Stephen Q. Dornbos¹, William Phelps², David J. Bottjer¹, Mary L. Droser², Brian Anderson³
(1) University of Southern California, Los Angeles, CA
(2) University of Southern California, Riverside, CA
(3) Brown and Caldwell, Boise, ID

Abstract: Effects of bioturbation on reservoir sandstone porosity and permeability: Studies of outcrop samples from the Upper Cretaceous, Book Cliffs, Utah

Heterogeneity in the porosity and permeability of subsurface reservoir rocks, which to a large extent determines fluid flow volumes and rates, is affected by a variety of depositional, diagenetic, and tectonic processes. One of the factors that is known to exhibit heterogeneity and to directly affect permeability and porosity is the extent of bioturbation, as reflected by ichnofabric, in a sedimentary rock. To render observations on extent of bioturbation more comparable the ichnofabric index (ii) methodology was developed, which allows the semi-quantitative ranking of ichnofabric, from no bioturbation (ii1) to sediment totally homogenized by bioturbation (ii6).

We determined the relationship between amount of bioturbation as recorded by ii and porosity and permeability through study of Upper Cretaceous nearshore and shallow marine sandstones that outcrop in the Book Cliffs region of Utah. Four stratigraphic sections were measured in which 78 samples were collected, and in the lab these samples were assessed by means of the ii method, as well as subject to standard porosity and permeability determinations. For these samples average porosity shows a slight decrease with increased bioturbation, from 21.25% (ii1) to 18% (ii5). Average permeability values show a relatively strong decrease with increased bioturbation, from 220 md (ii1) to 18 md (ii5). A variety of other primary and secondary features undoubtedly are factors which contribute to these trends, but these initial results provide some of the first quantitative evidence of the role that bioturbation plays in the development of porosity and permeability in sandstones.

AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana