Kerogen ratio

Schimmelmann A, Lewan MD, Wintsch RP (1999) D/H ratios of kerogen, bitumen, oil and water in hydrous pyrolysis of source rocks containing kerogen types I, II, IIS and III. Geochim Cosmochim Acta 63 3751-3766... 

The similarity of kerogen and trona acids is indicated by their elemental compositions shown in Table I. In both materials the carbon to oxygen ratio indicates the presence of considerable quantities of oxygen functional groups. 

Pyrolysis of kerogens and asphaltenes has demonstrated the nature and logic of this conversion sequence (58.63-64). Relative to elemental composition, initial asphaltenes have a much lower atomic O/C ratio, a slightly lower atomic S/C ratio, and almost the same H/C and N/C ratios as their source kerogens (53). 

Table II Carbon, sulfur, nitrogen and iron elemental data of kerogens (containing pyrite) isolated from Nflrdlinger Ries core samples. H/C, N/C and SQrg/Corg are atomic ratios.

Figure 4. S/C atomic ratios from kerogens of living plants, modem sediments, and buried sediments of the three main types of organic matter encountered in the Abu Dhabi recent sedimentary system, (A) the microbial mats, (B) the Avicennia mangrove, (C) the lagoonal seaweeds.

The Microbial Mat. The "intermediate fraction of the extracts and pyrolysates of the kerogens of a modem and buried mat were analysed by GC with coupled FID and FPD and GC-MS. In the extract of the modem mat, two isomers of a C20 thiophenic isoprenoid (compounds I and II) already reported in the literature (11-12) are the most abundant organic sulfur compounds (Figure 5B). The "intermediate" fraction of the pyrolysate of the corresponding sample, is also dominated by the same two isomers of thiophenic isoprenoids that exhibit a similar internal ratio as in the extract. The presence of a third isomer was detected in smaller quantity (compound III)(13). Other thiophenic compounds are present in the sample and were tentatively identified using mass spectra data, GC retention time, and literature (14-16). They are similar to the compounds found in the "intermediate" fraction of the pyrolysate of the buried mat (Figure 5D). 

The S/C atomic ratios of kerogens of modem and buried microbial mats covers a large range of values. The lowest S/C values were found for the modem microbial mat with thick carbonate-sand layers collected near the lagoon. Landward, as the carbonate layers get thinner and more muddy, the S/C values increase progressively, until they reach the maximum value which is found in the modem samples, in the massive mat with lumps filled with H2S. This could be explained, if we consider the aptitude of this thick mineral layers to allow water circulation, and so a better oxygenation of the system. On the contrary the massive mat with the much thinner mineral layers seems to form a much closer system as suggested by the occurrence of lumps filled with H2S. 

Partial pyrolysis-gas chromatograms of representative immature kerogens or coals from the four sequences studied are shown in Figure 2. The abundance of thiophenes relative to aliphatic and aromatic hydrocarbons in the partial FID chromatograms differs markedly for the four samples shown. This is reflected by the ratio of the peak area of 2,3-dimethylthiophene relative to those due to 1,2-dimethylbenzene and n-non-l-ene (Le. TR = [2,3-dimethylthiophene]/[l,2-dimethylbenzene+n-non-... 

Figure 7. Variation in the thiophene ratio (TR upper diagram) and Ternary plot showing the variation in the relative abundances of 2,3-dimethylthiophene, n-non-l-ene and 1,2-dimethylbenzene (lower diagram) with artificial maturation temperature for a suite of kerogen residues obtained from hydrous pyrolysis (72 hrs) of Kimmeridge kerogen (Dorset, U.K.). NB. Integrated peak areas for 2,3-dimethylthiophene also include 2-vinylthiophene.

Figure 9. Relationship between the thiophene ratio, TR, and Rock-Eval Tmax f°r samples °f varying kerogen type. NB. All samples are immature with respect to oil generation (i.e. vitrinite reflectance, R0 < 0.5%).

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. 

With increasing thermal stress, H/C and S/C elemental ratios of kerogens and S, C and H isotope ratios of pyrolysis products show systematic maturation trends. S isotope fractionation between solid, liquid and gaseous pyrolysates falls within a narrow range of about 2 per mil and mimics the variation observed in natural samples. These results confirm the utility of S isotopes towards source rock-oil correlations in sedimentary basins. 

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. 

Figure 3 Variation of carbon isotope ratios with H/C ratios in residual kerogens and bitumens generated during pyrolysis. 0 hours denotes starting kerogens and original sedimentary bitumen. 2, 10 and 100 denote heating times.

Figure 5 Variation of sulfur isotope ratios with H/C ratios in original and residual kerogens during pyrolysis.

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