Ankerite cement Oseberg Formation

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. 

Table 4. REE analyses (in ppm) of diagenetic calcite, siderite and ankerite cements in the Oseberg Formation, Oseberg field... 

Fig. 10. Chondrite-normalized REE patterns of (A) siderite, ankerite and (B,C) calcite in the Oseberg Formation. The numbers in the key boxes refer to the analysis numbers in Table 4. In (C) a same symbol shape denotes different samples coming from a single calcite-cemented layer in a same well.

Ankerite cement is very scarce in the Oseberg reservoir, except in one sample located at the boundary with the underlying shales. In this sample, fluid inclusion analysis indicates that ankerite formed at temperatures of 70-80 °C, i.e. during the latest Cretaceous or Early Tertiary. The Sr/ Sr ratios and REE patterns of ankerite are consistent with a significant influence of fluids and/or trace elements derived from shales. The 5 0 values indicate a marked contribution of meteoric water during ankerite formation. Ankerite from the fault zone and from the structural blocks have similar 5 0-5 C values, suggesting similar conditions of formation. 

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