Ronald R. McDowell1, David L. Matchen1, Katharine L. Avary1
(1) West Virginia Geological and Economic Survey, Morgantown, WV
ABSTRACT: Bioturbation and Reservoir Flow Characteristics: Where did the Permeability Go?
Effects of bioturbation on sediment permeability are not always intuitive. Bioturbation may alter sorting, disturb sedimentary layering, "pipe" sediment and fluids between sedimentary units, add or remove organic matter and clay, or change pore fluid chemistry. Permeability may increase only to disappear during diagenesis.
The Devonian Gordon sandstone of West Virginia and Pennsylvania represents a strandplain deposit from the westernmost advance of the Acadian clastic wedge. Core recovered from this unit in the Jacksonburg-Stringtown oil field in West Virginia comprises sandstone, shale, and conglomerate. The sandstones are the most important and most problematic. Some form prolific pay units, others are "tight" and unproductive.
Gordon sandstones consist monotonously of very fine- to fine-grained quartz sand with quartz pebble layers. Sandstones are differentiated by the presence or absence of distinctive sedimentary structures, primarily well-preserved trace fossils and ripple-scale crossbedding. Logs of the bioturbated sandstones show high densities; core permeabilities are <20 mD. Bioturbation probably enhanced porosity and permeability but left cross-stratal "pathways" open to mineralizing pore fluids. Silica and siderite cements filled all pore space. On the other hand, the "featureless" sandstones have low log densities; core permeabilities range from 50 to 500 mD. Pore space may have been maintained by early entry of petroleum or because pathways to cementing fluids were absent. A remaining question is whether the featureless sandstones were always devoid of sedimentary structures or had them removed by thorough bioturbation. This work was funded by U. S. Department of Energy contract DE-AC26-98BC15104.
AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado