Bioturbation Impact on Reservoir Quality of Potential Tight Gas Sandstone Reservoirs, Lower Carboniferous, Howick, Northumberland, UK

ABSTRACT The Lower Carboniferous strata of Breagh oil and gas fields contain abundant oil and gas reserves, and the tight sandstone reservoirs are characterised by extensive bioturbation. The Howick outcrop (Upper Limestone Group, Namurian, Northern England) was selected to investigate the effect of bioturbation on Lower Carboniferous reservoir quality, by analysing its petrological, diagenetic and petrophysical properties. Scanning electron microscopy with energy dispersive x‐ray spectroscopy (SEM‐EDX) and a petrographic microscope were used for porosity measurement, trace fossil morphological description and lithological analysis. Permeability was measured by spot minipermeametry. Five lithofacies and four bioturbation assemblages were identified. Medium‐ to coarse‐grained sandstones with large‐scale planar cross‐bedding (Facies S C ) are dominated by Skolithos ichnofacies. Bioturbated sandstone intercalated with mudstone (Facies S BM ) is characterised by Curvolithus ichnofacies. Nearshore Skolithos ichnofacies is most common in bioturbated ripple‐structured sandstone bed topped with mud (FaciesS B ), while nearshore Cruziana ichnofacies is commonly observed in bioturbated laminated sandstone with ripple and interrupted ripple structures (Facies S BL ) and bioturbation is rare in black coal seams (Facies C). Medium‐ to coarse‐grained sandstone has a higher porosity–permeability; ‘Bioturbation reduces the porosity and permeability of Facies S C by approximately 10% and 50%, respectively. Bioturbation improves the porosity more efficiently in originally finer sandstone, increasing it up to 30%. In fine‐ to very fine‐grained sandstone and mudstone, bioturbation not only improves the original porosity and reduces compaction, but also influences permeability by rearranging the distribution of pores and throats and creating new fluid passages for dissolution and cementation. In addition, bioturbation associated with lined or meniscate burrows can decrease reservoir porosity and permeability, while unlined burrows with a simple structure and filling enhance reservoir properties. The study results serve as an effective indicator in the reservoir description and quality analysis of a similar‐aged offshore field.