KEROGEN PRECURSORS: CHEMICAL AND BIOLOGICAL ALTERATION OF LIPIDS IN THE SEDIMENTARY SURFACE LAYER

Abstract

A study of the processes operating upon organic molecules in present day sediments can illuminate kerogen-petroleum inter-relationships. The starting point for kerogen formation is not the initial biological inputs to a sediment but rather those organic molecules which survive the active upper 1 to 2 cm of surface sediment where biological diagenesis is most marked. Depth profile studies on fatty acids, alcohols, sterols, and dicarboxylic acids emphasise the differing sensitivity of lipid classes to chemical and biological processes. Distribution patterns for these lipids in older sediments thus reflect the changed weightings which result from utilisation and preservation within the biologically active zone.Environmental effects are discussed in terms of stability of chlorophyll-a and bacteriochlorophyll-a molecules in sediments taken from temperate zones and contrasted with those in sediments from tropical zones. Whilst demetallation is an important degradative pathway for bacteriochlorophyll-a, this is not the case for chlorophyll-a. In this latter molecule the pigment ring system is chemically unstable, but the phytyl ester appears to remain substantially unhydrolysed. The data given in the tables bear on alkane genesis and on the origin of the isoprenoid alkane biological markers. The consequences of this type of study for side chain structures in the coal macromolecule and industrial conversion processes are noted.