16) e.g., steroid having formula (C30H50OH) as an alcohol.

Acyclic Isoprenoids:

compounds based on isoprene, chemical composition (C5H8), a compound may have multiple of isoprene.

Isoprenoids form long branched and saturated molecule, branching is every 4th carbon having methyl group attached.

Occurs in sediments with abundant derivatives from chlorophyll from plants and hemin from animals.

They are useful indicators of oil origin. Ratio of Pristane: Phytene, measured chromatographically. Indicates type of organic matter from which oil is derived and also measured maturity of source.

Pristane (2,4,6,10,14 Tetra Methyl Pentadecane; having nineteen carbon)

Phytene (Tetra Methyl Hexadecane; having twenty carbon)

If ratio is high, indicates organic matter derived from continent. Therefore ratio is a vital finger prints for crude oil.