Geochemical evidence for the history of diagenesis and fluid migration; Brent Sandstone, Heather Field, North Sea

The diagenesis of Brent Group sandstones at Heather Field was studied to reconstruct time-dependent variations in reservoir quality, hydrodynamic history, and oil emplacement. Depositional facies, isotopic and trace-element composition of authigenic minerals, and present-day formation-water chemistry indicate several major changes in porewater chemistry related to both gravitational and compactional flow systems that significantly impacted diagenesis. Early cementation by calcite was related to influx of meteoric water and completely occludes porosity in certain areas of the Field, especially in lower reservoir zones. Geochemical, petrographic, and structural evidence indicate that average calcite precipitated at low to moderate temperature from reducing isotopically-depleted water having high levels of radiogenic Sr (40°–50°C, δ 18O = −4 to −6‰, 87Sr/86Sr > 0·71). A major period of kaolinite precipitation and feldspar dissolution followed calcite cementation. The isotopic composition of pore-filling kaolinite shows Field-wide uniformity (δ 18O average 13·8‰, δD average −53·2‰), suggesting thorough flushing of the reservoir by meteoric water and precipitation at low to moderate temperature (45°–60°C). Tectonic, burial, and thermal histories suggest that meteoric flushing occurred during the late-Cimmerian sea-level low, possibly in response to gravitational flow of meteoric water from exposed parts of the adjacent East Shetlands Platform. Illite and quartz diagenesis post-date kaolinite cementation, with illite K-Ar ages indicating precipitation through much of the Paleogene (55–27 Ma), coincident with migration of hydrocarbons from neighbouring sub-basins of the East Shetlands Basin. Illite stable isotopic data indicate precipitation in a system resulting from partial mixing of trapped meteoric pore-fluids with saline compaction water. The intensity of sandstone diagenesis is influenced by differences in the fluid migration history, content of detrital K-feldspar, and the time of hydrocarbon emplacement and results in spatial and temporal variations in reservoir quality.