THE TEMPERATURE-DEPENDENCE OF HYDROGEN ISOTOPE FRACTIONATION BETWEEN CLAY-MINERALS AND WATER - EVIDENCE FROM A GEOPRESSURED SYSTEM

Delta-D values of water samples from two Gulf Coast geopressured fields vary linearly with temperature, while the delta-D values from the coexisting clays remain constant and are independent of temperature. Fluid flow rates on the order of millimeters per year, characteristic of Gulf Coast geopressured systems, provide adequate residence time for hydrogen isotope equilibrium to be achieved between the extant porewaters and clay minerals. Since equilibrium is achieved, these delta-D values are used to calculate the temperature dependence of the hydrogen isotope fractionation factor between illite-smectite and water, alpha(clay-water)H, between 0 and 150-degrees-C: 1000 ln alpha(clay-water)H = -45.3 x 10(3)/T + 94.7. This equation predicts values for alpha(clay-water)H at 96-degrees-C which are equal to those predicted by a similar equation derived by YEH (1980). Above and below 96-degrees-C, this new equation predicts values which diverge from those predicted by the equation of YEH (1980) with a difference of +9 at 150-degrees-C and -25 at O-degrees-C. The possibility is raised that the very slow upward component of flow characteristic of Gulf Coast geopressured sediments, combined with the slow downward subsidence of the clay-rich sediments, can result in a system which is rock-dominated with respect to hydrogen.