Diffusional release of helium-4 from mineral phases as indicators of groundwater age and depositional history.

摘要

Recent measurements of diffusional flux of 4He from aquifer material suggest that it may be used a quantifiable age indicator in groundwaters on the scale of tens to thousands of years. Measurements from three different geological systems (clay aquitard, fractured bedrock aquifer, and a reservoir sandstone associated with natural gas) support non-steady diffusional release 4He, indicating not only ages of fluids associated with aquitards but also time constraints on depositional timing and exposure of the sediments.;Measurements of helium content from aquifer and aquitard material from St. Clair Clay Plain and a fractured bedrock aquifer in western New York respectively, show that the amounts of helium stored in the sediment grains are orders of magnitude above the steady state concentration, balanced from production by uranium and thorium and diffusional release. Measurements of concentrations of excess 21Ne indicate that the helium concentrations are lower than the calculated production ratio (4He/21Ne excess). This indicates that the helium is diffusing out from the mineral phases in the sediment and accumulating with the associated groundwater. Modeling of helium diffusion constrained the diffusion coefficients involved and verified that two distinct mineral phases (calcite and quartz) control the release of helium to the sediments. Modeling also constrained the timing of helium accumulation in the associated media and controls on the depositional timing and degree of recycling of the sediments.;Noble gas measurements made on natural gas occurrences in western New York reveal that thermal temperatures associated with the formation of hydrocarbons cause a diffusional fractionation of the noble gases during release from the source rock. This fractionation produces a distinctly light isotope enriched gas caused by enhanced diffusional release of the light isotopes. The distinct isotopic signature in the ratios of the nuclear components (4He, 21Neexcess, and 40Arradiogenic) is preserved during migration, and is preserved in the quartz grains within the reservoir rock. With exposure, the quartz grains release the helium and retain the heavier components. Modeled release times for the helium from measured losses can be on the order of hundreds of thousands to millions of years of residence in a near surface setting.