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

• Main Author: Wrona, Thilo
• Other Authors: Jackson, Christopher A.-L
• Published: Imperial College London 2016
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.745227

Silica diagenesis has the potential to drastically change the physical and fluid flow properties of the host strata and may therefore play a key role in the development of sedimentary basins. To investigate the role of silica diagenesis in the North Viking Graben, northern North Sea, mineralogical-, well- and 3-D seismic data are combined in this study. Optical microscopy, scanning electron microscopy and X-ray diffraction are used to identify the opal-A/CT transformation in the Cenozoic succession of the North Viking Graben, northern North Sea. The effect of the opal-A/CT transformation on the host rock properties is investigated 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 opal-A content explains host rock porosity to a large extent and 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. Focussed 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 the spatial and temporal evolution of the opal-A/CT transformation to be modelled. Modelling results indicate that opal-A/CT transformation probably 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. Based on the basin modelling results and a detailed fault analysis, silica diagenesis could have led to the nucleation and growth of the polygonal faults in the North Viking Graben. This study highlights that silica diagenesis is a complex process that can significantly impact compaction and deformation of siliceous sedimentary successions.