Molecular characterisation of kerogen from the Kimmeridge clay formation by mild selective chemical degradation and solid state 13C-NMR

Abstract

Molecular characterisation of a kerogen from the Kimmeridge clay formation (Upper Jurassic, Dorset, U.K.) has been undertaken by integration of results from a variety of chemical techniques. Basic data have been obtained from elemental analysis, pyrolysis yields, pyrolysis-GC and a study of the solvent-soluble fraction. More detailed molecular information (particularly about the aliphatic structures present) has been gained from selective degradation using ruthenium tetroxide (RuO 4, which oxidises aromatic rings predominantly. The average molecular structure was obtained from solid state dipolar dephased 13C-CP/MAS NMR, using a peak synthesis approach based on chemical shifts of standard compounds. The RuO 4 oxidation gave a dichloromethane-soluble extract (42%), a solid residue (14%) and CO 2 (26%) based on original kerogen weight. THe extract contained a series of straight chain monocarboxylic acids (14%) in the range C 9–C 27 and α,ω-dicarboxylic acids (51%) in the range C 4–C 27, several branched chain mono- and diacids (9%), and particularly large amounts of acyclic isoprenoid acids (16%) (C 14–C 17, C 19–C 21). The NMR data revealed that the average molecular structure is mainly aliphatic (only 25.8% aromatic carbon) and contains predominantly CH 2 groups in rings or chains, or attached to oxygen. The aromatic species present are protonated (10%) (“benzene”-like), bridgehead or ring junction (8.1%) alkylated (5.8%) and phenolic (1.9%). 13C-NMR spectroscopy and ruthenium tetroxide oxidation provide conmplementary information on Kimmeridge clay kerogen, the former on the aromatic and the latter on the aliphatic component. The overall conclusion is that the carbon skeleton of the Kimmeridge clay kerogen cotnains mainly linear aliphatic chains with only small amounts of cyclic structures, a view which contrasts with some current models for Type II kerogens.