The Stø and Nordmela Formations of Early to Middle Jurassic age are represented in wells 7119/12-1, 7120/7-1and 7120/7-3 located in the Tromsø/Hammerfest transition zone area and in wells 7120/8-1, 7120/9-1, 7121/4-1, 7121/5-1 and 7120/6-1in the Hammerfest Basin area in the SW Barents Sea. The study deals with the petrographic understanding, diagenetic development and their influence on reservoir quality. Several techniques like microscopic analysis, scanning electron microscopy, x-ray diffraction analysis and petrophysical analysis have been applied during the study.
The sandstones of Stø Formation are mineralogically and texturally mature, while the Nordmela Formation is mineralogically mature like Stø but texturally less than this. The quartz content in the Stø Formation and overall formation thickness is high in the SW side of the study area. The area has been subjected to an average of 800m uplift suggested by the amount of quartz cement. The reservoir quality is influenced by this process. The degree of quartz cementation is higher in the deepest well (7119/12-1) as compared to shallowest well (7121/4-1) which reflects that the magnitude of uplift is higher towards the shallower well (NE) and results in decrease in temperature. Dissolution of detrital grains at stylolites is the main source of silica cementation. The quartz cementation in the study area is controlled by the grain size and the spacing of stylolites.
Cementation is the major reason of porosity loss in the Stø Formation, where both mechanical compaction and cementation are the major porosity reducing factors in the Nordmela Formation. The reservoir quality of Stø Formation is fair with the porosity ranges from 10.5% to 18.1% and some samples from sandy portion of the Nordmela Formation show a reasonable porosity but reservoir quality in the study area may not be good due to hetrolithic nature of the formation.