Influence of Diagenesis on the Physical Properties of Siliceous Mudstones: Insights from Geochemical, well and 3D Seismic Reflection data from the North Viking Graben

Thilo Wrona
Imperial College London, Department of Earth Science and Engineering, London, United Kingdom
[email protected]

Diagenetic reactions in siliceous sediments can strongly influence their physical properties, and may potentially affect fluid flow, seal and reservoir development within hydrocarbon-bearing sedimentary basins. The dissolution and re-precipitation of silica can cause large reductions in porosity and permeability. Two key silica diagenetic transformations are recognized: (1) biogenic silica (opal-A) to cristobalite and tridymite (opal-CT); and (2) a subsequent transformation of opal-CT to quartz (Q).

I use geochemical, well and 3D seismic data to map the silica transformation zone in the Oligocene to Miocene succession in the North Viking Graben. Well data indicate that the effective porosity decreases relatively abruptly by 27% due to silica diagenesis. The transformation zone is also cross-cut by a polygonal fault system. Quantitative analysis of fault throws shows that 65% of the polygonal faults nucleated at the same depth as the present-day opal-A/CT transformation zone (±50m), proposing that fault nucleation is related to silica diagenesis. I suggest that the dissolution of opal-A decreased the host rock cohesion and, coupled with an increase in pore fluid pressure following the release of water during silica diagenesis, initiated polygonal faulting.

I propose a model for the initiation of a natural, polygonal fault and fracture network, which is intimately related to silica diagenesis. Furthermore, I suggest that silica diagenesis decreases the porosity and permeability of the host strata, thus creating an effective seal. I am currently undertaking detailed geochemical analysis of borehole cuttings from the North Viking Graben to test the physical basis of both these predictions.

AAPG Search and Discovery Article #90183©2013 AAPG Foundation 2013 Grants-in-Aid Projects