Geochemical Fossils and Their Significance in Petroleum Formation

Geochemical Fossils and Their Significance in Petroleum Formation

“saturated hydrocarbons having branching chain structure produced by certain organism action”

These come from plant and animals: these are helpful in locating the source on the basis of structure

High molecular having formula: Cn H2n-x: ratio between C:H<2

There are various types of geochemical fossils

Long Chain Paraffin

Complex Ring Alcohols

Triterpentinoid Alcohols

Acyclic Isoprenoids

1. Long Chain Paraffin  Derived from waxes of higher plants

2.Complex Ring Alcohols

Based on cyclohexane and are derived from plants and animals

3.Triterpentinoid Alcohols

These are aromatic compounds based on terpene

Terpene: organic compound having basic structure (C10 H16) or Multiple of (C10 H16) e.g. Steroid having formula (C30 H50 OH) as an alcohol of terpene

4.Acyclic Isoprenoids

Compounds based on isoprene: chemical composition is (C6 H8)

Compounds may have multiple of isoprene

Isoprenoid: formed long branched and saturated molecule, branching is every 4th cation of isoprene having methyl group

Occurs in sediments with abundance derivatives from chlorophyl from plants and hemin form animals

Significance

They are useful indicators of oil origin

Typical example are Pristane and Phytene

Pristane: 2,6,10,14 Tetra methyl pentadecane, it has nineteen carbon in

skeleon

Phytene: Tetra methyl hexadecane, it has twenty carbon in its skeleton

 Ratio of Pristane and Phytene (Pristane:Phytene)

ratio measured chromatographically from oil or rock sample, it indicates the organic matter from which oil is originated or thermal maturation of source.

High ratio indicates large contribution form teristrial OM

The ratio is therefore a vital “finger prints” for any crude oil, containing both molecule.

Finger prints: because it has to identify the source of OM

Catagenesis

Principle Stage of oil formation

T continue to increase, bonds break down e.g., ester and some C-C bonds

H/Cs molecules, and particularly aliphatic chains, are produced from kerogen and from previously generated N, S, O compounds

Some H/Cs are C15-C30, which are comparable to geochemical fossils, entrapped in kerogen or linked by ester bonding.

However, most of new molecule has medium to low molecular weight

This is priciple zone of oil formation, as described by Vassoevich, 1969, however, also some significant amount of gas.

Principle Stage of Condensate and Wet Gas Formation

P-T continues to increase, breaking of C-C bonds occur more frequently

Light hydrocarbons are generated through cracking from remaining kerogen and in petroleum; the quickly dominated compound is methane.

Overall transformation is equivalent to disproportionation;

On one hand H/Cs increasing in Hydrogen content are genereated having average atomic ratio H/C is 1.5 to 2.0 in crude oil and 4.0 in methatne

 On the other hand, the residue kerogen depleted in hydrogen H/C of about 0.5 by the end of catagenesis.

Metagenesis: Dry gas Zone

After most liable material has been eliminated through catagenesis, however, in this stage no significance amounts of H/Cs are generated except some methane

Which may result from cracking of source rock H/Cs and reserviored liquid petroleum