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Asphaltenes are heteroatom rich macromolecules thought to be mainly derived from the early breaking of covalent bonds in kerogen and thus have been considered to be a lower molecular weight analog of the kerogen at the time of early oil generation. This paper reviews information on asphaltene structures and sources derived from a variety of chemical procedures including oxidation, desulfurization and various forms of pyrolysis. An additional focus is on the geochemical information that can be obtained from oxidation of asphaltene with different chemical reagents. This procedure yields not only the chemical units that make up the asphaltene but also the occluded hydrocarbons providing biomarker information on the oil from which the asphaltenes were isolated. The latter is particularly useful when studying biodegraded oils that have received a subsequent charge and where biomarker information on the first, biodegraded phase may have been preserved within the asphaltene structure and/or occlusions. A common observation is that occluded hydrocarbons show distributions of biomarkers having less mature stereochemistry than in the oil maltenes (pentane soluble fraction). Ruthenium ion catalyzed oxidation mineralizes aromatic carbons in asphaltenes leaving a residue of oxidized (acidic) alkyl and alicyclic fragments that show how the aromatic centers were bound together. In addition, occluded hydrocarbons are released with little or no oxidation. Mild oxidation with hydrogen peroxide/acetic acid degrades a limited number of bonds in the asphaltene, releasing occluded or physically trapped species with little contribution from the asphaltene structure itself. Reduction techniques include the use of alkali metal or Raney nickel and generally these are used to desulfurize asphaltenes and release hydrocarbon moieties that were bonded via sulfide linkages. Hydropyrolysis, thermal degradation in the presence of high pressure hydrogen and a Mo catalyst, results in the cleavage of saturated and aromatic asphaltene moieties comprising the asphaltene that are then available for typical biomarker analysis using gas chromatography–mass spectrometry and other approaches. These degradation products include biomarker compounds useful for making genetic correlations among samples, age dating, providing estimates of thermal maturity and deconvoluting mixtures of oils. [ABSTRACT FROM AUTHOR]

Fresh stainless steel (316) surfaces were blackened by heating at 250 °C for 2–20 h in the presence of water and argon. Headspace gas after heating contained up to 1 mol% molecular hydrogen. These results indicate that the water oxidized the metal surfaces causing the blackening and released molecular hydrogen. Oxidation must be considered as an alternative or complementary process to carburization during the seasoning of 316 stainless steel reactor vessels during hydrous pyrolysis. [ABSTRACT FROM AUTHOR]

Cyclopentanone is an important platform chemical for various industrial syntheses. It has been recently reported as a major product in biomass cellulose-derived biofuels but has rarely been reported in geochemical research. We report here the abundant occurrence of cyclopentanone, 2-cyclopenten-1-one and their alkyl derivatives in pyrolysates of organic-rich shales. Closed system pyrolyses at temperatures from 290 °C to 350 °C have been carried out on thermally immature organic-rich shale samples of Ordovician and Devonian age from Canada. GC–MS analyses of the pyrolysates show that cyclopentanones, 2-cyclopenten-1-ones and phenols dominate over hydrocarbons in the low temperature hydrous pyrolysates, and their abundance relative to hydrocarbons decreases with increasing thermal maturation. However, these low molecular weight cyclic ketones are only minor components compared with hydrocarbons in anhydrous pyrolysates, indicating that water has played an important role in the formation of these oxygen-containing compounds. In addition, n -alkan-2-ones in the range of C 6 –C 35 are also detected in the hydrous pyrolysates, but they are at much reduced concentrations compared with the cyclic ketones. The potential precursors in the shale and the likely chemical reaction mechanism for the formation of these cyclic ketones are discussed in detail in the paper. [ABSTRACT FROM AUTHOR]

Kerogen from the organic rich Eocene Huadian oil shale in NE China was subjected to sequential stepwise pyrolysis at 50 °C intervals from 310 °C to 610 °C. This allowed generation of compounds according to the strength of the bonds linking them to the kerogen and thus permitted identification of compounds that were only minor constituents of one step flash pyrolysis at high temperatures such as Curie point 610 °C. Pyrolysis at 310 °C and 360 °C yielded very small amounts of n -alkanes having a distribution similar to that of the extractable hydrocarbons, which were presumed to be occluded within the kerogen since pyrolytic products such as n -alkenes were only minor constituents. Also present was the C 18 isoprenoid ketone (6,10,14-trimethylpentadecane-2-one). The pyrolyzate at 410 °C showed a full suite of n -alkanes with a small amount of the corresponding n -alkenes, indicating that some cleavage of alkyl chains from the kerogen matrix had occurred. These showed a very low odd/even predominance apart from local maxima at n -C 23 and n -C 27 . The major component was prist-1-ene, which co-occurred with a small amount of the later eluting prist-2-ene. We speculate that the pristenes were generated from tocopherols (or their oxidized products) bound to the kerogen by single, easily cleaved, C O bonds. C 15 –C 20 isoprenoid alkanes were also observed and these might derive in part from breakdown of the isoprenoid-based algaenan produced by Botryococcus race L. The 460 °C pyrolyzate contained a dominant series of n -alkene/ n -alkane pairs as well as a series of novel long chain C 30 –C 37 mid-chain ketones showing no odd/even predominance. The C 35 ketone was most abundant, with only traces of higher homologues. Mass spectra showed only minor amounts of fragments due to different positions of the carbonyl group and the major isomer in all cases was assigned as the alkan-ω24-one. It seems likely that these ketones are derived from pyrolysis of algaenan that had been incorporated into the kerogen, although no previous reports have identified this chain length range or position of the carbonyl group. Some similarities with the pyrolysis products of algaenan derived from eustigmatophytes were noted. In addition, a series of n -alkan-2-ones were present, which also likely result from pyrolytic breakdown of algaenan. At pyrolysis temperatures of 510 °C and 560 °C, the pyrolyzates were dominated by n -alkene/ n -alkane pairs with a greater proportion of alkenes and short chain hydrocarbons at the higher temperature. Only very small quantities of hydrocarbons were liberated at 610 °C, indicating that lower pyrolysis temperatures can be used to study immature kerogens. Our results demonstrate that stepwise pyrolysis can be a useful means for studying components of kerogens that differ in bond strength and for elucidating specific substructures related to algaenan. The data confirm that preservation of algaenan in the Huadian oil shale is a key factor in explaining its high organic content. [ABSTRACT FROM AUTHOR]