Kerogen, Its types and Enviornment of Depositon

Kerogen, Its types and Enviornment of Depositon

“The organic content of sedimentary rock that is neither soluble in aqueous alkaline solvents nor in the common organic solvents”

It is important organic form on earth surface

1000 times more abundant than coal and petroleum in reservior

50 times more abundant than bitumen and other dispersed petroleum in non reservior rocks

In ancient rocks the kerogen is about 80-99% of total organic content within a rock rest bitumen (Fig. p:124)

Kerogen (O.M) may be humic: plant contributors or Sapropelic: amorphous (after destruction)

Composition

For every 1000 C atoms; there are;

500-800: H atoms

50-300: O atoms

5-30: S atoms

10-35: N atoms

Isolation of Kerogen

Two methods

Physical Method

Chemical Method

I. Physical Method

Two methods; (a) difference in specific gravity (sink-float method)

(b) differential wetting of kerogen and minerals by two immiscible liquid- oil + water (Quass method)

Advantage: no chemical alteration

Disadvantage: recovery incompelete; fractionation of kerogen may occur.

II. Chemical Method

Destruction of inorganic material by HCl/HF

Acid treatment in mild condition in Nitrogen atmosphere

Should avoid alteration of kerogen by chemical reaction; by process of elimination of pyrite

CCl4, CHCl3 preffered over HNO3 or LiAlH4 or NaBH4 to avoid alteration of kerogen

Pyrite, rutile, zircon left behind from treating the sample, they do no hinder the analysis.

Structure of Kerogen

A general structure of Kerogen; as proposed by various data collected from various analytical techniques

Three dimention macromolecule contains;

Nuclei; cross linked by chain like bridges

Bridges

a) Nuclei

The stacks, which are building blocks of kerogen, contains:

Two to four, more or less parallel aromatic sheets

Each sheet contains; less than 10 condensate aromatic rings; including occasional heterocycles containing; N, S, O

Nuclei bear; Alkyl chain (linear + branched), Nephthane rings, Various functional groups

b) Bridges

Contains structural arrangements of;

Aliphatic chain: -(CH2)n- linear or branched

Oxygen, sulphur functional groups

Ketones (-CO-)

Ester (-COO-)

Ether (-O-)

Sulfides (-S-)

Disulphide (-S-S-)

Combination of (-R) with functional group.

Superficial funtional, substitute on nuclei or chain such as -(OH), carboxly (-COOH), methoxy (-OCH)

Types of Kerogen and Depositional Environment

After isolation; can be studied with help of natural light reflectance and transmittance or UV florescence to determin the grade of evolution

Three basic elements (C, H, O), having graph H/C vs O/C determine the group of kerogen

Kerogen taken; from same formation within various depths, the group along curve called “evolution path” (related with environment of depositon)

Different cuves starts with different H:O ratios

The plot was first used by Van Krevelen (1961) to characterized coal and their coalification path: thus used for kerogen path and called as “Van Krevelen Diagram” (Fig)

 Feature of Kerogen-I

High H/C ratio (> 1.5)

Low O/C ratio (< 1.0)

Mostly lipids and aliphatic chain compounds; poly-aromatic nuclei and hetero-atomic bond is low compared to other types. Small amount of O present in ester bond

On pyrolysis (550-600); plenty of volatile extractable compounds (upto 80% by weight), high yeild oil are obtained

Enviornment of Deposition

High proportion lipids either:

(a) selective accumulation of algae material (which shows lacustrine

condition) or (b) severe biodegradation of OM other than (lipid +

waxes)

Type 1 kerogen is relatively rare as compared to other types

Feature of Kerogen-II

Frequent in many petroleum source rocks and oil shales

High H/C ratio

Moderate O/C ratio

Mostly poly-aromatic nuclei, hetro-atomic ketones and carboxylic group are important than they are in type-I, but less than type-III.

Ester bond abundant, saturated material along with abundant aliphatic chain of moderate length and napthenic rings.

Sulphur also present as substantial amount; located in heterocycles and probably also sulphid bonds and is associated with bitumen, the cyclic structure (napthanic, aromatic or thiophenic)

On pyrolysis yield oil but low compared to Type-I Kerogen, but still is commercial, do not contain much of volatile but contains sulpher

Environment of Deposition

Developed through organism (mixture of phytoplankton, zooplanktons and other micro-organism), which survive in marine conditions; mostly autochthonous

Feature of Kerogen-III

Low H/C ratio (> 1.0)

High O/C ratio (0.2-0.3)

Contains an important porportion of poly-aromatic compounds, less amount of hetro-atomic and carbonxylic compounds. Ester are totally absent from it

Non-Carboxylic oxygen may included in ether bond, contain aliphatic group (minor constitutents of OM; consists of few long chain, originated from high plant waxes, methyl group and other short chain)

On pyrolysis; it is confirmed that it is not suitable for the formation of commercial oil, however, it produces more gas which is commerical

Enviornment of Deposition

Based on organic remains, determin that it has been derived from land, i.e., continental origin.

Feature of Kerogen-IV  Low H/C ratioLow O/C ratio. On pyrolysis produces very low amount of oil but may produce enough gas.