Laboratory Oxidation of Fossil Organic Matter Studiedby in situ Infrared Spectroscopy, Rock-Eval Pyrolysis and Pyrolysis-Gas Chromatography-Mass Spectrometry

Fossil organic matter in Miocene and Silurian sediments was subjected to experimental oxidation, which was investigated by Rock-Eval pyrolysis, the "off-line" pyrolysis-gas chromatography-mass spectrometry combination, and continuous FTIR monitoring. The pyrolysate yield decreased during the oxidation particularly in the low-matured, predominantly aliphatic organic matter from the Miocene sediments (type I kerogen, algae-type kerogen). This suggests that aliphatic chains are preferentially oxidized, which is in agreement with the marked decrease in the intensity of the v(CH2) and v(CH3) IR bands. The n-alkane distribution in the chromatographic profile was unaffected by the oxidation; hence, the oxidation of the alkane chains was not selective. At the same time, the bands within the 1 900-1 550 cm-1 range grew in intensity for both the aliphatic and mixed-type (type II kerogen) organic matter. The oxidation of the aromatic (humic) type of organic fossil matter in the Miocene sediments (type III kerogen, coal-type kerogen) was only accompanied by very small changes in the FTIR spectra. The results of the "of-line" pyrolysis are consistent with those of the Rock-Eval pyrolysis. For all samples, the oxidation was accompanied by a gradual decrease in the hydrogen index (HI) as well as in the pyrolysis temperature maximum (Tmax). The changes in the S2/S1 ratio ("bound"-to-"free" hydrocarbons) indicate that the "free" hydrocarbons in the rocks are preferentially oxidized during the first 8-16 h of the experimental run. Subsequently, hydrocarbon chains involved in the kerogen macromolecule are attacked.