Size fractions (0.4 and 0.4–1.0 m) of Brent Group sandstones from the northern North Sea contain mostly illite-smectite mixed layers with kaolinite, whereas the same size fractions of Fulmar Formation sandstones from the south-central North Sea consist of illite-smectite mixed layers with minor chlorite. Transmission electron microscope observations show elongated illite laths or agglomerates consisting of small laths when larger individual laths are lacking.

The K-Ar data of the fractions less than 0.4 m of Brent Group samples plot on two arrays in a ⁴⁰Ar/³⁶Ar vs. ⁴⁰K/³⁶Ar diagram that have isochron characteristics with ages of 76.5 4.2 and 40.0 1.5 Ma, and initial ⁴⁰Ar/³⁶Ar ratios of 253 16 and 301 18, respectively. For the Fulmar Formation samples, the data points of the fractions less than 0.2 and less than 0.4 m also fit two isochrons with ages of 76.6 1.4 and 47.9 0.5 Ma and initial ⁴⁰Ar/³⁶Ar ratios of 359 52 and 304 2, respectively. Some of the coarser 0.4–1.0-m fractions also plot on the two isochrons, but most plot above indicating the presence of detrital components more than 0.4 m. The almost identical ages obtained from illite-type crystals of sandstones with different deposition ages that are located about 600 km (373 mi) apart record two simultaneous illitization episodes. These events were not induced by local burial conditions, but are related to episodic pressure and/or temperature increases in the studied reservoirs, probably induced by hydrocarbon injection. This interpretation is indirectly supported by notably different K-Ar illite ages from cores of a nearby reservoir at hydrostatic pressure.

Illite is not as well crystallized as expected for potential crystallization temperatures above 160C measured by fluid-inclusion determinations. In both the northern and south-central North Sea, the two illite generations remain unaffected after crystallization despite continued burial, suggesting notably higher crystallization temperatures than those estimated from geothermal gradients. No recent illite crystallization or alteration is recorded in the K-Ar ages, despite a dramatic regional acceleration of the subsidence in the southern North Sea.