Changes

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 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.

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>; Katz, 1983; Peters, 1986; Crossey et al., 1986; Langford and Blanc-Valleron, 1990).

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

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

Fig. 3. Van Krevelen-type diagram for organic facies A through D, using Rock-Eval parameters on whole-rock samples. Discrimination of organic facies CD and D is analytically less definitive than others, and thus these boundaries are represented by gray shading.

Fig. 3. Van Krevelen-type diagram for organic facies A through D, using Rock-Eval parameters on whole-rock samples. Discrimination of organic facies CD and D is analytically less definitive than others, and thus these boundaries are represented by gray shading.

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).

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 />; 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 /><ref name=Dmbcki1983 />

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.