Influence of Silica Diagenesis on Seal Development: Insights from 3-D Seismic Reflection and Well Data from the Norwegian Margin
Wrona, Thilo; Jackson, Christopher A.; Huuse, Mads; Taylor, Kevin G.
Silica diagenesis is an important but underappreciated mechanism for generating regional seals in hydrocarbon basins (e.g. Santa Maria Basin, offshore California), because the dissolution and re-precipitation of silica during the burial of siliceous sediments can cause a large reduction in their porosity and permeability. Two key silica diagenetic transformations are recognised: (1) biogenic silica (opal-A) to crystobalite and tridymite (opal-CT); and (2) a subsequent transformation of opal-CT to quartz (Q). Although the general processes that occur during these transformations are well-understood, variations in effective porosity reduction and the ability of silica diagenesis to initiate fracturing (i.e. permeability increase), remain poorly understood.
In this study, we use 3-D seismic and borehole data to map the opal-CT transformation zone in the Tertiary succession of the north-east margin of the northern North Sea. The top opal-A-CT reflection, which is expressed as a high-amplitude, strata-concordant reflection, occurs at 500-650 mbsf and the basal opal-CT-Q reflection, which has a more subtle seismic expression, occurs at 625-800 mbsf.
Well data indicate that the effective porosity decreases relatively abruptly by c. 37% (from 49% to 31%) across the opal-A-CT boundary. We suggest that dissolution of amorphous siliceous fossils and the precipitation of opal-CT in the pore space caused this porosity reduction. Both diagenetic boundaries are cross-cut by a regionally-developed polygonal fault system. Quantitative analysis of fault throws suggests that 65% of them nucleated at approximately at the same depth as the opal-A-CT boundary (+/- 50m), suggesting that fault nucleation and silica diagenesis may have been coeval. Changes to the physical properties of the host rocks (e.g. reduction in grain cohesion or internal friction) and an increase in pore fluid pressure, following the release of water during silica diagenesis, may have initiated polygonal faulting.
Our results suggest that the dissolution of opal-A decreased host rock strength and may have initiated a polygonal faulting, producing fracture permeability and potential fluid pathways. Conversely, these low-throw faults may have resulted in shale-shale juxtaposition, shear smear and very limited permeability enhancement. Additional 3-D seismic-based studies are required to better understand the role that silica diagenesis has in creating and destroying hydrocarbon seals.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013