Open and restricted hydrologies in Brent Group diagenesis: North Sea

Diagenetic cements in the mid Jurassic Brent Group sandstones of the North Sea have been studied in the Thistle, Murchison, Dunlin and Alwyn South (3/13a-1) oilfields. Volumetrically important cements start with early siderite, kaolinite and calcite, and continue to later kaolinite, ankerite, quartz and illite. The mineralogies of early siderites and calcites are homogeneous in formations with different depositional porewaters. Carbonate concretions nucleated on biotite grains, and their carbon supply was very local and facies controlled. Strontium was derived from dissolution of silicates and shell debris and ⁸⁷Sr/ ⁸⁶Sr rapidly became homogeneous in different formations. Oxygen was derived from meteoric water and was homogeneous throughout, even in marine deposits. These data suggest an open-flow system with uniform meteorically-derived pore fluids and local ion supply throughout the sandstones. Diagenetically later ankerite formed below 1.1 km and shows mineralogies which are less homogeneous, with ⁸⁷Sr/ ⁸⁶Sr variable. Cement ions could all have been very locally supplied from within the formation. Late kaolinite could also have been formed locally from feldspars within the formation. The exception to this was quartz cement, its large volumes indicating import across formations or from outside the Brent. Fluid inclusions in quartz show abnormally hot temperatures equating to 80-100°C at 2.2 km. These can be interpreted in two ways. First to indicate a large circulation of hot evolved meteoric water within the basin. This would have been induced by tensional fracturing of the crust related to the final phase of North Atlantic rupture in the Palaeocene and Eocene. However, fluid inclusion temperatures within quartz overgrowths show a systematic increase of temperature with present-day burial depth, suggesting that these temperature records could have been reset, and are now too hot. If these temperature data are rejected, a second interpretation is possible. The δ ¹⁸O isotopic signatures show that late diagenetic silicates and carbonates could have grown in meteoric water, unmodified in its δ ¹⁸O value, in a normal geothermal gradient. Illite grew in a slowly flowing fluid, which rapidly evolved to todays δ ¹⁸O values, as individual groups of pores attempted to equilibrate with the rock around them Oil accumulation halted diagenesis and was permitted by buoyant trapping as this porewater system became stagnant.