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Chemical evidence of kerogen formation in source rocks and oil shales via selective preservation of thin resistant outer walls of microalgae: Origin of ultralaminae

Geochimica et Cosmochimica Acta
Publication Date
DOI: 10.1016/0016-7037(91)90162-x


Abstract New structures, termed ultralaminae, were recently observed by Transmission Electron Microscopy, usually in high amounts, in a number of kerogens from oil shales and source rocks. Morphological similarities were noted between ultralaminae and the thin (ca. 15 nm) resistant outer walls, composed of non-hydrolyzable macromolecules (algaenans), commonly occurring in extant Chlorophyceae, especially in the cosmopolitan genus Scenedesmus. Identification of the pyrolysis products of S. quadricauda algaenan showed (i) a highly aliphatic structure based on a macromolecular network of long (up to C 32) polymethylenic chains probably cross-linked by ether bridges, and (ii) a close correlation based on the formation of n- alkylnitriles , between this algaenan and two ultralaminar kerogens, the Rundle Oil Shale (mainly composed of ultralamina accumulations) and the Green River Shale (ultralaminae dispersed within an amorphous matrix). These fossil ultralaminae, therefore, likely originated from the selective preservation of the thin, algaenan-containing, outer walls of Scenedesmus and/or of other Chlorophyceae containing outer walls of a similar morphology and composition. Relative distributions of n- alkylnitriles and of n- alkanoic acids, in the pyrolysates of S. quadricauda algaenan and of the Rundle Oil Shale, indicated that nitriles are not derived from secondary reactions of carboxylic acids but originate from preexisting nitrogen functions, likely amides. Previous evidence of kerogen formation via selective preservation of algaenans was restricted to rather uncommon kerogens; the present results, added to ultralamina common occurrence and abundance, point to a wide involvement and to a large contribution of the selective preservation of algaenan-containing thin outer walls of Chlorophyceae in the formation of kerogens in a number of lacustrine source rocks and oil shales. All the available information suggest that the three-way correlation (selective preservation of algaenans from thin resistant chlorophycean outer walls, formation of fossil ultralaminae, presence of n- alkylnitriles with a typical distribution in the pyrolysates of ultralaminar kerogens) observed in this study might be of a general character.

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