unusual geological conditions are considered necessary for commercial concentrations of helium in natural gas. helium accumulations are commonly in structural closures overlying bedrock highs. faults, fractures, and igneous intrusives are regarded by some geologists as important pathways for helium to migrate upward into the sedimentary section. the atomic radius of helium is so small that shale, which is effective in trapping methane, allows the helium to migrate upward through the shale pores. nonporous caprock such as halite (rock salt) or anhydrite is more effective in trapping helium. helium deposits occur mostly in paleozoic rocks.