Thilo Wrona

Silica diagenesis and physical properties of Cenozoic rocks in the North Viking Graben, northern North Sea

by Thilo Wrona, Christopher A-L. Jackson, Mads Huuse and Kevin G. Taylor

As an introduction, I thought it would be nice to present some of the work that I’ve done in my PhD. This covers quite a range of disciplines, so I hope there’s something in there for everyone.

Silica diagenesis describes two dissolution-precipitation reactions that can drastically change the physical and fluid flow properties of its host strata. To study the effect of silica diagenesis, we combine mineralogical-, well- and 3-D seismic data of the North Viking Graben, northern North Sea. We use optical microscopy, scanning electron microscopy and X-ray diffraction to identify the opal-A/CT transformation in the Cenozoic succession. We determine the effect of the opal-A/CT transformation on the host rock properties by combining quantitative mineralogical data obtained by X-ray diffraction with wireline data of sixteen exploration wells using multiple linear regression analysis. The analysis shows that: (1) opal-A content explains host rock porosity to a large extent and (2) opal-CT and pyrite content explain host rock porosity to a lesser degree. The overall decline in opal-A content with depth is interpreted to reflect increasing biogenic silica production between the Eocene and Miocene. Focused reductions in opal-A content that coincide with increased opal-CT contents are probably the result of opal-A/CT transformation. Because the observed opal-A/CT transformation does not coincide with major lithology variations, it is assumed that the transformation is primarily a function of time and temperature. This assumption allows us to model the spatial and temporal evolution of the opal-A/CT transformation. Modelling results indicate that the opal-A/CT transformation started in the Balder Formation in the Middle-to-Late Eocene, migrated upwards through the lower Hordaland Group, and fossilised as a result of Middle-Miocene sea-level fall and erosion. Moreover, we use these basin modelling results and a detailed fault analysis to study the impact of silica diagenesis on the formation of polygonal faults. Polygonal fault systems comprise large numbers of layer-bound, low-displacement normal faults, which in plan-view have a polygonal arrangement. Based on our results, we are able to demonstrate that the majority of the polygonal faults nucleated in the silica-rich strata. In summary, this study highlights that silica diagenesis is a complex process that can significantly change compaction and deformation of siliceous sedimentary rocks.