Kerogens and asphaltenes

Relationship between kerogens and asphaltenes have taken on new meaning in the last few years especially for relatively low maturity oils. Asphaltene composition can give inferences about the source kerogen composition when kerogen data are unavailable (53.58-59). Asphaltenes are defined as materials soluble or peptized in oil or bitumen that precipitate when... 

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). 

We should caution that the above concept of the genetic relationship between kerogens and asphaltenes differs from the more historic view that asphaltenes are condensation and/or alteration products of hydrocarbons and resins. Certainly, in some petroleum processing treatments and probably at higher maturation levels in nature, various reactions do form new products with asphaltene solubility characteristics. These new condensation products may be regarded as altered asphaltenes and intermediates in the coke or pyrobitumen formation process (62-64)- Contamination of original asphaltenes by subsequently formed or altered products, of course, will result in a less definitive correlation between an asphaltene and its source kerogen. 

Figure 5. Proposed structures of alkylthiophene moieties in kerogens and asphaltenes and their presumed flash pyrolysis products. Examples are give for alkylthiophene moieties with (a) linear, (b) isoprenoid, (c) branched and (d) steroidal side-chain carbon skeletons. Carbon skeletons are indicated with bold lines.

Immature kerogens and asphaltenes contain thiophene units in their macromolecular structure, which have mainly linear, isoprenoid, branched and steroidal side-chain skeletons. These units and possibly other sulfur-... 

The different properties of sulfur-rich kerogen and asphaltenes, on the one hand, and sulfur-rich resins on the other hand (flash pyrolysis behaviour) may be explained only by differences in degree of (sulfur) cross-linking and thus by differences in molecular size and in degree of condensation. 

Table 2. Proponeil kinetic models for the Draupne formation, hosed on kerogen and asphaltenes...

The study of the matrix on pyrolysis result has an additional use besides the understanding of the origin of pyrolysate components. This is related to the influence of the matrix on the generation of specific hydrocarbons from a certain starting organic substrate under the infiuence of heat and of catalysis [46,47]. However, most of these studies are not directly related to analytical pyrolysis. In these studies, furnace pyrolysers were commonly preferred to small sample and flash pyrolysis [46]. These and other pyrolysis appiications for the study of kerogens and also of oil related components such as asphaltenes [47] have been proven extremely useful in practice [19]. 

Hartgers W. a., Sinninghe Damste J. S. and de Leeuw J. W. (1994) Geochemical significance of alkylbenzene distributions in flash pyrolysates of kerogens, coals and asphaltenes. Geochim. Cosmochim. Acta 58, 1759-1775. 

T.I. Eglinton, S.R. Latter, and J.J. Boon, Characterization of kerogens, coals and asphaltenes by quantitative pyrolysis mass spectrometry, J. Anal. Appl. Pyrolysis, 20 25 5 (1991). 

Organic geochemical polymers petroleum asphaltenes and coals (40,41) and kerogens and bitumens (42)... 

FIGURE 1.51 Pi versus Flmo for kerogens, coals, and asphaltenes (see samples in Table 1.9) with addition of oxidized samples. (From D. Joseph. L oxydation des matieres carbonSes. Thfese d Etat Orleans 1982. D. Joseph and A. Oberlin. Oxidation of carbonaceous matter, (a) Parti Elemental analysis andIR spectrometry. Carbon 21,559-564 (1983) (b) Part II X-ray diffraction and TEM. Carbon 21, 565-571 (1983). With permission.)... 

---