The Influence of Bioturbation on Sandstone Brittleness: A Comparative Analysis of Massive Sandstone versus Bioturbated Sandstone (Ordovician Qasim Formation-Saudi Arabia)

Abstract

Bioturbation is the reorganization of sediment particles by living organisms, mainly infauna, as they move through and over sediment, feed, and construct dwellings. The biosedimentary structures produced by organisms alter the petrophysical and mechanical properties of the sediment, including porosity, permeability, and brittleness, such that the petrophysical and mechanical properties of bioturbated sedimentary strata commonly differ from that of unbioturbated strata. This directly impacts fluid flow through sedimentary strata. In this study, we investigate the impact of bioturbation on sandstone brittleness through the comparison of seemingly massive sandstone with bioturbated sandstone in the Ordovician Kahfa Member of the Qasim Formation in northwestern Saudi Arabia. Through a series of field observations (sedimentological descriptions and fracture intensity measurements) and laboratory analyses (thin-section petrography, SEM, XRD, XRF, spot permeability and hardness, as well as Vp and Vs measurements) the mechanical properties of bioturbated and unbioturbated sandstone are determined.  The results indicate that although both units have moderate to high porosity (17–25%), the bioturbated sandstone units exhibit decreased brittleness (~ 0.2 fractures·m-1) relative to their unbioturbated counterparts (~ 2 fractures·m-1). Young’s Modulus of bioturbated sandstone ranges from 4.0 to 5.7 GPa, while unbioturbated sandstone has much higher moduli, ranging from 15.1 to 17.6 GPa. This decreased brittleness in bioturbated sandstone is attributed to locally enhanced porosity, altered grain size distribution, and the incorporation of clay particles, all of which are associated with bioturbation. These modifications result in a less brittle sandstone and, by extension, a reduction in fracture density. Understanding the role of bioturbation in altering sandstone brittleness is not only significant for geotechnical and geological applications but also holds implications for civil engineering and environmental science. Further research into the specific mechanisms underlying bioturbation-induced changes in sandstone properties is recommended for comprehensive insights into this phenomenon.