Reservoir Quality in the Jurassic Sandstones reservoirs located in the Central Graben

Abstract

ABSTRACT This study investigates the diagenesis and reservoir quality of Upper Jurassic Sandstones from the Central Graben. Petrophysical and petrographical studies have been done on cored interval from well 2/1-6. Precipitation of quartz cement is the main porosity destroying process in deeply buried quartz rich sandstone reservoirs of the North Sea. Quartz cement precipitate in the form of syntaxial overgrowth over detrital grain of quartz. Grain coatings like micro-quartz and illite are the main reasons of preservation of porosity in the area. Grain coats preserve porosity by covering the grain and inhibiting the quartz overgrowth. Petrographical and petrophysical data in this study clearly indicates that grain coatings are present in the Central Graben. Micro-quartz grain coating is the most common grain coat in the Upper Jurassic Sandstones of Ula Formation. Micro-quartz grain coat are generated from the transformation of siliceous sponge spicules known as Rhaxella Perforata. Though micro-quartz is present in all low and high porosity zones but it could not preserve porosity in low porosity zones. Clay grain coats like illite and chlorite grain coats are also present but in variable amounts. Relation between Intergranular volume (IGV) vs matrix and quartz cementation vs porosity have been also been studied. IGV is strongly affected by mechanical compaction, grain size, grain shape, quartz cementation, and carbonate cement. Sandstones with high amount of matrix and fine grained grains have high IGV as compared to coarse grained sandstones because coarse grained sandstones are compacted more when they are subjected to mechanical compaction. Grain shape has also a pronounced affect on the porosity in the area. Angular grains loose porosity as they are subjected to stress. In angular grains contact forces are more concentrated because of small contact areas. This is the reason we have low porosity zones which have angular grains and were not influenced by micro-quartz grain coatings.