P-T conditions of sandstones silicification from the Brent Group (Dunbar, North Sea)
Guilhaumou, Nicole; Cordon, Sophie; Durand, Claudine; Sommer, Frédéric
European Journal of Mineralogy Volume 10 Number 2 (1998), p. 355 - 366
published: Mar 31, 1998
manuscript accepted: Oct 6, 1997
manuscript received: Jan 18, 1996
ArtNo. ESP147051002015, Price: 29.00 €
Abstract Silicification is one of the diagenetic phenomena that causes a drastic decrease of porosity in sandstone reservoirs. In several important hydrocarbon fields located in the Brent Province of the northern North Sea, silicification is found together with illitisation. The present paper attempts to define pressure and temperature conditions of silicification in the Dunbar field (Greater Alwyn, Great Britain). SEM cathodoluminescence observations were performed on sandstones. Raman FTIR microanalyses and microthermometric measurements were done on single inclusions precisely related to the quartz overgrowth generation. Two main types of aqueous and hydrocarbon-bearing fluid inclusions are observed and can be related to the beginning of the main silicification phase: (1) Trapping of fluid inclusions with highly variable methane contents in rehealed cracks; (2) Trapping of contemporaneous aqueous and hydrocarbon fluid inclusions, mainly at the boundary between detrital grains and overgrowth. Two phase aqueous fluid inclusions homogenize between 105 and 110°C in the Frontal Panel area and between 100-105 and 105-110°C in the West Flank area depending on the well. Salinities are between 2.5 and 4 wt% equiv. NaCl. Raman microspectrometric analyses and observation of earlier heterogeneous trapping suggest that the brine is methane-saturated in all samples with no other gas species detected. FTIR microspectrometry of hydrocarbonbearing fluid inclusions show that they contain light aliphatic mature oils (C8-O2 alkane equivalent) with variable amounts of dissolved CO2. Their homogenization temperatures vary between 80 and 90°C, depending on the well. Since the trapped brines are in the methane-saturated H2O-NaCl(±KCl) system, homogenization temperatures of aqueous inclusions are equal to trapping temperatures. The trapping temperatures are 15 to 20°C lower than the present-day formation temperatures, this argues against a present-day resetting of the inclusions. The deduced pressure values are near 400 ± 20 bars. Using basin modelling (GENEX) to match observed organic matter maturation parameters, the temperatures obtained from fluid inclusions correspond to a burial depth of 2.3 to 2.5 km sub-seafloor for the Frontal Panel and 2.4 to 2.8 km sub-seafloor for the West Flank. The pressure values obtained from fluid inclusions are higher than those derived from modelling which may indicate the existence of overpressures in the reservoir at the time of cementation.