Helium measurements of pore fluids obtained from the San Andreas Fault Observatory at Depth (SAFOD, USA) drill cores

⁴He accumulated in fluids is a well established geochemical tracer used to study crustal fluid dynamics. Direct fluid samples are not always collectable; therefore, a method to extract rare gases from matrix fluids of whole rocks by diffusion has been adapted. Helium was measured on matrix fluids extracted from sandstones and mudstones recovered during the San Andreas Fault Observatory at Depth (SAFOD) drilling in California, USA. Samples were typically collected as subcores or from drillcore fragments. Helium concentration and isotope ratios were measured 4–6 times on each sample, and indicate a bulk ⁴He diffusion coefficient of 3.5 ± 1.3 × 10^–8 cm² s^–1 at 21°C, compared to previously published diffusion coefficients of 1.2 × 10^–18 cm² s^–1 (21°C) to 3.0 × 10^–15 cm² s^–1 (150°C) in the sands and clays. Correcting the diffusion coefficient of ⁴He_water for matrix porosity (∼3%) and tortuosity (∼6–13) produces effective diffusion coefficients of 1 × 10^–8 cm² s^–1 (21°C) and 1 × 10^–7 (120°C), effectively isolating pore fluid ⁴He from the ⁴He contained in the rock matrix. Model calculations indicate that <6% of helium initially dissolved in pore fluids was lost during the sampling process. Complete and quantitative extraction of the pore fluids provide minimum in situ porosity values for sandstones 2.8 ± 0.4% (SD, n = 4) and mudstones 3.1 ± 0.8% (SD, n = 4).