Abstract: Improvement of Sandstone Porosity During Bioturbation

William C. Dawson

The ability of burrowing organisms to alter the composition and texture of sediments was studied using results obtained from sandstone containing Upper Cretaceous trace fossils. Specimens were collected from a variety of shallow-marine lithofacies exposed in northeast Texas. Depositional environments were established independently of the traces using physical sedimentary structures and mollusk communities.

The Cruzina assemblage includes Thalassinoides, Rhizocorallium, Pseudobilobites, Stipsellus, Gyrolithes, Planolites, and numerous vertical tubes. The mineralogy, grain size, and fabric of 200 specimens were studied by standard thin-section and sieving techniques. The amount of cement (determined by point counting) served as an indicator of minimum initial porosity. Burrow fillings and host sediments were analyzed separately and compared. Stipsellus, Planolites, and small thalassinoidean burrows were filled with fine, well-sorted, quartz sand and inoceramid prisms. Elongate grains were oriented paralleling burrow walls, and porosity averaged 20%. By contrast, host rocks were composed of poorly sorted, clayey-micaceous sandstones having 5 to 7% porosity; oriented grain fabrics were not developed. Rhizocorallium, Pseudobilobites, Gyrolithes, and large thalassinoidean structures were filled with sediment that is mineralogically and texturally similar to the host rock.

Some burrowing organisms segregated sand and clay while grazing. Resulting burrow fills had porosities 10 to 15% higher than nonbioturbated rock. When grazing organisms actively backfilled their burrows, oriented grain fabrics were created. If hydrocarbons migrated into moderately bioturbated sandstones before cementation, burrows could provide "tubular permeable conduits" transforming an otherwise nonreservoir into a reservoir.

AAPG Search and Discovery Article #90961©1978 AAPG Annual Convention and Exhibition, Oklahoma City, Oklahoma