Effect of Bioturbation on the Porosity and Permeability of Shelf Carbonates: Examples From the Ulayyah Member of the Hanifa Formations, Central Saudi Arabia

Abstract

Previous workers have shown that the presence of high burrow connectivity enhances such petrophysical properties as porosity and permeability. Only a few studies, however, have documented and modeled burrow heterogeneities in carbonate reservoirs and outcrop analogs to provide predictive models for their impact on reservoir quality. Through outcrop investigation and numerical modeling, this study documents and models Thalassinoides burrow networks of the Glossifungites Ichnofacies in firmground, mud-dominated carbonate strata of the Lower Jurassic Tuwaiq Mountain and Middle Jurassic Hanifa formations in central Saudi Arabia. This study provides insights into how the complex geometries of coarse-sediment infilled Thalassinoides burrow networks enhance reservoir connectivity and, therefore, reservoir quality of bioturbated carbonates.

Here we focus on the Ulayyah Member of the Hanifa Formation because of its large number of extensively bioturbated intervals with three-dimensional (3D) outcrop exposures. To evaluate how 3D facies models of the Glossifungites Ichnofacies may vary with Thalassinoides burrow network density, a series of facies models were constructed by multipoint statistics in Petrel™ 2016. These models utilized train images of slabbed rock with Glossifungites Ichnofacies and have been constructed for different Thalassinoides burrow network densities of 20–70% (portion of volume with interconnected burrows). The results show that Glossifungites Ichnofacies with 60–70% burrow density are connected throughout the 3D facies volume. Glossifungites Ichnofacies with 30–50% burrow density will show connectivity when a set of fractures is introduced into the model. The models were tested by comparison with previously studied carbonates that show super flow units associated with the Glossifungites Ichnofacies, and the results have shown interesting parallels.

These results show that the Glossifungites Ichnofacies in firmground, mud-dominated lithofacies are important for petrophysical characterization because the coarse-sediment infill of the 3D Thalassinoides burrow networks provide permeability pathways in an otherwise less permeable medium. This study is of broad significance because the resulting geostatistical models provide a better understanding of the geometrical distribution and impact of Glossifungites Ichnofacies on carbonate reservoirs.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018