From mud to shale: rock stiffening by micro-quartz cementation

For the first time, direct petrographic evidence of fine-grained (1–3 μm) crystals of pore-filling quartz cement in mudstones is documented. The cathodoluminescence responses of the micro-quartz give a clear indication of an authigenic origin. The detection of micro-quartz cement in drill-bit cutting samples at depths around and deeper than 2500 m (80–85°C ) of Upper Cretaceous mudstones in the northern North Sea suggests that most of the silica released during mudstone diagenesis is not exported to adjacent sandstones but precipitated locally. The locally precipitated micro-quartz has most likely been sourced by silica released from the dissolution of smectite, resulting in precipitation of illite (and possibly chlorite) during progressive burial. The continuous nucleation–precipitation process, which takes place in the chemical compaction regime close to and above 80–85°C, explains why the micro-quartz crystals are found as isolated grains, short chains, and small nests/clusters of micro-quartz embedded in the fine-grained matrix. The interconnected micro-quartz networks (skeletons) and aggregates probably increase the mudstone stiffness significantly, as indicated by an abrupt increase in P-wave velocity close to 2500 m (80–85°C).