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

• Published in: Hydrogeology journal Vol. 19; no. 1; pp. 237 - 247
• Main Authors: Ali, S, Stute, M, Torgersen, T, Winckler, G, Kennedy, B. M
• Format: Journal Article
• Language: English
• Published: Berlin/Heidelberg Berlin/Heidelberg : Springer-Verlag 01-02-2011; Springer-Verlag; Springer Nature B.V
• Subjects: Aquatic Pollution; Diffusion; Diffusion coefficient; Earth and Environmental Science; Earth Sciences; Fluid dynamics; Gases; Geochemistry; Geology; Geophysics/Geodesy; Helium; Hydrodynamics; Hydrogeology; Hydrology; Hydrology/Water Resources; Pore fluids; Porosity; Rocks; stable isotopes; Tortuosity; Waste Water Technology; Water Management; Water Pollution Control; Water Quality/Water Pollution; United States; Pore fluids; USA; Porosity; Tortuosity; Stable isotopes
• ISSN: 1431-2174; 1435-0157
• DOI: 10.1007/s10040-010-0645-6

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 ⁴Hewater 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).