Diagenesis of the Oseberg Sandstone Reservoir (North Sea): An Example of Integration of Core, Formation Fluid and Geochemical Modelling Studies: ABSTRACT

Abstract

A detailed multidisciplinary integrated study of the Middle Jurassic Oseberg reservoir in 20 wells of the Oseberg field, Norwegian North Sea, was carried out in collaboration with Norsk Hydro and Oseberg partners. The objectives were to reconstruct the tinting, conditions and spatial variation of diagenetic transformations; to characterize the nature and origin of diagenetic fluids; and to develop a geochemical model of the observed diagenesis. The 20-60 m thick Oseberg Formation occurs at depths of 2.5 to 3.2 km, and at present temperatures of 100 to 125[degrees]C. The detrital assemblage is mainly composed of quartz, K-feldspar, albite, muscovite and lithic clay clasts, and is very homogeneous throughout the field. The diagenetic sequence includes: minor siderite and pyrite, K-feldspar rims, ankerite, pervasive feldspar dissolution, abundant vermiform kaolinite, quartz overgrowths, poikilotopic ferroan calcite, and dickite. Diagenetic temperatures were derived from fluid inclusions in ankerite, quartz and calcite, and combined with the modelled burial/thermal history to constrain approximate ages and duration of diagenetic events. Isotopic compositions of carbonates and kaolinite indicate that meteoric water and seawater were two major constituents of diagenetic fluids. Present formation waters are fairly similar chemically and isotopically at reservoir scale and represent mixing of three end members: seawatermore » ([approximately]54%), meteoric water ([approximately]40%) and primary evaporative brine ([approximately]6%). Stability diagrams and chemical geothermometers indicate that formation fluids are close to equilibrium with the host sandstone at present reservoir temperatures.« less