The purpose of the thesis is to characterize the reservoir quality in the Middle and Upper Jurassic sandstone reservoirs in the South Viking Graben, with emphasis on the differences between the Hugin and Draupne formations. The focus of the study is to understand the diagenetic processes that have affected the deeply buried sandstones, both the mechanical and chemical compaction. The study involves eight wells situated in the eastern part of the South Viking Graben, in block 15/3, 24/12 and 15/5, all investigated using the following methods; well correlation, petrophysical assessment, mineralogical and petrographic analysis.
Authigenic quartz cement will start to nucleate on detrital quartz grain surfaces at temperatures above 75-80°C. The Middle and Upper Jurassic sandstones of the Hugin and Draupne formations are buried at depths exceeding 4 km. As the precipitation of quartz cement follows a time/temperature-integral, it is expected to have filled all pore-space in the deeply buried sandstones in the South Viking Graben. Mechanisms such as fluid overpressure, hydrocarbon emplacement and grain coating minerals may preserve porosity at great depths.
The porosity distribution of the Middle and Upper Jurassic sandstones in the South VikingGraben showed three subpopulations of porosity; 0-10%, 10-20% and 20-30%. According to porosity-depth curves reported from the study area, 0-15% porosity is normal. Porosity above this level is considered anomalous, and may be related to porosity preserving mechanisms.
The hydrocarbon emplacement appears to have had a negligible effect, while the grain coating minerals are believed to have been an important porosity preserving factor in the studied samples. The illite-coating was found in both of the studied formations, while the presence of microcrystalline quartz was restricted to samples from the Draupne formation and corresponded to the highest observed porosities. The microcrystalline coating is therefore considered to have been the main porosity preserving mechanism.
The content of authigenic clay minerals was investigated both in optical microscope and using scanning electron microscopy. The proximal Hugin formation contained an abundance of kaolin, typically connected to meteoric water flushing. A variation of K-feldspar content is believed to be the cause of the varying degree of illitization within the formation. The Draupne formation was deposited in a distal setting and the sediments were isolated from meteoric water flushing. Correspondingly the Draupne formation has a low amount of kaolin and a correspondingly low amount of illite. The low amounts of illite are considered to be positive for the permeability of the formation, as the fibrous morphology of illite is known to reduce permeability.
A stabilization of the intragranular volume at 25% was observed, as well as stylolites, indicating that quartz cement from dissolution at stylolite has been the main porosity destructive mechanism in the Middle and Upper Jurassic sandstones in the South Viking Graben.