RADIOGENIC AND STABLE-ISOTOPE EVIDENCE FOR AGE AND ORIGIN OF AUTHIGENIC ILLITES IN THE ROTLIEGEND, SOUTHERN NORTH-SEA

Aeolian sandstones of the Lower Permian Leman Formation (Rotliegend Gr oup) provide the best gas reservoir in the southern North Sea, but per meability is greatly reduced by the presence of authigenic fibrous ill ites. New radiogenic (K/Ar) and stable (oxygen and hydrogen) isotope d ata are presented for fibrous illite cements (< 0.1 mum), so that the absolute timing and controlling diagenetic factors for their formation can be more fully evaluated. Thus, the expected quality of gas reserv oirs in the southern North Sea might be better predicted. Samples have been analysed from five wells in areas with contrasting structural ev olution: the Sole Pit Basin, and the Indefatigable Shelf. The K/Ar age s of between 160 and 190 Ma have been obtained from the Indefatigable Shelf illites, and between 120 and 160 Ma for those from the Sole Pit Basin, reflecting different times of basin inversion. These K/Ar ages are interpreted by reference to burial/thermal models for each well. T he temperature of illite precipitation falls between 88 and 140-degree s-C. Calculated pore-fluid compositions derived from oxygen and hydrog en isotopic analyses give values of approximately + 1 to +9 parts per thousand (SMOW) deltaO-18 and +1 to -50 parts per thousand (SMOW) delt aD. The illite deltaD values have probably been affected by isotopic e xchange and fractionation with the surrounding gaseous hydrocarbon. Th e deltaO-18 values reflect the degree to which evaporative concentrati on had affected Zechstein marine waters which subsequently invaded the Leman Sandstone. Comparisons between deltaO-18 and deltaD values in c lays and in formation water for the Leman Field suggest that oxygen is otope exchange might have taken place, and that the initial K+ and rad iogenic Ar-40 contents within illites may have been modified.