Identification of alkenylbenzenes in pyrolyzates using GC-MS and GC-FTIR techniques: evidence for kerogen aromatic moieties with various binding sites

Pyrolysis products obtained from heating a shale under vacuum have been ana lyzed for alkenylbenzenes using GC-MS and GC-FTIR, The shale pyrolysate con tains a homologous series (C-11-C-23) Of alkenylbenzenes with terminal unsa turations concomitant with lesser amounts of hitherto unknown trans alk-1-e nylbenzenes. These latter compounds are unique constituents of kerogen pyro lyzates and are suggested to be derived from alkylbenzene moieties bound to kerogen at the benzylic position. The presence of these unsaturated compou nds in pyrolyzates has been interpreted as evidence for site-specific bindi ng of alkylbenzene moieties in kerogen. Formation of alkenylbenzene moietie s is suggested to result from diagenetic cyclization/aromatization reaction s of fatty acid precursors. Incorporation of structurally different lipid p recursors such as saturated, mono- and polyunsaturated fatty acids is sugge sted to reflect the nature of the different alkenylbenzene products generat ed upon pyrolysis. The application of 'compound specific' GC-FTIR technique s, provides a new and complementary tool to the study of geochemical sample s. Compound specific IR spectra in conjunction with electron impact mass sp ectra are shown to facilitate the structural identification of hydrocarbons in geochemical samples. (C) 1999 Elsevier Science Ltd. All rights reserved .