ABSTRACT

The deeply buried (3.8-4.3 km) Brent reservoir sandstones of the Hild Field, northern North Sea, have undergone intense diagenetic alteration. Fluid inclusion, O-C-H-Sr isotope and K-Ar age data combined with petrography and burial history constrain the conditions and timing of the main diagenetic transformations as well as the nature and origin of diagenetic fluids.

Non-ferroan calcite cement formed at shallow depth (< 2 km) and low temperature (20-70°C) from predominantly marine-brackish connate water. Subsequently, pore-filling vermicular kaolinite and pore-lining illite, developed at shallow-intermediate burial depth and temperatures (< 100°C) during the Cretaceous, from water that could not be precisely constrained. The late diagenetic events include precipitation of quartz overgrowths, pore-filling ferroan calcite, pore-filling dickite, and pore-bridging, fibrous illite. They occurred at depths of 2.5 to 4.3 km and temperatures of 95 to 155°C from the Early Tertiary to the present. The late, deep-burial cements formed from evolved basinal waters, isotopically similar to present-day formation water, that originated from deeply buried mudstones in the nearby Viking graben and, to a lesser extent, in the East Shetland Basin. Introduction of these compactional waters in the reservoir began in the Late Cretaceous-Early Tertiary and both preceded and accompanied hydrocarbon migration.

The reconstructed diagenesis of the Hild Brent reservoirs displays notable differences with diagenetic models developed in many fields of the northern North Sea. In particular, diagenesis in Hild is characterized by the prevailing influence of deep basinal water, a feature related to the immediate proximity of the Viking Graben. This study further illustrates that diagenetic processes related to compaction-derived fluids, commonly reported in sediments located within rift basins, should also be expected to occur in neighboring sub-basins along rift margins.