Bioturbation Enhances Porosity and Permeability of Mudrocks: Insights from the Ordovician Qasim Formation, Northern Saudi Arabia

Abstract

Mudstones are typically characterized by tight rock textures with low permeability, often acting as vertical barriers when interbedded with sandstone. However, this study presents a different perspective based on an integrated analysis of bioturbated mudstone intervals in the Kahfah Member, Qasim Formation, northwestern Saudi Arabia. Using field observations, high-resolution imaging, and laboratory measurements, the study suggests that bioturbation can enhance vertical fluid transmission through mudstones, allowing them to function as part of the flow unit communicating sandstone units above and below. The mudstone intervals exhibit a bioturbation index of 2–3, with burrows such as Skolithos, Taenidium, Arenicolites, and Palaeophycus filled with silt to fine sandstone. CT scans reveal a three-dimensional network of burrows and laminae, with burrow filling porosity estimated at 15–25% and permeability ranging from 100 to 250 mD. In contrast, the surrounding clay-rich matrix remains low in permeability (< 1 mD). The interbedded sandstone shows porosity of 25–30% and permeability values between 25 and 126 mD. These findings challenge the assumption that mudstones serve as impermeable barriers. Instead, the interconnected burrow networks may facilitate vertical communication between sandstone units. Traditional well log interpretations frequently underestimate the fluid flow potential of mudstones, often classifying them as tight, low-permeability formations. However, bioturbation can significantly enhance the permeability of mudstones, suggesting that their fluid flow potential may be underestimated. This highlights the need for integrating ichnological, petrophysical, and well log data to avoid underestimating permeability and storage capacity, critical for effective reservoir management, aquifers, and injection sites.