CLOSED PYROLYSES OF THE ISOPRENOID ALGAENAN OF BOTRYOCOCCUS-BRAUNII, L RACE - GEOCHEMICAL IMPLICATIONS FOR DERIVED KEROGENS

Algaenans, i.e., highly aliphatic, nonhydrolysable, insoluble macromol ecular constituents, have been identified in a number of microalga cel l walls and their selective preservation shown to play a major role in the formation of numerous kerogens. All the algaenans so far examined comprise a network of long polymethylenic chains, except for the L ra ce of Botryococcus braunii. The resistant macromolecular material isol ated from the latter, termed PRE L, is based on C-40 isoprenoid chains with a lycopane-type skeleton. Recent comparative studies of PRE L an d of Botryvococcus-derived sediments provided the first example of ker ogen formation via the selective preservation of an ''isoprenoid'' alg aenan. The present study is concerned with PRE L pyrolyses in sealed g old tubes under various temperature/time conditions (260-350 degrees C , 0.5-69 h). For the conversion rates thus obtained, ranging from 30 t o 100%, a complete mass balance of the different families of pyrolysis products was established; most of the C-1 to C-40 pyrolysate constitu ents were identified and the abundances of the above compounds and the ir variations with conversion progress were determined. This study thu s allowed us (1) to derive further information about PRE L chemical st ructure (location of the ether bridges, contribution of linear chains and their relationships with the C-40 isoprenoid ones), (2) to determi ne the behaviour of this isoprenoid algaenan to thermal stress (timing of the formation of the different groups of products then released, n ature of the primary cleavages, origin and mode of formation of the se condary products, and further degradations), and (3) to show, in conne ction with previous studies, that PRE L-derived kerogens should exhibi t pronounced differences relative to standard type I kerogens, the lat ter being based on polymethylenic chains, regarding not only the struc ture of the generated products but also the timing of oil generation ( upward shift of the catagenesis zone).