Temperature and Time Effects on Free Radical Concentration in Organic Matter: Evidence from Laboratory Pyrolysis Experimental and Geological Samples

Abstract

The free radicals concentration (N g ) of organic matter evolves with the temperature, time, kerogen types and other factors for the individual kerogens. This paper focuses on the N g evolution of immature type I and II kerogens and a type III coal under the laboratory pyrolysis temperature and heating time. The experiments were carried out in a closed system pyrolysis at an isothermal reactor in the temperature range from 300 to 500°C over 30 to 480 minutes. A temperature time index (TTI) is applied and the TTI values were calculated in order to indicate the pyrolysis temperature and time at which the kerogen had experienced. Some equations were established to show the relationship between the TTI and N g of type I, II and III kerogens, based on the tested data from laboratory anhydrous pyrolysis experiments of three kerogen types and geological type I kerogen samples. However, the evolution of free radicals concentration with temperature and time shows some differentials between different types of kerogen samples. This paper also discusses the different TTI-N g relation of type I kerogen between laboratory pyrolysis experimental and geological samples, which are collected from core samples with depth interval of 1137 to 4090.6 m. The TTI-N g equation from pyrolysis experimental samples should be calibrated by the data from geological samples when the equation is to be extended to the geological timescales. As the free radical concentration in organic matter is strongly maturity dependent, the relationship between TTI-N g may prove to be a valuable method to study paleotemperature of sedimentary basins.Keywords: Free radicals, Vitrinite reflectance (R o ), Kerogen, Temperature Time Index (TTI), Thermal history, Pyrolysis experiment INTRODUCTIONFree radicals are units with an unpaired electron in organic matter. Kerogen, the portion of sedimentary organic matter not soluble in organic solvents, is believed to have analogous structures in which numerous aromatic rings form angularly conjunctive aromatic sheets, with substituents (such as paraffinic side chains) attached at the sheet margins (Tisssot and Welte, 1984). Thermal cracking of paraffinic chains from the kerogen molecules initially forms alkyl and kerogen free radicals (Wang and Chen, 1988). Smaller alkyl radicals are highly unstable and are thus rapidly quenched by hydrogen extracted from the surrounding environment. However, the larger kerogen free radical is stabilized by the extended aromatic system and can be stable through geological time. Although there have been various structure models for different types of kerogen at different level maturity, the main chemical structure of kerogen include the core structures and linkages. Kerogen is defined to be a complex geopolymer and a heterogeneous macromolecule comprised of randomly cross-linked core structures that may be aliphatic, naphthenic, aromatic and/or heteroatomic . The chemical structures of kerogen and the free radicals reactions have been studied widely (Siskin et al., 1995;Sun and P...

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