This presentation provides an overview of a novel approach using noble gas tracers to derive formation-scale hydraulic conductivities of key aquitards in the Gunnedah Basin, New South Wales. Concentrations of helium in quartz were measured as proxy for helium in aquitard pore-water, and used to calibrate a fluid flow model encompassing a 500-m thick aquitard. Specifically this study:
Modelling suggests a vertical fluid velocity on the order of 0.002–0.02 mm/year (~10-13 - 10-12 m/s), which is very slow and indicative of diffusion-dominated transport, especially because the aquitard thickness exceeds 500 m. The shape of the helium profile suggests helium concentrations in the underlying Maules Creek Formation varied over recent geological time.
The future usage of this method depends on the depth and thickness of the aquitard of interest. Deeper aquitards are at higher temperatures, which enhances the diffusion of helium in quartz, resulting in faster equilibrium, and thus more reliable results. Subsurface temperatures should be 40 °C at a minimum. In thicker aquitards (tens to hundreds of metres), internal helium concentrations are slow to adjust when helium concentrations change in adjacent formations. Therefore, equilibrium of helium between quartz and pore water can be assumed for such thick aquitards.