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Shale Fabric and Sedimentary Processes – A Review

Neal R. O’Brien

State University of New York at Potsdam, Geology Department

Potsdam, New York 13676-2294

Presented here is a review of common fabric signatures in gray and black shales and the sedimentary processes responsible for their formation. Macro– and microfabric features are revealed in x- radiographs and thin-section and scanning electron microscope photomicrographs. Macrofabric analysis shown by x-radiography reveals: (1) well developed lamination indicating absence of bioturbation; (2) indistinct lamination resulting from slight disruption of sediment features by bottom-flowing currents and/or minor bioturbation; or (3) absence of lamination indicative of extensive bioturbation. Petrographic analysis of organic-rich gray to black shales reveals four specific lamination types: (1) fine, (2) thick, (3) wavy, (4) lenticular – each useful indicators of sedimentary processes. These techniques when combined with scanning electron microscope analysis show fabric signatures of shale forming processes such as: (1) the influence of suspension settling and/or bottom flowing currents; (2) the role played by clay flocculation or dispersion upon fabric, (3) the impact of benthic microbial mats and bioturbation upon final particle orientation. Fabric evidence combined with other geological features is thus useful in determining not only the original sedimentary processes but also in predicting properties of shale which influence primary hydrocarbon migration.

AAPG Search and Discovery Article #90059©2006 AAPG Eastern Section Meeting, Buffalo, New York