Ankerite cement formation temperature

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. 

The stability relationship between dolomite and ankerite depends on the temperature and activity ratio of Fe /Mg " (Fig. 5c). Formation waters from Norwegian North Sea reservoirs (Egeberg Aagaard, 1989) have log (aFe2+/aMg2+) of -3 to -2 and fall within the stability field of dolomite and ankerite (Fig. 5c). Both of these minerals are widely reported as mesogenetic cements in these sediments (Saigal Bjorlykke, 1987 Morad et al., 1990). 

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