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Title: Reservoir quality in the late Jurassic Fulmar sandstone : sponge spicules and silica diagenesis
Author: Humphrey, Neil.
Awarding Body: University of Reading
Current Institution: University of Reading
Date of Award: 2003
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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.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available