Carbonate precipitation late-diagenetic

Several lines of evidence suggest the involvement of TSR. Partially dissolved remnants of early diagenetic anhydrite cement are found embedded in quartz cement (Fig. 12). Fluid inclusion data indicate that nodular quartz cement with anhydrite inclusions (Fig. 15) formed at elevated temperatures, suggesting that anhydrite dissolution may have coincided with high temperature TSR. Post-pyrobitumen carbonate and pyrite cements and nodules are also present in the Norphlet and may represent late diagenetic precipitates associated with TSR. Pyrite cement from the Norphlet has sulphur isotope ratios that are identical to those of the Pine Hill Anhydrite (Table 3) and within the range reported for Jurassic seawater (Hoefs... 

Fig. 11. Plot of oxygen and carbon stable isotopic compositions of bulk carbonate samples. Note the small range in isotopic compositions of calcite in lenses despite variable ratios between early and late calcite cement. This points to similar physical and chemical conditions during replacement of early diagenetic CaC03 cement by low-Mg calcite and late calcite cement precipitation.

Fig. 6. (Opposite) Thin-section photomicrographs of textural occurrences of diagenetic carbonates in the Oseberg Formation. (A) Spherulitic precipitates of early diagenetic siderite (S) embedded in late poikilotopic calcite (C).

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