Diffusive anisotropy in low-permeability Ordovician sedimentary rocks from the Michigan Basin in southwest Ontario

• Diffusive anisotropy was studied for sedimentary rocks.
• Effective diffusion coefficients (De) were determined on cm-scale paired samples.
• SEM technique was used to visualize diffusion pathways at the tens of μm scale.
• Several characteristic diffusion pathways were revealed by SEM imaging.
• Tracer specific effect on De value was evaluated.

Diffusive anisotropy was investigated using samples from Upper Ordovician shale and argillaceous limestone from the Michigan Basin of southwest Ontario, Canada. Effective diffusion coefficients (De) were determined for iodide (I−) and tritiated water (HTO) tracers on paired cm-scale subsamples oriented normal (NB) and parallel to bedding (PB) prepared from preserved drill cores within one year from the date of drilling. For samples with porosity > 3%, an X-ray radiography method was used with I− tracer for determination of De and porosity accessible to I− ions. A through-diffusion method with I− and HTO tracers was used for most siltstone and limestone samples with low-porosity (< 3%). The De values range from 7.0 × 10− 13 to 7.7 × 10− 12 m2·s− 1 for shale, 2.1 × 10− 13 to 1.3 × 10− 12 m2·s− 1 for limestone, and 5.3 × 10− 14 to 5.6 × 10− 13 m2·s− 1 for siltstone and limestone interbeds within the Georgian Bay Formation shale. The sample-scale anisotropy ratios (De-PB:De-NB) for De values obtained using the I− tracer are 0.9 to 4.9, and the anisotropy ratios for the HTO tracer are in the range of 1.1 to 7.0.The influence of porosity distribution on diffusive anisotropy has been investigated using one-dimensional spatially-resolved profiles of I−-accessible porosity (shale only) and the use of AgNO3 for fixation of I− tracer in the pores, allowing for SEM visualization of I−-accessible pore networks. The porosity profiles at the sample scale display greatest variability in the direction normal to bedding which likely reflects sedimentary depositional processes. The SEM imaging suggests that diffusion pathways are preferentially oriented parallel to bedding in the shale and that diffusion occurs dominantly within the argillaceous component of the limestone. However, the fine clay-filled intergranular voids in the dolomitic domains of the limestone are also accessible for diffusive transport.