1. 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

2. Re-cap cont’d
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

3. Methodologies 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

4. Methods cont’d 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

5. Screening for potential
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 Tmax (temp at which maximum pyrolyzable OM evolves)
microscopic characterization
qualitative
proportions of woody OM, amorphous OM etc
measurement of the Thermal Alteration Index
fluorescence
vitrinite relectance (%Ro)

6. Historical information
Can not be determined by Rock Eval/microscopic techniques

10. 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”

11. Kerogen Type 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

12. Which technique?
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

13. Different techniques Elemental analysis
Determination of H/C and O/C atomic ratios

14. Elemental Analysis
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)

15. Purely chemical analysis

16. 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 maturation
can only be recognized at fairly low maturation levels <0.8% Ro

18. 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 maturation
can only be recognized at fairly low maturation levels <0.8% Ro

19. Purely chemical analysis
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 ~ (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

21. Purely chemical analysis
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 ~ (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

22. Purely chemical analysis
Type III kerogens
H/C < 1 (relatively low)
O/C ~ (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

24. Purely chemical analysis
Type III kerogens
H/C < 1 (relatively low)
O/C ~ (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

25. Purely chemical analysis
Type IV / Residual Type / Inertinite
H/C always < 0.5
maturation line near the bottom of the van Krevelan axis

27. Purely chemical analysis
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

28. Purely chemical analysis
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

29. Much more to read
Chapter 14 of Organic Geochemistry (Engel and Macko) pp
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)