MASS-BALANCE OF ILLITIZATION AND QUARTZ OVERGROWTH IN THE SANDSTONE RESERVOIR OF THE ELLON FIELD (ALWYN, NORTH-SEA)

In the Ellon field (Alwyn area, North Sea), the Brent sandstones show a heterogeneous calcite cementation. The uncemented sandstones shows a higher grade of diagenesis than the cemented sandstones. Potassic fel dspar, muscovite and kaolinite are preserved from alteration in cement ed sandstones. They dissolved in uncemented sandstone while quartz ove rgrowth and illite developed. Mass balance computations show that sili con, aluminium and potassium are immobile during illite precipitation and quartz overgrowth. Therefore these elements are not brought by flu id flow. The potassium source for illite is feldspar dissolution while the main silica source for overgrowth is quartz dissolution by pressu re-solution. The calcite cement would prevent fluid flow. By filling t he porosity it will prevent chemical reactions by dissolution-precipit ation.