Microscale bioturbation as a first order control on distal prodelta to offshore mudrock reservoirs

Mud-dominated reservoirs exhibit significant heterogeneity driven by depositional processes and post-depositional modification, yet the role of micro-scale bioturbation remains poorly constrained. This study focuses on the impact of diminutive trace fossils (Multina and Phycosiphon) within event beds of the Lower Silurian Qusaiba Member (Qalibah Formation, Saudi Arabia) using micro-computed tomography (micro-CT), petrography, and QEMSCAN analyses. In the studied rocks, these diminutive trace fossils form dense, shallow-tier networks restricted to the upper portions of thin event beds and constitutes at least 1% of the sediment volume. Micro-CT imaging reveals complex three-dimensional burrow architectures that disrupt primary sedimentary fabrics and generate pronounced micro-scale heterogeneity. Mineralogical analyses indicate systematic differences between burrow fills and host sediment, including redistribution of clay minerals and localized enrichment of diagenetic phases such as pyrite. Consequently, the trace fossil producers modified sediment composition through selective feeding and induced early diagenetic processes. The findings demonstrate that micro-scale bioturbation is a fundamental component of mudrock fabric, potentially exerting a first-order control on porosity, permeability, and geomechanical properties. This has important implications for unconventional reservoir characterization, where subtle textural variations can significantly influence fluid flow and reservoir performance.