Kerogenoperational 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
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
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
Methods cont’d
No magic bullets
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
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
measurement of the Thermal Alteration Index
fluorescence
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”
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
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
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
Different techniques
Elemental analysis
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)
Purely chemical analysis
Purely chemical analysis
Type I kerogen
H/C > 1.25
O/C < 0.15
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
Purely chemical analysis
Type I kerogen
H/C > 1.25
O/C < 0.15
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
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 ~ 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
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 ~ 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
Purely chemical analysis
Type III kerogens
planktonic remains are virtually absent in ‘reference’ Type III samples
significant higher plant and ‘woody’ material contributions
‘woody’, ‘coaly’, ‘vitrinitic’ or ‘humic’
Gas prone
planktonic remains are virtually absent in ‘reference’ Type III samples
significant higher plant and ‘woody’ material contributions
‘woody’, ‘coaly’, ‘vitrinitic’ or ‘humic’
Gas prone
H/C always < 0.5
maturation line near the bottom of the van Krevelan axis
Purely chemical analysis
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
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
Much more to read
Chapter 14 of Organic Geochemistry (Engel and Macko) pp. 289-353
describes all different analytical techniques including
microscopic techniques
pyrolysis techniques
infrared spectroscopy
Isotopic techniques