Kerogen isotopic composition

The isotopic composition of carbon in carbonaceous organic material (kerogen) from ancient sedimentary rocks gives information on whether photosynthetic organisms were present during rock formation or not. It can also provide information on biological activities if cellular structures had already been destroyed. Sulphur can be used in a similar way (Schopf, 1999). 

Combined stable isotope analysis ( C, D, N, has been used successfully in petroleum exploration (Stahl 1977 Schoell 1984 Sofer 1984). The isotopic composition of crude oil is mainly determined by the isotopic composition of its source material, more specifically, the type of kerogen and the sedimentary environment in which it has been formed and by its degree of thermal alteration (Tang et al. 2005). Other secondary effects like biodegradation, water washing, and migration distances appear to have only minor effects on its isotopic composition. 

Kerridge JF (1983) Isotopic composition of carbonaceous-chondrite kerogen evidence for an interstellar origin of organic matter in meteorites. Earth Planet Sci Lett 64 186-200 Kerridge JF, Haymon RM, Kastner M (1983) Sulfur isotope systematics at the 21°N site. East Pacific Rise. Earth Planet Sci Lett 66 91-100... 

Kerridge JF, Chang S, Shipp R (1987) Isotopic characterization of kerogen-hke material in the Murchison carbonaceous chondrite. Geochim Cosmochim Acta 51 2527-2540 Kharaka YK, Berry FAF, Friedman I (1974) Isotopic composition of oil-field brines from Kettle-man North Dome, California and their geologic implications, Geochim Cosmochim Acta 37 1899-1908... 

Redding CE, SchoeU M, Monin JC, Durand B (1980) Hydrogen and carbon isotopic composition of coals and kerogen. In Douglas AG, Maxwell JR (eds.) Phys Chem Earth 12 711-723 Rees CE (1978) Sulphur isotope measurements using S02and SFg. Geochim Cosmochim Acta 42 383-389... 

Sulfur, carbon and hydrogen stable isotope ratios of pyrite, kerogens, and bitumens of two high-sulfur Monterey formation samples from the onshore Santa Maria Basin in California were determined. Kerogens from these were pyrolyzed at 300°C for periods of 2, 10 and 100 hours in closed systems and the yields and isotopic compositions of S-containing fractions (residual kerogens, bitumens and hydrogen sulfide) were determined. 

Two Type II-S kerogens (as defined by Orr (i)) from the onshore Santa Maria Basin Monterey formation were pyrolyzed in this study to determine (a) the distribution of sulfur and its isotopic composition among the various products formed during artificial maturation, and (b) maturation trends reflected in the sulfur isotopic and elemental S/C ratios of kerogens, and in the variation of C and H isotopes. In addition, S isotopes in pyrites, kerogens and bitumens from the two Monterey shale samples were examined to speculate on the mode of S incorporation into Santa Maria Basin sediments. 

The samples were ground and extracted with the solvent mixture 9 1 dichloromethane/methanol for 48 hours in a soxhlet system. The extracted samples were then demineralized to isolate kerogens using a detailed procedure described in (3). Hie kerogen was next solvent extracted a second time and treated with 0.2 N nitric acid at room temperature to separate pyrite from the organic matter and the solution was kept for determination of pyrite S content and isotopic composition. X-ray diffraction was used to verify the removal of pyrite from the kerogen. 

Sulfur isotope studies have also provided insights into the transition from Archean low Po to higher values in the Proterozoic. In the same studies that revealed extremely low Archean ocean sulfate concentrations, it was found that by —2.2 Ga, isotopic compositions of sedimentary sulfates and sulfides indicate bacterial sulfate reduction under more elevated seawater sulfate concentrations compared with the sulfate-poor Archean (Habicht et al., 2002 Canfield et al., 2000). As described above, nitrogen isotope ratios in sedimentary kerogens show a large and permanent shift at —2.0 Ga, consistent with denitrification, significant seawater nitrate concentrations, and thus available atmospheric O2. 

Sulfur is shown on Figure 3 as SO3, which includes organic and inorganic sulfur. Direct observations and isotopic composition of sulfur (5) indicate that kerogen-bound sulfur accounts for 60-80% of the total sulfur in the rock while the remainder is pyritic and gypsum sulfur. 

Fig. 5.62 Maturity-related variations in the relative abundance and isotopic composition of methane (after Clayton 1991).The effects of increasing maturation and migration are shown by arrows the maturity of labile kerogen is represented by the degree of gas generation, and of refractory kerogen by vitrinite reflectance.

Fig. 6.8 Variation in carbon isotopic composition of carbonate and kerogen over the past 3.8 Gyr (after Schidlowski 1988).The samples from Isua, Greenland, have experienced metamorphism.

Fig. 11. Methane carbon isotopic profiles of associated gases from end-member undersaturated and saturated oils and representative gases from gas-condensate reservoirs. The grey region represents range of carbon isotopic compositions calculated after Clayton (1991) as a function of maturity extending from GGI = 0.1 (lower limit) to GGI = 1.0 (annotated), assuming the parent kerogen to be —24.5%o PDB.

If decarboxylation were the dominant mechanism then the (reprecipitated) carbonate cement in the cementation zone should be characterized by a lighter (5 C isotopic composition, inherited from the CO derived from kerogen, compared to that from a carbonate cement of sedimentary mineral origin. If dissociation were the dominant process then it should be controlled by the generation of H and the carbonate cement would be characterized by an S C isotope composition similar to that of marine carbonates. Furthermore, ions derived from the organic acids and in particular acetates should be encountered in the formation waters. 

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