Geology Reference
In-Depth Information
Chapter 5
Molecular Transformation of Plant
Biopolymers in High P-T Conditions
Abstract Experimental heating of plant tissues (350 °C, 700 bars) generated a
resistant non-hydrolysable aliphatic macromolecule similar to that comprising
organic matter in ancient sediments and fossil leaves. Comparison of the products
derived from such heating of different pre-treated plant tissues clearly demonstrates
that solvent-extractable and hydrolysable lipids are precursors of the generated
macromolecular material. Thus, these experiments indicate that labile alkyl com-
pounds can be a source of the insoluble aliphatic component of fossil organic matter
in the absence of a resistant aliphatic precursor (e.g. cutan) in the living organism.
Keywords Maturation ￿ Aliphatic macromolecule ￿ Fossil organic matter ￿ Kerogen ￿
Pyrolysis
Introduction
Ancient sedimentary organic matter is formed by diagenetic and catagenetic
alteration of biological material, typically yielding a non-hydrolysable, organic
solvent insoluble macropolymer called kerogen (Tissot and Welte 1984 ). The com-
position and type of kerogen is heavily dependent on the nature of the biological
input, the environment of deposition and the diagenetic pathway (de Leeuw and
Largeau 1993 ) but many kerogens are highly aliphatic (especially Type I/II) and
serve as a source of petroleum products during thermal maturation. Thus, the path-
ways and mechanism of kerogen formation and preservation are critical to the for-
mation of fossil fuel deposits, have important implications for the global carbon
cycle and are essential processes in the preservation of macroscopic and morpho-
logically intact organic materials in the fossil record.
Kerogen formation is generally attributed to neogenesis (Tissot and Welte 1984 )
in which sedimentary organic matter is preserved by random intermolecular poly-
merisation and polycondensation of biological residues, natural vulcanisation which
involves the reaction between reduced sulfur and various functional groups in organic
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