ILLITE DATES RECORD DEEP FLUID MOVEMENTS IN PETROLEUM BASINS

K-Ar age dates of authigenic illite from sandstones in the UK south Ce ntral Graben have a bimodal distribution. In contrast to established h ypotheses of thermal triggers, this illite growth is explained by chan ges in hydrogeological history. Fluid motion during burial can hence b e dated. Illite growth at 84-58 Ma on the graben margins was coinciden t with rapid subsidence of the Graben axis and consequent expulsion of pore fluids onto the margins. This event pre-dated both overpressure, and the secondary migration and accumulation of hydrocarbon in the re gion. Illite growth was probably caused by increased solute transport rates during pore fluid motion. Illite growth on an axial high (33-30 Ma) occurred during overpressured conditions. This was triggered eithe r by: (1) downward fluid migration as a consequence of overpressure re lease. These fluids carried carboxylic acids or hydrocarbons from the highly overpressured Kimmeridge Clay Formation into the less pressured Fulmar Sandstone Formation; or (2) a decrease in water-rock ratios as fluid flow declined within the sandstones, as a consequence of overpr essure build-up. Published experiments show that illite growth is favo ured by low water-rock ratios.