High impedance mudstone associated with sand injection complexes: significance for basin‐scale fluid retention and escape

High impedance (hard) mudstones are sometimes observed in association with sand injection complexes in the Paleogene petroleum province of the northern North Sea. A hard mudstone surrounding a water‐bearing sandstone can give a similar acoustic response to an oil‐bearing sandstone surrounded by low impedance (soft) mudstone. The presence of hard mudstone thus impacts the ability to predict hydrocarbon presence directly from seismic data during exploration. To establish the mechanism of ‘hardening’ to better predict the presence of variable mudstone characteristics, we examine three cored wells from the Beryl Embayment. Well logs and core were examined to characterise the structure, petrology, petrophysical properties and spatial distribution of both hard and soft mudstones. The results indicate that mudstone hardening is most likely associated with mechanical compaction and efficient dewatering of mudstones into the sand injection complex. This process is enhanced where sand injection complexes transect primary overpressure zones, that promote dewatering from basal overpressured mudstone into the injection network. This study highlights that seismic response needs careful investigation in the context of the complexity of the injectite complex along with variable mudstone attributes. Additionally, this process highlights the role sand injection complexes play in efficient dewatering through lateral transfer in overpressured basins.