Inorganic Gases in Natural Gas as an Indicator of Subcrustal Events in Southeastern Colorado

NELSON, JON S., and E. C. SIMMONS, Colorado School of Mines, Golden, CO

Helium, nitrogen, and argon are small, chemically unreactive molecules which can diffuse through a gas-field cap rock relatively rapidly compared to hydrocarbons. To maintain the concentration of these species over geologic time spans, the rate of influx of these gases must at least equal their diffusional loss rate.

Nitrogen in natural gas is generally believed to result from the thermal degradation of nitrogen-bearing organic compounds. Helium and argon are usually thought to be of crustal radiogenic origin. An alternate source of all three gases may be subcrustal outgassing.

Anomalously high concentrations of He, Ar, and N2 occur in the natural gases of southeastern Colorado. The diffusional flux of He and Ar from gas fields and the generation rate of these through radioactive decay indicate insufficient concentrations of uranium, thorium, and potassium exist within the normal compositional range of the earth's crust. For nitrogen, there is insufficient organic carbon in the sedimentary column to generate the observed volumes of N2 unless the thermal degradation of organics is a recent event. Anomalous concentrations of He, Ar, and N2 suggest the presence of a subcrustal outgassing and thermal event under southeastern Colorado.

The occurrence of a subcrustal event could correlate with the anomalously high heat flow in eastern Colorado, the recent (less than 5 Ma) epeirogenic uplift of the Great Plains, the thickest continental crust in the Mid-Continent, and recent nearby vulcanism.

AAPG Search and Discovery Article #91008©1991 AAPG Mid-Continent Section Meeting, Kansas Geological Society, Wichita Kansas, September 22-24, 1991 (2009)