Structure and Modeling of Complex Petroleum Mixtures by Chunming Xu & Quan Shi

Author:Chunming Xu & Quan Shi
Language: eng
Format: epub
Publisher: Springer International Publishing, Cham

3.4 Kerogen

Kerogen is considered as the petroleum precursor; hence, the molecular structure of kerogen is likely to resemble that of heavy petroleum. Due to the heterogeneity and complexity of kerogen, selective chemical degradations with various reagents were employed to fragment a large kerogen molecule into various smaller units in order to gain insights into the types of moieties and the nature of the bonds linking them to form the macromolecular matrix [64].

RICO reaction system has been used for structural investigation of kerogen molecules [40, 63, 64, 71, 89–97]. The degradation products from kerogen were similar to those from petroleum asphaltenes [62], comprising normal and branched α,ω-dicarboxylic acids, normal and branched monocarboxylic acids, tricyclic terpenoid acids, hopanoic acids, gammacerane carboxylic acids, pregnanoic acids, sterane carboxylic acids, and benzenemono- to hexa-carboxylic acids. However, the amounts of these compounds varied significantly from sample to sample, indicating that kerogen has a wide range of molecule compositions. Eglinton and coworkers [95, 96] subjected a variety of type I, II, and III kerogens to RICO and found that, in all cases, straight-chain acids were the predominant products. However, other studies reported a different composition of the kerogen RICO product: α,ω-dicarboxylic acids were more abundant than monocarboxylic acids [63, 67, 97].

Boucher et al. investigated a type II clay kerogen by a variety of chemical techniques. The dichloromethane-soluble extract of the kerogen RICO product contained a series of straight-chain monocarboxylic acids (14%) in the C9–C27 range, α,ω-dicarboxylic acids (51%) in the C4–C27 range, several branched chain mono- and diacids (9%), and relatively large amounts of C14–C17 and C19–C21 acyclic isoprenoid acids (16%). The RICO and 13C-NMR spectroscopy provided complementary compositional information on aliphatic and aromatic components, respectively. The finding was that the carbon skeletons of the kerogen were mainly linear aliphatic chains with relatively small amounts of cyclic structures.

Messel shale kerogen was readily used in many RICO studies. Standen et al. [98], Reiss et al. [94], Blokker et al. [99], and Dragojlović et al. [71] reported that the sources of α,ω-dicarboxylic acids (C10–C13, with a maximum at C10) in the kerogen RICO product could have been α,ω-disubstituted or α,ω-dienic aliphatic chains in the kerogen matrix and/or originated from oxidation of Cn–2 polymethylene bridges linking aromatic structural units. However, Barakat et al. [89] suggested that α,ω-dicarboxylic acids could be derived from diterminal oxidative cleavage of functionalized polymethylene chains within the kerogen (e.g., polymethylene chains linked to the kerogen matrix by ether groups) or from oxidation of functionalized aliphatic moieties (e.g., n-alkan-1-ols, aldehydes, carboxylic acids) linked to the kerogen structure and not removed by Na2Cr2O7 oxidation [89].

Yoshioka et al. [91] performed RICO on marine and lacustrine kerogens and indicated that short chain (C2–C5) α,ω-dicarboxylic acids were major compounds in the kerogen RICO products. They suggested that the precursor molecules of α,ω-dicarboxylic acids were melanoidins or unsaturated fatty acids. The maximum C9 α,ω-dicarboxylic acids were identified only in the oxidized product from the lacustrine kerogen. It could be attributed to the difference between marine and lacustrine kerogens with respect to the precursor of unsaturated fatty acids.

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