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Abstract: Reconstruction and Geochemical Modelling of the Diagenetic History of the Middle Jurassic Oseberg Sandstone Reservoir, Oseberg Field, Norwegian North Sea

Jean-Pierre Girard, Bernard Sanjuan, Christian Fouillac, Karen Ziegler, Max Coleman

A detailed multidisciplinary integrated study of the Middle Jurassic Oseberg reservoir in 13 wells of the Oseberg field, Norwegian North Sea, was carried out in order to: (1). reconstruct precisely the timing, conditions and spatial variation of diagenetic transformations; (2). characterize the nature and origin of the diagenetic fluids and (3). 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 ranging from 100 to 125° C. The detrital assemblage is mainly composed of quartz, K-feldspar, albite, muscovite and lithic clay clasts, and is very homogeneous throughout the study area. The chronological sequence of diagenetic phases established from petrographic observations includes: minor siderite and pyrite, K-feldspar overgrowths, ankerite, feldspar dissolution, vermiform kaolinite, quartz overgrowths, poikilotopic Fe-rich calcite, and dickite. Diagenetic temperatures were determined from fluid inclusions in ankerite, quartz and calcite. Combination with modelled burial/thermal history permitted to constrain approximate ages and duration of major diagenetic events. Isotopic co positions of diagenetic cements indicate that meteoric water was (and still is) a major constituent of diagenetic fluids.

Present formation waters are fairly similar chemically and isotopically at reservoir scale and represent mixing of three endmembers: seawater, meteoric water and primary evaporative brine. Stability diagrams and chemical geothermometers suggest that formation fluids are close to equilibrium with the host sandstone at present reservoir temperatures. Geochemical modelling of the diagenetic evolution of water-reservoir interactions was carried out using the EQ3/6 code and the AllanTM/Neptunix integrated simulator system. Results emphasize the importance of circulations of large volumes of fluid within the reservoir throughout the diagenetic history.

AAPG Search and Discovery Article #90956©1995 AAPG International Convention and Exposition Meeting, Nice, France