Variation in kaolinite morphology with growth temperature in isotopically mixed pore-fluids, Brent Group, UK North Sea

Diagenetic kaolinite in reservoir sandstones of the Brent Group precipitated following the dissolution of detrial feldspar. Two distinct morphologies of kaolinite occur: (1) early diagenetic vermiform kaolinite which is often associated with expanded detrital micas; (2) later diagenetic ‘blocky’ kaolinite. Combined hydrogen and oxygen isotopic studies suggest that vermiform kaolinite precipitated at 25–50°C, and blocky kaolinite at 50–80°C, from pore-waters of a similar isotopic composition (δ18O = −6.5 to −3.5‰). These pore-waters are interpreted to be either a mixture of meteoric and compactional waters, or alternatively a meteoric water that had evolved isotopically due to water-rock interaction. Kaolinite precipitation occurred predominantly during the late Cretaceous to early Eocene. Influx of meteoric water into the Brent Group, probably occurred during the Palaeocene. Fluid flow across the entire basin was driven by a hydrostatic head on the East Shetland Platform palaeo-landmass to the west. The development of the two kaolinite morphologies is possibly related to the degree of supersaturation at the time of precipitation. At low degrees of supersaturation, vermiform kaolinite precipitated slowly upon detrital mica surfaces. Blocky kaolinite precipitated more rapidly into open pore-space at higher degrees of supersaturation. Precipitation of blocky kaolinite was perhaps triggered by the decay of oxalate.