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

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 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

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)

Historical information 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”

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

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 Determination of H/C and O/C atomic ratios

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 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

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

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 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

Purely chemical analysis 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

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

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

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