Print this pageCharacteristics of Coal bed Methane Seeps from Fruitland Formation Outcrops in the San Juan Basin, Colorado; The Need for Hazard Awareness and Implications for Near Surface Gas Surveys
FONTANA, JOHN V.
In the summer and fall of 1995, a soil gas survey was conducted to identify
and characterize methane gas seeps along the Fruitland Formation Outcrop in La
Plata County, Colorado. The study was funded by four local gas producing
companies with assistance from local government agencies. The study 
area
covered
a 25 mile long narrow outcrop strip of Fruitland Formation Outcrop on the
northern edge of the San Juan Basin. The formation is a major coal bed methane
producer in the basin. Historical seeps have been previously identified, but
their extent into more remote terrain was not known.
Survey techniques were developed using portable gas detection equipment,
shallow subsurface probes, and GPS to locate and map the seeps. Areas of outcrop
were identified with soil gas concentrations that were considered above safe
levels in a confined space. Since methane has a very low specific gravity
relative to air, it disperses rapidly in the atmosphere density. Therefore, most
of the gas seeps do not present any physical hazard, unless a structure is built
over a seep. Areas of stressed vegetation were noted in the strongest seep
areas. Some areas revealed evidence of past vegetation stress with new growth,
indicating major changes in the gas flux.
Detailed profiles showed that the character of a seep changes depending on
the seeps apparent relative
location to the groundwater depth, stream drainage
and soil composition. Some conditions result in very narrow seeps, while others
result in a wider dispersion of the soil gas. This characteristic feature leads
to implications for exploration soil gas surveys and geologic hazard surveys.