Kerogen

Kerogen - a re-cap

complex, HMr, diseminated organic matter in sediments

operational definition: OM that is insoluble in non-polar solvents (benzene/methanol, toluene, methylene chloride) and nonoxidizing mineral acids (HCl and HF)

major starting material for most oil and gas generation

sediments are subjected to heating in the subsurface - oil and gas is generated from the kerogen

most abundant form of organic carbon on earth (1000 x more than coal)

made up from altered remains of marine and lacustrine microorganisms, plants and animals - with differing amounts of terriginous debris

Kerogen

kerogen

~1% of OM originating from biological sources - forms after all degradation processes discussed earlier in this course

structured, terriginous portions of kerogen have an elemental composition similar to coal

may contain significant contributions from biopolymers altered during degradation pathways

substantial incorporation of biological macromolecules that have been transformed prior to and after burial

contains info about the depositional, geological, and geothermal history of sediments

Re-cap cont’d

Chemical and optical methods utilized

Kerogen does not migrate - so, sediment matrix and ‘kerogen’ are from same depositional and thermal history

microscopic methods - work well for structured kerogen

chemical methods - work well for ‘amorphous’ OM (usually present in greater abundance than structured)

- why do we want to know?

so we can find out the ‘petroleum-generating’ potential

Methodologies

No magic bullets

combination of chemical methods

chemical techniques provide

routine analyses in oil and gas exploration

information with regard to the origin and subsequent geological history of kerogen

can’t do both with one technique

Methods cont’d

Determination of total oil and gas generation potential directly linked to availability of hydrogen rich linkages

how easy it is to release the CH moieties

Rock Eval pyrolysis - measures gas generating potential and thermal maturation via T

max (temp at which maximum pyrolyzable

OM evolves)

microscopic characterization

qualitative

proportions of woody OM, amorphous OM etc

measurement of the Thermal Alteration Index

fluorescencevitrinite relectance (%R

o)

Screening for potential

Can not be determined by Rock Eval/microscopic techniques

Historical information

End member determination/ delineation

The more we know about the modern ‘depositional environment’ - the better it is to look at the past

Problems - when major depositional systems have changed

eg., ocean circulation patterns are different today than when most of all oil was generated

>50% of world’s petroleum was generated in the Jurassic and the Cretaceous

Chemical and optical properties tend to merge at higher maturities

GEOLOGICAL/STRATIGRAPHIC/SEDIMENTOLOGICAL reconstruction likely gives a reasonable estimate of past generation potential

Use multiple chemical/microscopic/geological techniques to understand “origin” vs. “maturation” vs. “biodegradation”

Historical information

How to know more and more about less and less.....

elemental and isotopic analysis

average-bulk structure and composition of all OM in a given sediment

qualitative, semi-quantitative, quantitative analysis

structural, spectral properties

degradative techniques

detailed characterization of well-defined subunits

pyrolysis-gas chromatography/mass spectrometry

chemical degradative schemes

Bulk gives us an average, details give us fine definition of only a very small - possibly non-representative portion of the kerogen

Kerogen Type

ask yourself “what do I need to know” - use the correct number of techniques to find the answers

How much oil and gas will be generated?

most important process is hydrogen transport

“how much elemental hydrogen is bonded to the kerogen?”

Rock Eval pyrolysis

What geological processes have been involved in the kerogen formation?

detailed chemical methods about the ‘minor fractions’ of kerogen

Py-GC/MS coupled with microscopic techniques

Which technique?

Elemental analysis

Determination of H/C and O/C atomic ratios

Different techniques

During thermal maturation/catagenesis, all kerogen types lose hydrogen and oxygen containing functional groups

progression is towards the lower left hand corner of the following plots

During low temp maturation/diagenesis ALL kerogens expel hydrogen and oxygen predominantly as water and carbon dioxide

During high temperature maturation ALL kerogens expel hydrocarbons (HC)

Elemental Analysis

Purely chemical analysis

Type I kerogen

paraffinic kerogens (produce ‘light oils’)

H/C > 1.25

O/C < 0.15

found in boghead coals and shales

contain abundant Botyococcus algae

derived from lacustrine sedimentation or tasmanite (marine equivalent)

using this criteria some Persian Gulf Cretaceous limestones are included as Type I

Type I - primarily oil prone on maturation - very rare

probably because the Type I curve merges with Type II during maturationcan only be recognized at fairly low maturation levels <0.8% R

o


Type I kerogen

paraffinic kerogens (produce ‘light oils’)

H/C > 1.25

O/C < 0.15

found in boghead coals and shales

contain abundant Botyococcus algae

derived from lacustrine sedimentation or tasmanite (marine equivalent)

using this criteria some Persian Gulf Cretaceous limestones are included as Type I

Type I - primarily oil prone on maturation - very rare

probably because the Type I curve merges with Type II during maturationcan only be recognized at fairly low maturation levels <0.8% R

o


Type II Kerogens

original reference for Type II kerogens came from the Lower Toarcian Shale of the Pris Basin

H/C < 1.3 (lower than Type I)

O/C ~ 0.03 - 0.18 (equivalent or greater than Type I)

organic-rich ancient and recent low-maturity marine sediments have predominantly Type II kerogen associated with them

the ‘reference’ kerogens generate a mix of oil and gas on maturation

immature analogs of the major kerogen types found in highly productive oil and gas fields


Type II Kerogens

original reference for Type II kerogens came from the Lower Toarcian Shale of the Pris Basin

H/C < 1.3 (lower than Type I)

O/C ~ 0.03 - 0.18 (equivalent or greater than Type I)

organic-rich ancient and recent low-maturity marine sediments have predominantly Type II kerogen associated with them

the ‘reference’ kerogens generate a mix of oil and gas on maturation

immature analogs of the major kerogen types found in highly productive oil and gas fields


Type III kerogens

H/C < 1 (relatively low)

O/C ~ 0.03 - 0.3 (relatively high)

planktonic remains are virtually absent in ‘reference’ Type III samples

significant higher plant and ‘woody’ material contributions

‘woody’, ‘coaly’, ‘vitrinitic’ or ‘humic’

Gas prone


Type III kerogens

H/C < 1 (relatively low)

O/C ~ 0.03 - 0.3 (relatively high)

planktonic remains are virtually absent in ‘reference’ Type III samples

significant higher plant and ‘woody’ material contributions

‘woody’, ‘coaly’, ‘vitrinitic’ or ‘humic’

Gas prone


Type IV / Residual Type / Inertinite

H/C always < 0.5

maturation line near the bottom of the van Krevelan axis


Type II-S

high-sulfur (8-14%) type II kerogen

source for heavy sulfur oils from the onshore and offshore Monterey Formation in California

generated at much lower maturities than observed for other kerogens

distinguished from Type II due to the higher S/C

not visually different from Type II


Problems

Type II “Systematic elemental analysis performed on a set of amorphous kerogens from various origins has shown that, although some of them belong to type II, the chemcial composition of the amorphous kerogen may spread over the entire van Krevelen diagram” Tissot 1984

Type III although chemical determinations say ‘wood’ or ‘higher plant’ from microscopic techniques it is not obvious that the higher O/C comes from plant remains/fragments


Chapter 14 of Organic Geochemistry (Engel and Macko) pp. 289-353

describes all different analytical techniques including

microscopic techniques

pyrolysis techniques

infrared spectroscopy

nuclear magnetic resonance spectroscopy (NMR)

Electron Spin Resonance (ESR) spectroscopy

Isotopic techniques

Pyrolysis -GC and Py-GCMS

Electron microscopy (diffraction)

Much more to read

Documents

Kerogen