Abstract
The Upper Jurassic sub-arkosic turbidite sandstones of the Magnus oil field, UK North Sea, are dominated by quartz, ankerite and kaolinite cements that all grew immediately prior to, and during, the early stages of oil-filling. The sandstones have no evidence of siliceous bioclasts or pressure dissolution yet contain ≤15% quartz cement with relatively more in the water leg than in the oil leg. Petrographic analysis revealed that the sandstones have undergone major K-feldspar alteration, especially in the water leg. XRF whole-rock data show that sandstones have uniform Si/Si+Al throughout the reservoir indicating that silica is not imported into the sandstone during quartz cementation. Carbon isotope data from the ankerite require an influx of organically derived CO2. Geochemical modelling of a CO2-influx into arkosic sandstone suggests that quartz, ankerite and kaolinite cements are likely to be genetically related. Reaction of K-feldspar with water at low pH will produce quartz and kaolinite. Low pH resulted from CO2-influx and was maintained through ankerite precipitation. The reaction led to bulk loss of potassium (most likely to the surrounding mudrocks). K-feldspar dissolution is thus the result of an influx of externally (source rock) derived CO2 although the silica in the quartz cement is derived internally in the sandstone. While there is no need to invoke large scale-import of silica-laden waters, quartz cementation is the result of fluid influx.
Published Version
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