Abstract

Economic helium accumulations, like hydrocarbon accumulations, result from predictable processes of generation and migration. Concepts for helium generation and migration can be used to explore for helium much like the principles of petroleum generation and migration can be used to explore for oil and gas.

Helium forms by radioactive decay of uranium (U) and thorium (Th). At diagenetic temperatures, helium diffuses rapidly from mineral grains into pore water. Helium concentration in the water increases with (1) increasing U and Th concentration in rocks enclosing the pore water, (2) increasing age of the pore water, and (3) decreasing porosity in the reservoir. If water is static, helium can only move by diffusion, but diffusion disperses the helium. Generation of helium does not invariably lead to high helium accumulations.

Helium is concentrated into economic accumulations where pore water interacts with a gas phase. Helium is concentrated in the gas as described by Henry’s Law. Any gas migrating through pore water will quickly extract almost all helium from the water. High helium concentration in the gas is favored by (1) high helium concentration in the pore water, (2) low volume of gas that interacts with the water, and (3) low pore pressure where the gas interacts with the water. Once helium is entrained in the gas, it migrates with the gas to accumulate in traps just like other gas accumulations.

As long as helium remains dissolved in the pore water, helium can only migrate with the water. This makes it difficult for helium in economic accumulations to be derived from lower and middle crust sources. Deeply generated helium cannot migrate to traps in overlying strata unless water or gas moves up through the basement. Formation of a gas phase at mid-crust depth is quite difficult, because the deeper crust is already de-volatilized. Basement faults or fracture zones may be potential pathways for migration from the deep crust, but little helium will migrate from the deep crust along these potential pathways in the absence of a migrating fluid. Fortunately, older sediments can act as efficient helium source rocks and have sufficient helium generation potential to account for most known economic helium accumulations. A deep basement source is not necessary.

The following guidelines are proposed to aid exploration for high helium gases. (1) Helium source rocks are older rocks with elevated U and Th concentrations and relatively low porosity but sufficient permeability to act as gas migration pathways. Clastic sediment, not deep basement, is the most probable source rock for economic accumulations. Fractured shales, arkoses, granite wash, and shallow fractured basement are good potential source rocks. (2) The pore water must be old and static prior to gas migration to allow significant helium accumulation prior to gas migration. Helium generated after gas migration will not accumulate, so recent gas migration through old pore water is most likely to develop high helium concentrations. (3) The depth of interaction of gas and water should be relatively shallow. Most high helium gases occur at relatively shallow depth. (4) The total volume of gas that interacts with the pore water should be relatively small; otherwise, helium concentration is diluted by later gas charge. Prolific generation of hydrocarbon gases will quickly dilute helium to sub economic levels. Explore in petroleum systems with non-hydrocarbon gases or marginal hydrocarbon gas generation. Alternately, explore near the updip edges of supercharged petroleum systems. Less gas will have migrated though carrier beds near the end of the migration pathway, so helium concentrations will not be diluted.

Nitrogen follows the same general concentration mechanism as helium, but its sources are different. Many high nitrogen gases in the greater Permian Basin are relatively early diagenetic gases associated with U- and Th-poor evaporites that have low helium source potential. This source accounts for high nitrogen, low helium gas, such as that in the Yates Formation, for example.

Volcanogenic CO₂ appears to have a major control on helium accumulation, especially in settings with few hydrocarbon gases. CO₂ gas sweeps helium from carrier beds, but later disappears as it reacts with the reservoir rocks. The CO₂ transport mechanism can be detected by helium isotopes even after CO₂ has reacted with the reservoir. Traps with fetch areas that contain intrusives may be especially favorable for high helium accumulation, assuming pore water was old at time of intrusion and trap seals are still intact.

The validity of these controls are demonstrated by geochemical interaction models and correlations of regional- and field-scale helium concentrations in the southwest US.