Oral Presentation Australasian Groundwater Conference 2017

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:

• Measured a vertical profile of helium concentrations in the Watermark/Porcupine Formation aquitard sequence.
• Constrained rates of helium diffusion to ensure helium reached equilibrium between pore water and quartz.
• Simplified the diffusion modelling process to allow the quantitative assessment of helium equilibrium between pore water and quartz.
• Modelled the formation-scale transport, production and partitioning of helium between pore-water and solid phase (mainly quartz) in the aquitard sequence.
• Compared results to independent core-scale measurements of aquitard permeability.

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.