Hydrogen isotope fractionation factor for mixed-layer illite/smectite at 60 degrees to 150 degrees C: New data from the northeast Texas Gulf Coast

Abstract

The deltaD values of water and clay collected from 0.6 to 5.5 km deep wells in 9 oil and gas fields within the Houston salt basin located in the northeast Texas Gulf Coast were used to reevaluate the temperature dependence of hydrogen isotope fractionation between mixed layer illite/smectite (I/S) and water, and the equations of Yeh (1980, Geochim. Cosmochim. Acta, 42:140-143) and Capuano (1992, Geochim. Cosmochim. Acta, 56:2547-2554) for the calculation of alpha(I/S)-water, both of which are commonly cited in the literature. deltaD values of water and clay in the normally pressured and geopressured sections are different. In the normally pressured section (<2.6 km), deltaD(water) is constant (-15+/-1 (1sigma)parts per thousand) while deltaD(clay) increases linearly from -59 to -43parts per thousand with increasing depth. In contrast, in the geopressured section deltaD(water) decreases linearly from -7 to -26parts per thousand with increasing depth while deltaD(clay) is nearly constant (-36 +/- 3 (1sigma)parts per thousand). The opposite trends are a product of D/H exchange between clay and water in a water-dominated system within the normally pressured section and rock-dominated system in the geopressured section. Assuming hydrogen isotope equilibrium between sediments and enclosed pore water, the new deltaD(water) and deltaD(clay) data were used to calculate the hydrogen isotope fractionation factor between I/S and water (alpha(I/S-water)), which showed a good fit to the equation derived by Capuano (1992), but not to the equation derived by Yeh (1980), both for the normally and geopressured data despite that both data sets are strikingly different. This suggests that hydrogen isotope equilibrium has been achieved in these two different regimes and that the equation by Capuano (1992) is valid and has wider applicability. Copyright (C) 2004 Elsevier Ltd.