ABSTRACT: Organic-Matter Distribution in Fine-Grained Deltaic Sediments: A Physical-Geochemical Investigation of Kerogen Microhabitat

SEGALL, M. P., University of Victoria, Victoria, British Columbia, Canada, J. K. RAFALSKA, Conoco Inc., Ponca City, OK, and S. A. KUEHL, University of South Carolina, Columbia, SC

Submicroscopic examination of fine-grained deltaic deposits is coupled with geochemical analyses to evaluate the type and spatial distribution of organic matter within the mineral groundmass. Samples with total organic carbon (trace amounts to 1.2%) were chosen from modern muddy sediments presently accumulating on the Bengal shelf and shallow marine Cretaceous black shales (TOC to 8%). Organic content increases offshore in the direction of highest sediment accumulation. Both the unconsolidated muds and Cretaceous shales exhibit resistant (protruding) and less-resistant

(recessed) microlaminae under scanning electron microscopic examination. Backscattered-electron imaging coupled with multielemental mapping indicate the resistant microlaminae are very fine grained (less than or equal to 2 micrometer particle diameter), essentially monominerallic (illite or smectite), with grains preferentially oriented in a closely packed, stepped, face-to-face arrangement. The less-resistant microlaminae are poorly sorted with respect to size and mineralogy and are characterized by large interconnected pore spaces. In Cretaceous samples, these pore spaces are rimmed by elemental concentrations of V, Ti, Fe, Ni, and S. The contact between the resistant and less-resistant microlaminae is characterized by authigenic pyrite octahedra or organic matter. Rock-Eval and opt cal examination suggest that organic components originally adhered to particles in the more porous microlaminae. Subsequent redistribution of the hydrogen-rich, amorphous organic matter resulted in its concentration at the interface between the resistant and less-resistant microlaminae. These observations of organic-mineral microfabric may have implications for understanding shale compaction and the expulsion of hydrocarbons from the kerogen framework in fine-grained source rocks.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)