Clay diagenesis in the sandstone reservoir of the Ellon Field (Alwyn, North Sea)

The nature, composition, and relative abundance of clay minerals in the san dstones of the Brent Group reservoir were studied between 3200-3300 m in a well of the Ellon Field (Alwyn area, North Sea). The sandstones have a hete rogeneous calcite cement which occurred during early-diagenesis. Clay diage nesis of the cemented and uncemented sandstones was investigated using opti cal microscopy, scanning electron microscopy (SEM), X-ray diffraction analy ses (XRD), and infrared spectroscopy (IR). The influence of cementation on clay neoformation is demonstrated in this study. Detrital illite and authig enic kaolinite are present in both the calcite-cemented and uncemented sand stones suggesting that kaolinite precipitated before calcite cementation. I n the uncemented sandstones, blocky dickite replaces vermiform kaolinite wi th increasing depth. At 3205 m, authigenic illite begins to replace kaolini te and shows progressive morphological changes (fibrous to lath-shape trans ition). Al 3260 m, all sandstones are not cemented by calcite. Illite is th e only clay mineral and shows a platelet morphology. In the cemented samples, vermiform kaolinite is preserved at all depths, su ggesting that dickite transformation was inhibited by the presence of the c alcite cement. This observation suggests that calcite cement would prevent fluid circulation and dissolution-precipitation reactions.