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>; Orr, 1983, Dembicki, et al., 1983; 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>, Dembicki, et al., 1983; 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.)