Stratigraphic and regional clay mineral variations and biogenic silica distribution, North Sea Cenozoic overburden – impact on seal integrity

Background
Traditionally, and often still, the shales and mudstones have been treated as a one type of lithology in many of our subsurface evaluations. However, they exhibit significant heterogeneity in terms of different clay mineral composition (including smectite, smectite/illite, illite, chlorite, kaolinite), grain size (quartz, silt) and content of siliceous (biogenic silica), volcanic, organic and carbonate content.

Moreover, the composition of mudstones and shales changes during progressive burial due to mechanical and chemical compaction (diagenesis). The transformation of mud to shale is well known in sedimentary basins. This mud lithification (rock stiffening or from ductile to brittle) process is identified by change in the physical properties of during progressive burial.

The Cenozoic succession (overburden) in the Norwegian North Sea (NS) and surrounding areas serves as a valuable natural laboratory for studying the mudstones and shales of the overburden and its variations in petrophysical properties and sealing capacity. In general, the Cenozoic succession in the North Sea encompasses a diverse range of lithologies, including Paleogene marine swelling clays, sandy turbidites and sand injectites/sinkites, as well as Neogene deltaic sands, along with offshore marine clays with a broad range of mineralogy. This is mainly due to stratigraphically and geographically changes in the sediment (mineralogy) composition of these mudstones and shales. Regionally and stratigraphically heterogeneities in mudstones and shale are reflected in different rock properties and compaction trends mainly due to different provenance or erosional areas through Cenozoic times. For example, it has been documented that coarse grained glacial Pliocene mudstones undergo relatively rapid mechanical compaction, whereas fine grained Eocene smectite-rich clays formed from volcanic material or Pliocene fine-grained marine smectite-rich clays compact much more slowly.

While this variations of lithologies present opportunities for CO₂ storage reservoirs in the Cenozoic, it also poses challenges with respect to the sealing capacity of the mudstones and shales within the overburden. The quality or the integrity of the overburden are critical factors in facilitating safe, permanent, and secure CO₂ storage in the subsurface. However, ensuring seal integrity is a complex process influenced by many factors, like pressure, temperature and mechanical and geomechanical properties, controlled by the mineral composition of the mudstones and shales. Seal failure due to caprock fracturing (hydraulic seal) of the primary or secondary seals is one of the major risks during CO₂ storage and injection.

Goal
To unlock shales and mudstones of the North Sea Cenozoic overburden from a mineral perspective.

Overview of regional and stratigraphic clay mineral and biogenic silica variations of shales and mudstones of the North Sea Cenozoic overburden.

Generate a knowledge basis for selecting use cases, key wells and study areas to developing and testing a novel model of rock stiffening due to biogenic silica diagenesis.

Activities
The first activity in the project is to collect and compile information about the Cenozoic clay and bulk (whole rock)) XRD mineral data from various sources, including literature, public dataset, unpublished reports, and master theses. The objective is to obtain a comprehensive overview of the status of publicly available clay mineral data and other relevant mineral available from the Cenozoic overburden in North Sea and surrounding areas. This will involve creating a regional and stratigraphic overview or a database of the wells and intervals already analysed in the North Sea. The second activity is to identify regionally and stratigraphically intervals with the biogenic silica-rich mudstones within the Oligocene and Miocene sediments. These two first activities lay the foundation for the project’s main objective and the third activity; to provide the knowledge basis for developing and testing an innovative hypothesis on biogenic silica diagenesis* and its impact on geomechanical properties.

*One significant source for quartz cementation in shale and mudstones comes from microfossils composed of silica (opal-A), such as diatoms, silicoflagellates and sponge spicules. These microfossils are often deposited alongside fine-grained smectite-rich mudstones. During burial, opal-A may undergo dissolution and precipitation to opal-CT (cristobalite/tridymite) at temperatures of c. 50-60 ºC; and later recrystallised to quartz cement at >60-80 ºC. The presence of opal-A and opal-CT, as well as biogenic-induced quartz cement, may significantly affect the physical and geomechanically properties of the ooze-rich mudstones, influencing the sealing capacity. However, these processes are still underexplored and little investigated.

The compiled mineral dataset and associated petrophysical properties may also serve as a basis for selecting key wells/use cases for further studies, as wells for generating AI models, digital maps or atlas etc. However, these activities are beyond the scope of this particular project.

Results
To Be Announced

Further Work
The knowledge base established through this scheme project will serve as the foundation for a larger R&D project.

Literature
Bjørlykke, K. (2015). Petroleum Geoscience. From Sedimentary Environments to Rock Physics Springer 662 pages.
Marcussen. Ø., Thyberg, B. Peltonen, C., Jahren, J. and K. Bjørlykke (2009). Physical properties of Cenozoic mudstones from the northern North Sea: Impact of clay mineralogy on compaction trends. AAPG Bulletin, 93(1), 127-150.
Thyberg, B. Thyberg, B. & Jahren, J. (2011). Quartz cementation in mudstones: sheet-like quartz cement from clay mineral reactions during burial. Petroleum Geoscience, 17, 53-63.