Reservoir quality in the late Jurassic Fulmar sandstone : sponge spicules and silica diagenesis
Siliceous spicules from the extinct sponge Rhaxella are found within the Mid to Late
Jurassic Fulmar Formation. Rhaxella palaeoecology and palaeobiology are investigated in
this study, along with the controls on Rhaxella sponge distribution. The way in which the
presence of Rhaxella spicules affects both diagenesis and reservoir quality is also analysed.
Spicules are observed in depositional settings ranging from the shoreface to the offshore
transitional zone. Sponge colonies originally grew in shoreface settings. The spicules
found in more distal environments represent re-worked, allochthonous accumulations. At
the shoreface the sea was warm and contained low amounts of suspended argillaceous
material. Energy levels were low, but substantial enough to provide a constant supply of
nutrients to the filter feeding sponges.
Rhaxella sponges first colonised the Outer Moray Firth area during the Callovian, then
spread south through the North Sea into the northern margins of the Tethys Ocean. This
expansion was followed by a retreat back into the North Sea Central Graben. Volcanic
activity and water depth are proposed as important controls on the distribution of sponge
colonies. Elevated silica levels were used by Rhaxella sponge colonies in the construction
of their skeletons. Three of the wells studied display a trend between relative sea level and
spicule volume. It is found that spicules are more commonly observed in shallower-water
regimes, and are less common in sediments deposited in deep-water settings.
The quantity of authigenic silica is highly variable throughout the study wells, but
statistical analysis indicates a strong positive correlation between the abundance of
intergranular silica cement and siliceous spicules. Seven types of silica cement have been
identified, with two diagenetic pathways for silica cementation being recognised. It is
proposed that the initial volume of spicules has a controlling influence in the type of silica
cement that developed.
Reservoir quality is significantly improved by the occurrence of spiculitic moldic porosity
in all of the studied wells. The occurrence of spicules may however, also have detrimental
effects on reservoir quality. Results presented in this thesis suggest that reservoir quality
has been significantly reduced throughout intervals that exhibit abundant preserved
spicules (>20% total volume), coupled with a low volume of 'spiculitic moldic porosity'.
In such units all moldic porosity has been occluded and pore throats are significantly
blocked, lowering permeability.