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When an upper
reservoir has been pressure depleted by production, upwardly moving
gases from the lower reservoir are partially captured or
"thieved" into the upper reservoir (blue). The total
fluxing hydrocarbon at the surface is depressed because of the
"thieving" but the hydrocarbon anomaly is detectable against
background and it shape is retained. Older geochemistry methods,
having no ability to concentrate the soil vapor sample, resulted in
non-detectable hydrocarbon above a producing pool. This lead to
the concept of the "halo" where the pool itself was not
detectable, only a thin hydrocarbon ring around the pool.
Gas-sieve geochemistry eliminates the halo effect and the pool is
visible as a positive hydrocarbon feature.
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