Characterization of lchnofabrlcs within Lowstand Valley-Fill Systems Tracts: Outcrop Examples from the Upper Cretaceous Sego Sandstone, Western Interior, USA

Brian G. Anderson, Mary L. Droser, and David J. Bottjer

Lowstand systems tracts offer a favorable reservoir target for hydrocarbon exploration; however, biogenic reworking can significantly alter porosity and perrneability. In order to test the utility of ichnofabrics as a tool for characterizing variability of biogenic fabrics within valley-fill successions ichnofabric indices were logged for extent of bioturbation frorn 5 measured sections in the Campanian Sego Sandstone of east-central Utah.

Results demonstrate that ichnofabric index values vary both vertically and

laterally within valley-fill successions. lchnofabric indices >1 for discrete Ophiomorpha are pervasive in marginal and nearshore marine sandstones However, ichnofabric index = 1 occurs throughout mudstone and siltstones lithofacies. Ichnofabric indices representing nondescript mottling range fron 1 to 5, although higher values tend to be associated with lithofacies characteristic of low to moderate physical energy levels.

When integrated with sedim entologic and petrophysical information ichnofabrics provide an alternative source of data that can be used to evaluate biogenic sedimentary fabrics in valley-fill systems tracts. Future studies should provide a comparison of downhole geophysical logs with ichnofabric index data collected from core material to quantify the effect of bioturbation on reservoir porosity and permeability.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California