The Age of Illite Cement Growth, Village Fields Area, Southern North Sea: Evidence from K-Ar Ages and 18O/16O Ratios

ABSTRACT In this paper we describe K-Ar radiometric ages and oxygen isotope ratio measurements of illite cements from eolian sandstone samples of the Permian lower Leman Sandstone Formation (Rotliegendes Group). The samples come from a large (about 100 × 100 km) part of the Southern North Sea basin (Village Fields area) and from a range of burial depths (2.8-3.4 km subsea bed). Mean illite ages in the gas fields of Ravenspurn North (164 Ma ± 20.1 [2σ], n = 11), Hyde (161 Ma ± 12.3, n = 7), Cleeton (155Ma ± 20.2, n = 3), and Hoton (152 Ma ± 14.0, n = 14) fields, and in two wells located in Block 47/5A (162 Ma ± 16.1, n = 4) are all in the Middle to Late Jurassic, apart from two Early Cretaceous ages considered to represent unreliable analyses. The mean for all ages is 158 Ma ± 18.6 [2a], n = 39. We largely avoided contamination with (old) detrital K-bearing minerals by careful sample selection and monitoring. To a first approximation, the assumptions of K-Ar dating appear to be fulfilled, so the measured ages are likely to date illite cement precipitation. Oxygen isotope ratios measured on the dated illite samples show small but significant variations among fields and a weak correlation with depth during the Middle Jurassic when illite grew. Both K-Ar dating and oxygen isotope ratios are compatible with a regional Middle to Late Jurassic phase of illite growth during which the mineral precipitated over a large area and over a range of temperatures governed by the depth of the formation at that time. The age of illite cementation coincides with rifting, which ultimately may have caused illite growth by driving pore-water flow.