Abstract

The natural gas emanating from three seeps in and near Yellowstone National Park is distinctly different from the gas at six seeps in Grand Teton National Park. All the seep gases, however, contain hydrocarbons ranging from methane to at least the butanes along with variable amounts of carbon dioxide, nitrogen, oxygen, and argon. This report is the first to describe hydrocarbon gases in seeps in Grand Teton National Park.

The Yellowstone Park seeps contain less than 1% hydrocarbons, and the principal gas is carbon dioxide (91.0 to 95.4%), with carbon isotopic compositions (δ¹³C) ranging from -2.1 to +0.5°/00 (relative to the PDB standard). These carbon dioxide-dominated seeps are typical of geothermal regions. The minor hydrocarbons are probably derived from the thermal decomposition of sedimentary organic matter.

In Grand Teton National Park, the seep gases have variable compositions and fall into two categories: nitrogen-dominated seeps, with nitrogen contents from 69.4 to 89.1%, and methane-dominated seeps, with 83.7 to 94.5% methane (δ¹³C = -49.7° /00). Carbon dioxide content ranges from about 1 to 10% (δ¹³C from -6.6 to -20.4° /00). Four seeps sampled on the west edge of Jackson Lake along the trace of the Teton Fault contain from about 8 to 84% methane (δ¹³C from -59.2 to -61.9° /00) accompanied by up to 40 ppm of heavier hydrocarbons. In these four seeps and one other, the hydrocarbons have molecular and isotopic compositions suggesting a mixture of gas from microbial and thermal sources. Where nitrogen is the principal gas component, it is believed to be from atmospheric gas that was introduced previously into groundwaters during recharge of the plumbing system of the seeps. Variable compositions indicate that processes of gas generation in Grand Teton National Park are more complex than originally anticipated.