P-T CONDITIONS OF SANDSTONES SILICIFICATION FROM THE BRENT-GROUP (DUNBAR, NORTH-SEA)

Silicification is one of the diagenetic phenomena that causes a drasti c decrease of porosity in sandstone reservoirs. Ln several important h ydrocarbon fields located in the Brent Province of the northern North Sea, silicification is found together with illitisation. The present p aper attempts to define pressure and temperature conditions of silicif ication in the Dunbar field (Greater Alwyn, Great Britain). SEM cathod oluminescence observations were performed on sandstones. Raman FTIR mi croanalyses and microthermometric measurements were done on single inc lusions precisely related to the quartz overgrowth generation. Two mai n types of aqueous and hydrocarbon-bearing fluid inclusions are observ ed and can be related to the beginning of the main silicification phas e: (1) Trapping of fluid inclusions with highly variable methane conte nts in rehealed cracks; (2) Trapping of contemporaneous aqueous and hy drocarbon fluid inclusions, mainly at the boundary between detrital gr ains and overgrowth. Two phase aqueous fluid inclusions homogenize bet ween 105 and 110 degrees C in the Frontal Panel area and between 100-1 05 and 105-110 degrees C in the West Flank area depending on the well. Salinities are between 2.5 and 4 wt% equiv. NaCl. Raman microspectrom etric analyses and observation of earlier heterogeneous trapping sugge st that the brine is methane-saturated in all samples with no other ga s species detected. FTIR microspectrometry of hydrocarbon-bearing flui d inclusions show that they contain light aliphatic mature oils (C-8-C -10 alkane equivalent) with variable amounts of dissolved CO2. Their h omogenization temperatures vary between 80 and 90 degrees C, depending on the well. Since the trapped brines are in the methane-saturated H2 O-NaCl(+/-KCl) system, homogenization temperatures of aqueous inclusio ns are equal to trapping temperatures. The trapping temperatures are 1 5 to 20 degrees C lower than the present-day formation temperatures, t his argues against a present-day resetting of the inclusions. The dedu ced pressure values are near 400 +/- 20 bars. Using basin modelling (G ENEX) to match observed organic matter maturation parameters, the temp eratures obtained from fluid inclusions correspond to a burial depth o f 2.3 to 2.5 km sub-seafloor for the Frontal Panel and 2.4 to 2.8 km s ub-seafloor for the West Flank. The pressure values obtained from flui d inclusions are higher than those derived from modelling which may in dicate the existence of overpressures in the reservoir at the time of cementation.