OCCURRENCE AND ORIGIN OF ULTRALAMINAR STRUCTURES IN AMORPHOUS KEROGENS OF VARIOUS SOURCE ROCKS AND OIL SHALES

Forty low maturity, type I-II kerogens from source rocks and oil shales, ranging in age from Infra-Cambrian to Miocene, were examined by transmission electron microscopy (TEM). When previously observed by light microscopy and/or UV fluorescence microscopy the above samples, like a very large number of kerogens, seemed homogeneous and amorphous. In fact, TEM revealed the presence of "ultralaminar" structures in 22 of these "amorphous" kerogens. The abundance of "ultralaminae" depends on the considered sample and some of the tested kerogens are nearly exclusively composed of "ultralamina" accumulations. Based on thickness, three main groups of "ultralaminae" can be identified. Parallel studies on extant algae indicated that some of the above "ultralaminae" likely derived from the resistant outer walls of microalgae. The fossilization of such outer walls should occur via the "selective preservation" pathway and result in the formation of highly aliphatic "ultralaminar" kerogens. The lack of "ultralaminae" in Infra-Cambrian kerogens is consistent with a cyanobacterial origin.