NUMERICAL MODELING OF DIAGENETIC QUARTZ HYDROGEOLOGY AT A GRABEN EDGE - BRENT OILFIELDS, NORTH-SEA

Oilfields in the Middle Jurassic Brent Group of the East Shetland Basin contain abundant quartz cement, which reduces porosity by 10-15%. This quartz cement is believed to have started to grow at depths of around 2-3km. Oxygen isotope signatures in diagenetic quartz imply that it grew from meteoric pore waters; however, fluid inclusion temperatures are 30-50 degrees C higher than temperatures which should have prevailed at the inferred palaeo-depth. Large-scale circulation of hot water has been proposed to explain this ''temperature anomaly'', but this is geologically difficult in terms of heat and fluid budgets. Finite-element-fluid-flow models of a generic fault-block rift- basin stratigraphy a-re used to evaluate two alternative hydrogeological models for deep quartz diagenesis in Brent Group sandstones: (1) Meteoric waters may have penetrated several kilometres into the crust before rising rapidly as a hot, isotopically-evolved fluid; (2) Cool meteoric waters from the palaeo-land, fed directly into the Brent Group aquifer, moving tens of kilometres laterally into the basin. Numerical models show that in this case gravitational penetration of moving fluids to considerable depth in the crust was indeed feasible with a driving head of only 200 m. Fluid velocities were so low (approximate to 0.01 m/yr) that the heating effects of the fluids were minimal. Long-range lateral transport of heat was also inefficient, for fluids tend to emerge along the first major fault plane encountered. Large-scale mixing within aquifers resulted in a uniform fluid during the start of quartz diagenesis in the Brent Group. This mixing was not sensitive to permeability anisotropy. The slow infiltration of meteoric fluids into Brent Group aquifers is inferred to have maintained a low-salinity, meteoric isotopic dominance to the pore fluids. These slow, large-scale lateral flows may have exited from the Brent aquifer at ''leak points'' on shallow-buried structures, such as Gullfaks. Degraded oil on the downdip side of Gullfaks also independently suggests an active deep aquifer in the Early Tertiary. Consequently, diagenetic quartz grew at equilibrium temperatures and adopted a meteoric-dominated isotopic signature. Quartz supply was decoupled from such fluids, and was probably locally-sourced.