FORMATION OF ULTRALAMINAE IN MARINE KEROGENS VIA SELECTIVE PRESERVATION OF THIN RESISTANT OUTER WALLS OF MICROALGAE

Transmission Electron Microscopy (TEM) observations recently revealed the common occurrence of very thin lamellar structures, termed ultralaminae, in kerogens from source rocks and oil shales so far considered as amorphous. Ultralaminae in lacustrine kerogens were shown to derive from the selective preservation of the algaenans occurring in the very thin outer walls of various freshwater Chlorophyceae. The chemical correlation between such algaenans and fossil lacustrine ultralaminae was chiefly based on the production on pyrolysis of n-alkylnitriles with a typical distribution (bimodal, maxima at C28 and C-16). The origin of marine ultralaminae is investigated in this work. To this end, spectroscopic and pyrolytic studies were carried out on (i) the algaenan forming the very thin outer walls of Nanochlorum eucaryotum (an extant marine Chlorophycea) and (ii) two ultralaminae-containing marine kerogens from the Lower Toarcian shales of Paris basin (Fecocourt and Bray). A high contribution of polymethylenic chains probably linked via ether bridges was observed in these bio- and geopolymers. Furthermore, on pyrolysis they afforded the same n-alkylnitriles with the same distribution (unimodal, maximum at C-13, lack of C-17+ compounds). Fossil ultralaminae in marine kerogens therefore derive from the selective preservation of the alpenan-composed very thin outer walls of marine Chlorophyceae such as N. eucaryotum. Alpenans from marine and freshwater Chlorophyceae and, therefore, marine and lacustrine ultralaminae, should be characterized by sharply distinct n-alkylnitrile distributions.