Kerogen - a re-cap

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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

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Re-cap contd

• 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

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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

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Methods contd

• 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
• cant do both with one technique

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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)

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Historical information

• Can not be determined by Rock Eval/microscopic
  techniques

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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
• gt50 of worlds 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

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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

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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

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Different techniques

• Elemental analysis
• Determination of H/C and O/C atomic ratios

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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)

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Purely chemical analysis
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Purely chemical analysis

• Type I kerogen
• paraffinic kerogens (produce light oils)
• H/C gt 1.25
• O/C lt 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 lt0.8 Ro

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Purely chemical analysis

• Type I kerogen
• paraffinic kerogens (produce light oils)
• H/C gt 1.25
• O/C lt 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 lt0.8 Ro

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Purely chemical analysis

• Type II Kerogens
• original reference for Type II kerogens came from
  the Lower Toarcian Shale of the Pris Basin
• H/C lt 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

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Purely chemical analysis

• Type II Kerogens
• original reference for Type II kerogens came from
  the Lower Toarcian Shale of the Pris Basin
• H/C lt 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

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Purely chemical analysis

• Type III kerogens
• H/C lt 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

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Purely chemical analysis

• Type III kerogens
• H/C lt 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

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Purely chemical analysis

• Type IV / Residual Type / Inertinite
• H/C always lt 0.5
• maturation line near the bottom of the van
  Krevelan axis

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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

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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

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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
• nuclear magnetic resonance spectroscopy (NMR)
• Electron Spin Resonance (ESR) spectroscopy
• Isotopic techniques
• Pyrolysis -GC and Py-GCMS
• Electron microscopy (diffraction)