Geochemical constraints from formation water analyses from the North Sea and the Gulf Coast Basins on quartz, feldspar and illite precipitation in reservoir rocks

Abstract

Abstract Formation water analyses from North Sea and Gulf Coast Basin reservoirs provide important constraints on burial diagenetic processes. Thermodynamic calculations show that aluminium solubilities are low (< 1 ppm at 130°C) even in the presence of organic acids. Illite therefore forms mainly at the expense of dissolving aluminous minerals such as smectite and kaolinite. The potassium values of the formation waters are always lower than K-feldspar saturation during burial diagenesis (> 1.5 km burial depth), suggesting that K-feldspar is continually being dissolved. The potassium concentrations indicate, in most cases, super-saturation with respect to illite. This is evidence that the failure of illite to form is probably because aluminium is not available. In the North Sea Basin, potassium concentrations approach equilibrium with kaolinite and illite at higher temperatures (140°C), indicating that illitization at that point keeps up with K-feldspar dissolution. In the subsiding parts of the Gulf Coast Basin, where kaolinite and smectite are being heated, the potassium values are clearly lower than in the uplifted (cooling) parts of the basin where illite is not forming. This suggests that the potassium concentrations are to a large extent controlled by the rate of illite precipitation, which again depends on the availability of aluminium from dissolving Al-minerals. There is considerable evidence that both illitization and quartz cementation may continue in the water-saturated parts of the pore after oil emplacement. This is also consistent with an almost closed-system burial diagenesis. The primary mineralogical composition of the sediments and early diagenetic processes strongly influence deeper burial diagenesis and in particular the precipitation of illite.