Reservoir quality in the Jurassic sandstone reservoirs located in the Central Graben, North Sea

Abstract

Abstract This study investigates the diagenesis and reservoir quality of Upper Jurassic Sandstones of Ula Formation from the Central Graben. Petrophysical and Petrographical studies have been done on cored interval from well 2/1-6 of Ula Field. Precipitation of quartz cement is the main porosity destroying process in deeply buried, quartz-rich sandstone reservoirs of the North Sea. Quartz cement precipitates in the form of syntaxial overgrowth over detrital grain of quartz. Grain coatings, like micro-quartz and illite, are the main reasons for preservation of porosity in sandstones as they cover the grain and inhibit the quartz overgrowth. Petrographical data in this study clearly indicates that grain coatings are present in the studied samples. Micro-quartz grain coating is the most common grain coat in the Upper Jurassic Sandstones of Ula, which is generated from the transformation of siliceous sponge spicules known as Rhaxella Perforata. Clay grain coats like illite and chlorite are also present. Relation between Intergranular Volume (IGV) versus Matrix and Quartz Cementation versus Porosity have also been studied. IGV is strongly affected by mechanical compaction, grain size, grain shape, quartz, and carbonate cements. Sandstones with high amount of matrix and fine grains have high IGV as compared to coarse grains because coarse grains are compacted more when they are subjected to mechanical compaction. Grain shape also has a pronounced effect on porosity. Angular grains tend to lose porosity easier as they are subjected to stress.