Carbonate Cementation in the Middle Jurassic Oseberg Reservoir Sandstone, Oseberg Field, Norway: A Case of Deep Burial–High Temperature Poikilotopic Calcite
- Sadoon Morad
Published Online: 17 APR 2009
Copyright © 1998 The International Association of Sedimentologists
Carbonate Cementation in Sandstones: Distribution Patterns and Geochemical Evolution
How to Cite
Girard, J.-P. (2009) Carbonate Cementation in the Middle Jurassic Oseberg Reservoir Sandstone, Oseberg Field, Norway: A Case of Deep Burial–High Temperature Poikilotopic Calcite, in Carbonate Cementation in Sandstones: Distribution Patterns and Geochemical Evolution (ed S. Morad), Blackwell Publishing Ltd., Oxford, UK. doi: 10.1002/9781444304893.ch13
- Published Online: 17 APR 2009
- Published Print: 29 MAY 1998
Print ISBN: 9780632047772
Online ISBN: 9781444304893
- carbonate cementation in Middle Jurassic Oseberg reservoir sandstone, Norway;
- deep burial –high temperature poikilotopic calcite;
- petrography and minerology;
- isotopes and rare earth elements (REEs);
- petrography and spatial distribution of carbonate cements
Diagenetic carbonate cement in reservoir sandstones of the Oseberg Formation (Brent Group) in the Oseberg field, Norwegian North Sea, occurs as disseminated siderite and ankerite, and as massively calcite-cemented intervals. Other diagenetic features include extensive feldspar dissolution and K-feldspar, quartz, kaolinite and dickite cements. Conditions of carbonate cementation are constrained on the basis of textural, geochemical and fluid inclusion evidence.
Siderite formed early in the diagenetic history at low temperature (20–40°C) from mixed marine/meteoric waters. Ankerite formed at a higher temperature (70–80°C) in the Latest Cretaceous—Early Tertiary from waters of meteoric and marine origin with a significant influence of shale-derived fluids. Siderite and ankerite are sporadically distributed and affect only minor volumes of sediments. Conversely, calcite cement is volumetrically significant at field scale (≈1%). The calcite is ferroan and occurs as inter- and intragranular poikilotopic cement. Textural relationships indicate that it postdates all other diagenetic cements but dickite. Aqueous and hydrocarbon fluid inclusions yield formation temperatures (90–110°C) close to present-day reservoir temperatures. Combined with the reconstructed thermal and oil-filling history of the reservoir, this implies that calcite cementation occurred in the past 40 Myr and simultaneously with oil emplacement. Calcite is characterized by consistently low δ18O (≈−15‰ PDB) and δ13C (≈−12‰ PDB) values, reflecting primarily a high formation temperature and organic sources of carbon, and by elevated 87Sr/86Sr ratios (av. 0.7119). δ18O and 87Sr/86Sr values indicate that calcite-forming fluids were isotopically similar to present-day formation fluids. They acquired their isotopic signature following pervasive dissolution of feldspars prior to calcitization and, possibly, as a result of the introduction of oil-accompanying basinal brines in the reservoir.
Calcite-cemented intervals are heterogeneously distributed within the field. They are a few metres thick, with lateral extents no greater than a few kilometres. The amount of Ca necessary to form the <1% calcite present in the reservoir is small and compatible with either internal or external sources. The exact source of Ca could not be pinpointed, nor could the mechanism of reconcentration in specific intervals. REE compositions of calcite samples, however, suggest that trace elements originated from local and variable sources, and support the prevalence of relatively closed conditions during calcite cementation.