RE: A bit Geology for a rainy day8 May 2021 17:01
HELIUM MIGRATION FROM THE BASEMENT
Helium generated in the deep crust must migrate to the shallow subsurface to charge traps and mix with hydrocarbon gases. There are three possible mechanisms:
• Diffusion. Widely accepted to be ineffective due to long distances due to km-scale migration distances.
• Bulk fluid flow. Two possible fluids: melt and water. Not effective in cratonic areas due to absence of melt and
water flow from crust and dehydrated nature of deep crust.
• Two-phase flow. Buoyancy drives flow of low density fluid in pore spaces. This is the most likely mechanism for flow from the deep crust. Requires formation of a fluid less dense than water, such as gas or CO2 . Such a fluid forms where their fugacities sum to the total pressure. Results for such models are shown at right for low temperature gradients characteristic of cratons.
Two conditions are modeled:
(1) helium, steam, and CO2 re the only volatile species, and (2) some unknown gas species is available such that helium fugacity has to be only 10% of the total pressure to form a gas phase (unlikely but optimistic). The vertical axis is the time needed to generate sufficient helium to saturate water in a rock with 0.01% porosity, and the horizontal axis is the depth. Model 1 requires time greater than the age of the earth to form a gas phase at 1 km or deeper by helium generation. Model 2 requires helium generation for about 2.5 billion years to form a gas phase at depths greater than 10 km. This is older than the age of the basement in the midcontinent. In other words, a helium-rich gas phase cannot form in the deeper crust in the stable craton.
Helium cannot migrate to shallow depths in a gas phase.
Helium can migrate to the surface from the deep crust and mantle in magmas and immiscible CO2 -rich fluid where heat flow is high. Both require the high heatflow typical of continental margins and active rifts. These transfer mechanisms appear to be ineffective for forming economic high helium gases, because no economic high helium gas accumulation has been found in arc systems or in active rift systems.
Helium exploration requires the same skills and data as conventional petroleum exploration: evaluation of source, migration,
and trapping. However, sources are different, and interaction with water is much more important. Here are some general guidelines for helium exploration:
• Explore in old sediment with old pore waters. Use He/N2 and lithology (GR logs) as a guide to the helium generation potential.
• Explore for relatively shallow traps. He partitions into gas better at shallow depth, higher salinity pore water, and cooler
temperatures.
• High grade prospects with long fetch areas, because long migration maximizes gas exposure to helium-bearing water.
• Avoid central parts of supercharged petroleum systems because helium will be too diluted by methane charge. Explore along the edges of these systems or in reservoirs bypassed by most migration.
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