Reservoir Quality of the Stø Formation in the Hammerfest Basin, Southwestern Barents Sea - The role of initial composition of sediments and diagenesis

• Main Author: Løvstad, Kristoffer
• Format: Master Thesis
• Language: English
• Published: 2016
• Subjects: Barents Sea; Stø; Hammerfest; Hammerfest Basin
• Online Access: http://hdl.handle.net/10852/52354; http://urn.nb.no/URN:NBN:no-55784

Reduction of porosity in sandstones with increasing burial is a function of mechanical compaction causing reorganization and crushing of grains with increasing pressure, and chemical compaction, mainly precipitation of authigenic quartz cement as temperatures reaches 70-80 ºC and above. The amount of mechanical compaction is governed by the initial lithological and textural features of the sediments. The precipitation of authigenic quartz can be reduced if efficient grain coats exist on detrital quartz grains. This study investigates the parameters related to the reservoir quality of the Stø formation, the processes occurring during burial, the initial composition of the sediments and how they relate. Well data and thin section samples from the Stø formation the two wells, 7120/2-3S and 7120/12-2, situated in the western parts of the Hammerfest basin in the Southwestern Barents Sea were examined using a petrographical and petrophysical approach. The well logs were utilized in a petrophysical investigation, and thin sections were investigated using optical microscopy and scanning electron microscope. The sandstones of the Stø formation were seen to consist of highly matures quartz arenites with the major difference in initial composition between the units being the clay content, related to the depositional setting. The depositional settings observed ranged from upper shoreface beach/barrier bar deposits to more distal offshore deposits. The Stø formation in well 7120/2-3S have been subjected to more extensive burial and higher temperatures resulting in the formation of more quartz cement than the formation in well 7120/12-2. The Stø formation in well 7120/2-3 consisted of a lower unit of consistently clean sands with porosities around 12-15% and significant quartz cement, and an upper unit with higher porosities of around 15-18% and less quartz cement interpreted to be caused by allogenic grain coats corresponding to the higher clay content. The Stø formation in well 7120/12-2 consisted of two clean sandstone units in the upper and lower parts of the formation with good porosities of 20-25% and a more extensive unit with higher clay content and frequent clay laminated horizons most likely reducing vertical permeability.