Diagenesis and reservoir quality of Upper Jurassic sandstones in the South Viking Graben : A mineralogical, petrographical and petrophysical approach

Abstract

The deeply buried Upper Jurassic sandstone reservoirs of the South Viking Graben are differentiated by highly variant reservoir quality. A combined petrophysical and petrographical approach is incorporated to analyze these variations. Quartz cementation developed as syntaxial overgrowth on detrital quartz grains, is the dominant porosity destroying mechanism in these reservoirs. The phenomenon of quartz cementation has proven to be precipitation rate controlled and is insensitive to pressure variations in North Sea sandstone reservoirs. Microquartz grain coatings developed as minute quartz crystals are found to be the fundamental porosity preserving mechanism in these reservoirs despite of great burial depth (>4km) and high temperature (>120oC). The microquartz coatings developed at lower temperature (60-80oC) thus hinders the quartz cementation. Petrographic Upper Jurassic data evaluated during this study indicate that microquartz grain coats are common in deep marine sandstones of the Vilje sub-basin in South Viking Graben. Grain coating microquartz is generated from the dissolution of silica sponge spicules of the Rhaxella Perforata. These spicules observed in both eastern and western part of South Viking Graben indicate its occurrence in the Vilje Subbasin extensively. The porosity preserved in both Intra-Draupne Formation Sandstone and Brae Formation can be attributed to the presence of microquartz coatings that hindered the quartz overgrowth. Grain coating illite occurrence is observed only in the deep marine Kimmeridgian-Tithonian sandstones of western part of the South Viking Graben. The authigenic clays mineral content has been evaluated both in the optical microscope and scanning electron microscopy. The deep marine Intra-Draupne Formation sandstone contains illite in abundance with detrital kaolin occurrence in traces. The Brae Formation contains both illite and kaolin as porefilling clay mineral along with partially leached feldspar grains. The occurrence of kaolin attributed to the meteoric water flushing in the Brae Formation. The lower content of illite is responsible for the better permeability of the Brae Formation because its fibrous morphology has been reported to reduce the permeability. The intergranular volume is observed to be stabilizing at 25% and the stylolization phenomenon demonstrates that dissolution at stylolites acted as source for quartz cementation, which is the mechanism behind the destruction of porosity in the Upper Jurassic sandstone reservoirs in the South Viking Graben.