Quartz cementation in Late Cretaceous mudstones, northern North Sea: Changes in rock properties due to dissolution of smectite and precipitation of micro-quartz crystals

摘要: Abstract Late Cretaceous mudstones from two wells located in the northern North Sea and Norwegian have been examined with respect to quartz cement. Two different types of cement (Type 1 Type 2) identified using SEM/EDS/CL-analysis drill-bit cuttings at depths 2370–2670 m (80–85 °C). appears as relatively large aggregates (30–100 μm) depth/temperature related crypto- or microcrystalline macrocrystalline irregular formed by local re-crystallization biogenic silica. The CL-responses give a clear indication an authigenic origin. 2 represents high amounts extremely fine-grained micro-sized (1–3 μm) crystals embedded discrete, short chains small clusters/nests within illitized clay matrix. micro-quartz indicate is most probably sourced silica released during smectite illite dissolution–precipitation reaction. petrographic evidence indicates that reaction has not exported out mudstones. produce subtle inter-connected network interlocked crystals. This cementation process causes significant sharp change mudstone stiffness onset chemical compaction regime. indicated abrupt increase well log velocity (Vp) seismic facies close 2500 m (80/85 °C).