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The following descriptions of kerogen types indicate their biological input, stratigraphy, and depositional processes that control their oil-generative properties. Kerogen types are defined on H/C and O/C values (or HI and OI from Rock-Eval). In thermally immature samples, the chemically extreme kerogen types I and IV (and therefore the equivalent organic facies A and D) contain macerals having relatively uniform chemical properties. These end-members are dominated by the most and least hydrogen-rich constituents. Other kerogen types (and therefore their equivalent organic facies) are frequently mixtures of macerals. Microscopy is the method of choice for distinguishing the constituents of mixed organic matter assemblages.

The following descriptions of kerogen types indicate their biological input, stratigraphy, and depositional processes that control their oil-generative properties. Kerogen types are defined on H/C and O/C values (or HI and OI from Rock-Eval). In thermally immature samples, the chemically extreme kerogen types I and IV (and therefore the equivalent organic facies A and D) contain [[maceral]]s having relatively uniform chemical properties. These end-members are dominated by the most and least hydrogen-rich constituents. Other kerogen types (and therefore their equivalent organic facies) are frequently mixtures of macerals. Microscopy is the method of choice for distinguishing the constituents of mixed organic matter assemblages.

Before enumerating the criteria for discriminating kerogen types, it is important to consider the "mineral matrix effect." Some mineral (polar clay) constituents retard the release of hydrocarbons from powdered whole rock samples during Rock-Eval pyrolysis, under-evaluating the quantity, quality, and thermal maturation data. Although this factor, the mineral matrix effect, is well known to organic geochemists, it is frequently overlooked when interpreting Rock-Eval-dependent values used to determine kerogen type and organic facies. The mineral matrix effect occurs when polar clays react with polar organic molecules during the nonhydrous Rock-Eval procedure (<ref name=Esptl1980>Espitalie, J., M. Madec, and B. Tissot, 1980, [http://archives.datapages.com/data/bulletns/1980-81/data/pg/0064/0001/0050/0059.htm Role of mineral matrix in kerogen pyrolysis: Influence on petroleum generation and migration]: American Association of Petroleum Geologists Bulletin, v. 64, p. 59-66.</ref>; <ref name=Hrsfld1980>Horsfield, B., and A. G. Douglas, 1980, The influence of minerals on the pyrolysis of kerogens: Geochimica et Cosmochimica Acta, v. 44, p. 1110-1131.M</ref>; <ref>Orr, W. L., 1983, Comments on pyrolitic hydrocarbon yields in source-rock evaluation, in M. Bjoroy et al., eds., Advances in Organic Geochemistry 1981, p. 775-787.</ref>, <ref name=Dmbcki1983>Dembicki, H., B. Horsfield, and T. Y. Ho, 1983, Source rock evaluation by pyrolysis-gas chromatography: American Association of Petroleum Geologists Bulletin, v. 67, p. 1094-1103.</ref>; Katz, 1983; Peters, 1986; Crossey et al., 1986; Langford and Blanc-Valleron, 1990).

Before enumerating the criteria for discriminating kerogen types, it is important to consider the "mineral matrix effect." Some mineral (polar clay) constituents retard the release of hydrocarbons from powdered whole rock samples during Rock-Eval pyrolysis, under-evaluating the quantity, quality, and thermal maturation data. Although this factor, the mineral matrix effect, is well known to organic geochemists, it is frequently overlooked when interpreting Rock-Eval-dependent values used to determine kerogen type and organic facies. The mineral matrix effect occurs when polar clays react with polar organic molecules during the nonhydrous Rock-Eval procedure.<ref name=Esptl1980>Espitalie, J., M. Madec, and B. Tissot, 1980, [http://archives.datapages.com/data/bulletns/1980-81/data/pg/0064/0001/0050/0059.htm Role of mineral matrix in kerogen pyrolysis: Influence on petroleum generation and migration]: American Association of Petroleum Geologists Bulletin, v. 64, p. 59-66.</ref><ref name=Hrsfld1980>Horsfield, B., and A. G. Douglas, 1980, The influence of minerals on the pyrolysis of kerogens: Geochimica et Cosmochimica Acta, v. 44, p. 1110-1131.M</ref><ref>Orr, W. L., 1983, Comments on pyrolitic hydrocarbon yields in source-rock evaluation, in M. Bjoroy et al., eds., Advances in Organic Geochemistry 1981, p. 775-787.</ref><ref name=Dmbcki1983>Dembicki, H., B. Horsfield, and T. Y. Ho, 1983, [http://archives.datapages.com/data/bulletns/1982-83/data/pg/0067/0007/1050/1094.htm Source rock evaluation by pyrolysis-gas chromatography]: American Association of Petroleum Geologists Bulletin, v. 67, p. 1094-1103.</ref><ref name=Ktz1983>Katz, B. J., 1983, Limitations of Rock Eval pyrolysis for typing organic matter: Organic Geochemistry, v. 4, p. 195-199.</ref><ref>Peters, K. E., 1986, [http://archives.datapages.com/data/bulletns/1986-87/data/pg/0070/0003/0300/0318.htm Guidelines for evaluating petroleum source rocks using programmed pyrolysis]: American Association Petroleum Geologists Bulletin, v. 70, p. 318-329.</ref><ref>Crossey, L. J., E. S. Hagan, R. C. Surdam, and P. W. Lapointe, 1986, Correlation of organic parameters derived from elemental analysis and programmed pyrolysis of kerogen: Society of Economic Paleontologists and Mineralogists, p. 36-45</ref><ref>Langford, F. F., and M. M. Blanc-Valleron, 1990, [http://archives.datapages.com/data/bulletns/1990-91/data/pg/0074/0006/0000/0799.htm Interpreting Rock-Eval data using graphs of pyrolizable hydrocarbons vs. total organic carbon]: American Association Petroleum Geologists Bulletin, v. 74, p. 799-80</ref>

Fig. 2. Modified Van Krevelen diagram for organic facies A through D. (After Jones, 1987.)

[[File:KerogenTypeFig2.png|thumb|350px|{{figure number|2}}Modified Van Krevelen diagram for organic facies A through D. (After Jones.<ref>Jones, R. W., 1987, Organic Facies, in J. Brooks and D. H. Welte, eds., Advances in Petroleum Geochemistry, v. 2, Academic Press, London, p. 1-90.</ref>)]]

Pioneers of pyrolysis found that some minerals inhibit hydrocarbon expulsion during whole-rock pyrolysis and not during kerogen pyrolysis.<ref name=Esptl1980 /><ref name=Hrsfld1980 /><ref name=Dmbcki1983 /> The effect of different matrix constituents<ref name=Esptl1980 /><ref name=Hrsfld1980 /><ref name=Dmbcki1983 /><ref name=Ktz1983 /> varies from strongest to weakest: illite > Ca-bentonite > kaolinite > Na-bentonite > calcium carbonate > [[gypsum]].<ref name=Esptl1980 /> Variations in the mineral matrix effect related to organic richness occur in whole-rock samples with TOC values less than 10%.<ref name=Esptl1980 /><ref name=Hrsfld1980 /><ref name=Dmbcki1983 />

 

Pioneers of pyrolysis found that some minerals inhibit hydrocarbon expulsion during whole-rock pyrolysis and not during kerogen pyrolysis (<ref name=Esptl1980 />; <ref name=Hrsfld1980 />; Dembicki et al., 1983). The effect of different matrix constituents (<ref name=Esptl1980 />; <ref name=Hrsfld1980 />; Dembicki et al., 1983; Katz, 1983) varies from strongest to weakest: illite > Ca-bentonite > kaolinite > Na-bentonite > calcium carbonate > gypsum.<ref name=Esptl1980 /> Variations in the mineral matrix effect related to organic richness occur in whole-rock samples with TOC values less than 10% (<ref name=Esptl1980 />; <ref name=Hrsfld1980 />; Dembicki et al., 1983).

Geological thermal maturation processes differ from those of Rock-Eval pyrolysis. Whole-rock Rock-Eval samples are heated rapidly in an anhydrous environment. Geological burial processes cause clays to undergo physical and chemical alteration usually preceding the slow and systematic thermal conversion (generation) of kerogen to petroleum. These changes occur in hydrous environments, which probably reduce the reactive capabilities of clays, usually before significant hydrocarbon generation has occurred. Nevertheless, some degree of mineral matrix effect probably does persist under geological conditions.

Geological thermal maturation processes differ from those of Rock-Eval pyrolysis. Whole-rock Rock-Eval samples are heated rapidly in an anhydrous environment. Geological burial processes cause clays to undergo physical and chemical alteration usually preceding the slow and systematic thermal conversion (generation) of kerogen to petroleum. These changes occur in hydrous environments, which probably reduce the reactive capabilities of clays, usually before significant hydrocarbon generation has occurred. Nevertheless, some degree of mineral matrix effect probably does persist under geological conditions.

===Kerogen Type I===

===Kerogen Type I===

Kerogen type I is predominantly composed of the most hydrogen-rich organic matter preserved in the rock record. Often the organic matter is structureless (amorphous) alginite and, when immature, fluoresces golden yellow in ultraviolet (UV) light. A large proportion of type I kerogen can be thermally converted to petroleum and therefore is rarely recognizable in thermally mature or postmature rocks. Sometimes in thermally immature rocks, morphologically distinct alginite is structurally or chemically assignable to specific algal or bacterial genera. These organic-walled microfossils have high H/C values because they formed hydrocarbons biologically. Some examples of pure assemblages with type I kerogen properties include the following: (1) the lacustrine alga Botryococcus braunii, which sometimes retains its diagnostic cup-and-stalk colonial morphology and/or its unique chemical compound, botryococcane (Moldowan and Seifert, 1980); (2) Tasmanites spp., which are low-salinity, cool water, marine algal phyto-plankton with unique physical features (Prauss and Reigel, 1989); and (3) the Ordovician marine organic-walled colonial microfossil Gloeocapsomorpha prisca, with its diagnostic physical appearance and unique chemical signature (Reed et al., 1986). Where kerogen type I is widespread, it is mapped as organic facies A. It usually forms in stratified water columns of lakes, estuaries, and lagoons.

[[Type I kerogen|Kerogen type I]] is predominantly composed of the most hydrogen-rich organic matter preserved in the rock record. Often the organic matter is structureless (amorphous) alginite and, when immature, fluoresces golden yellow in ultraviolet (UV) light. A large proportion of type I kerogen can be thermally converted to petroleum and therefore is rarely recognizable in thermally mature or postmature rocks. Sometimes in thermally immature rocks, morphologically distinct alginite is structurally or chemically assignable to specific algal or bacterial genera. These organic-walled microfossils have high H/C values because they formed hydrocarbons biologically. Some examples of pure assemblages with type I kerogen properties include the following: (1) the lacustrine alga Botryococcus braunii, which sometimes retains its diagnostic cup-and-stalk colonial morphology and/or its unique chemical compound, botryococcane;<ref>Moldowan, J. M., and W. K. Seifert, 1980, First discovery of botryococcane in petroleum: Chemical Communications, v. 34, p. 912-914.</ref> (2) Tasmanites spp., which are low-salinity, cool water, marine algal phyto-plankton with unique physical features;<ref>Prauss, M., and W. Riegel, 1989, Evidence from phytoplankton associations for causes of black shale formation in epicontinental seas: Neues Jahrbuch fur Geologie und Palaontologie, Monatshefte, v. 11, p. 671-685.</ref> and (3) the Ordovician marine organic-walled colonial microfossil Gloeocapsomorpha prisca, with its diagnostic physical appearance and unique chemical signature.<ref>Reed, J. D., H. A. Illich, and B. Horsfield, 1986, Biochemical evolutionary significance of Ordovician oils and their source: Organic Geochemistry, v. 10, p. 347-358.</ref> Where kerogen type I is widespread, it is mapped as organic facies A. It usually forms in stratified water columns of lakes, estuaries, and lagoons.

Kerogen type I is concentrated in condensed sections where detrital sediment transport is low and primarily pelagic. Condensed sections occur in offshore facies of transgressive systems tracts in marine and lacustrine settings. Although this extension of terminology from marine to lacustrine environments may be unfamiliar at first, lacustrine rocks are formed by the same dynamic processes that form marine rocks (i.e., sediment supply, climate, tectonics, and subsidence), although changes in lake levels often reflect local changes in runoff, evaporation, and sediment basin filling rather than the global and relative sea level changes postulated for marine sediments (Haq et al., 1988).

Kerogen type I is concentrated in condensed sections where detrital sediment transport is low and primarily pelagic. Condensed sections occur in offshore facies of transgressive systems tracts in marine and lacustrine settings. Although this extension of terminology from marine to lacustrine environments may be unfamiliar at first, lacustrine rocks are formed by the same dynamic processes that form marine rocks (i.e., sediment supply, climate, tectonics, and subsidence), although changes in lake levels often reflect local changes in runoff, evaporation, and sediment basin filling rather than the global and relative sea level changes postulated for marine sediments.<ref>Haq, B. U., J. Hardenbohl, and P. K. Vail, 1988, Mesozoic and Cenozoic chronostratigraphy and cycles of sea-level Change, in C. K. Wilgus et al., eds., Sea-Level Changes: An Integrated Approach, SEPM Special Publication 42, p. 71-108.</ref>

===Kerogen Type II===

===Kerogen Type II===

Kerogen type II in its pure (monomaceral) form is characterized by the relatively hydrogen-rich maceral exinite. Examples include spores and pollen of land plants, primarily marine phytoplankton cysts (acritarchs and dinoflagellates), and some land plant components such as leaf and stem cuticles. As with kerogen type I, the occurrence of kerogen type II depends on high biological productivity, ow mineralic dilution, and restricted oxygenation. The pure exinitic kerogen type II is preserved in condensed sections and represents macerals that are slightly less hydrogen rich than kerogen type I.

[[Type II kerogen|Kerogen type II]] in its pure (monomaceral) form is characterized by the relatively hydrogen-rich maceral exinite. Examples include spores and pollen of land plants, primarily marine phytoplankton cysts (acritarchs and dinoflagellates), and some land plant components such as leaf and stem cuticles. As with kerogen type I, the occurrence of kerogen type II depends on high biological productivity, ow mineralic dilution, and restricted oxygenation. The pure exinitic kerogen type II is preserved in condensed sections and represents macerals that are slightly less hydrogen rich than kerogen type I.

Kerogen type II can also be formed from partial degradation of type I kerogen or from varying mixtures of type I and types II, III, and IV. For example, organic matter formed in different provenances can be combined, such as when planktonic algal material falls into sediments containing transported woody macerals (kerogen type III). Kerogen type II is recorded in transgressive systems tracts, sometimes landward of type I kerogen deposition.

Kerogen type II can also be formed from partial degradation of type I kerogen or from varying mixtures of type I and types II, III, and IV. For example, organic matter formed in different provenances can be combined, such as when planktonic algal material falls into sediments containing transported woody macerals (kerogen type III). Kerogen type II is recorded in transgressive systems tracts, sometimes landward of type I kerogen deposition.

===Kerogen Type III===

===Kerogen Type III===

Kerogen type III contains sufficient hydrogen to be gas generative but not enough hydrogen to be oil prone. In its pure form, it is composed of vitrinite, a maceral formed from land plant wood. As with other intermediate kerogen types, however, various maceral mixtures or degradational processes can contribute to kerogen type III formation. Coal-forming environments represent several different kerogen types. Most coals form in paralic swamps and abandoned river channels. Vail et al. (in press) find that in regions where sediment supply is low, incised valleys contain these sediments as estuarine or coastal plain deposits.

[[Type III kerogen|Kerogen type III]] contains sufficient hydrogen to be gas generative but not enough hydrogen to be oil prone. In its pure form, it is composed of vitrinite, a maceral formed from land plant wood. As with other intermediate kerogen types, however, various maceral mixtures or degradational processes can contribute to kerogen type III formation. Coal-forming environments represent several different kerogen types. Most coals form in paralic swamps and abandoned river channels. Vail et al. (in press) find that in regions where sediment supply is low, incised valleys contain these sediments as estuarine or coastal plain deposits.

===Kerogen Type IV===

===Kerogen Type IV===

Kerogen type IV is a term not universally employed by organic geochemists because it is difficult to distinguish type IV from type III using only Rock-Eval pyrolysis. It is an inert (does not generate hydrocarbons) end-member on the hydrocarbon generative spectrum. Kerogen type IV is composed of hydrogen-poor constituents such as inertinite, which is detrital organic matter oxidized directly by thermal maturation including fire (charcoal) or by biological or sedimentological recycling.

[[Type IV kerogen|Kerogen type IV]] is a term not universally employed by organic geochemists because it is difficult to distinguish type IV from type III using only Rock-Eval pyrolysis. It is an inert (does not generate hydrocarbons) end-member on the hydrocarbon generative spectrum. Kerogen type IV is composed of hydrogen-poor constituents such as inertinite, which is detrital organic matter oxidized directly by thermal maturation including fire (charcoal) or by biological or sedimentological recycling.

==See also==

==See also==

[[Category:Evaluating source rocks]]

[[Category:Evaluating source rocks]]

[[Category:Geochemistry]]

[[Category:Geochemistry]]