Ankerite cement

Boles J. R. (1978) Active ankerite cementation in the subsurface Eocene of southwest Texas. Contrib. Mineral Petrol 68, 13-22. 

The fluid inclusions in the microcrystalline and coarse blocky Fe-dolomite and ankerite cement are extremely rare and too small (< 2 im) to be used for reliable determination of homogenization temperatures. 

Fig. 8. Composition of eodiagenetic (Cl and Dl) and mesogenetic (C2, D2 and D3) calcite and dolomite/ankerite cements.

Fig. 12. Plot of manganese and iron contents in Dl, D2 and D3 dolomite/ankerite cement in the distal and middle domains.

Ankerite occurs as disseminated intergranular rhombs in wells 30/6-7, 30/6-8 and 30/9-1. It was identified by XRD and EPMA (Table 1). Ankerite rhombs ate often precipitated within exfoliated mica flakes or on detrital surfaces of adjacent quartz grains (Fig. 6B). Ankerite abundance does not exceed a few percent, except in sample 30/9-1 2781.0, where it reaches 33% of the total rock volume. This is the only occurrence of abundant ankerite cement found in the samples investigated in this study. 

Only six inclusions were found in the ankerite cement of one sample (Table 2). These exhibit prismatic shapes, sometimes approaching negative... 

Evidence for ankerite cement is that homogenization temperatures are consistent and 20-40 C lower than present reservoir temperature (Fig. 7, Table 2). As ankerite formed in the middle part of the diagenetic sequence, it has prpbably been sub-... 

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

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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