Abstract
The North Sea accommodated diatoms (siliceous phytoplankton) during the Eocene (56-33.9 Ma) and Miocene (23-5.3 Ma).
Diatom fossils accumulated at the seabed and formed diatomite sediments that are buried below up to 3 km of younger sediments and located above the chalk reservoirs.
Diatom fossils (Fig. 1a) are composed of amorphous opal-A which diagenetically alters to opal-CT lepispheres (Fig. 1b).
Diagenesis occurs through a dissolution–precipitation process when the accumulated heat from the burial activates it.
The transformation to opal-CT allows a substantial decrease in porosity, which can result in subsidence, overpressure, and fracturing of the sedimentary rock.
Fracturing in diatomite intervals increases permeability, causing potential problems after abandonment.
Figure 1: a) Diatom fossil composed of amorphous silica (opal-A) and b) lepispheres composed of more crystalline silica (opal-CT). Figures from Maria Al Taie, 2008, MSc thesis, DTU.
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