--> Abstract: Characteristics of Coalbed Methane Seeps from Fruitland Formation Outcrops in the San Jaun Basin, Colorado; The Need for Hazard Awareness and Implications for Near Surface Gas Surveys, by J. V. Fontana; #90937 (1998)

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Abstract: Characteristics of Coalbed Methane Seeps from Fruitland Formation Outcrops in the San Jaun Basin, Colorado; The Need for Hazard Awareness and Implications for Near Surface Gas Surveys

FONTANA, JOHN V., Direct Geochemical, Golden, Colorado

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 strip of Fruitland Formation Outcrop on the northern edge of the San Juan Basin. The formation is a major coalbed methane producer in the basin. Historical seeps have been previously identified, but their extent into more remote terrain was not known. Other gas seeps that are less extensive also been documented coming from formations that appear unrelated to the Fruitland Formation coals.

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 density (0.6) relative to air, it disperses rapidly in the atmosphere. Therefore, most of the gas seeps do not present any physical hazard in remote areas, unless a structure is built over a seep. Scattered housing and other structures in the area have had problems with methane in basements and crawl spaces.

Areas of stressed vegetation were noted in the strongest seep areas. Visual stress indicators included included dead or dying trees and shrubs, and bare spots in grass areas. Strong seeps in creek bottoms did not appear to affect the vegetation in those the wet 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 environmental and geomorphic conditions result in very narrow seeps with sharply defined edges, while others result in a wider dispersion of the soil gas seep pattern. The majority of the methane gas is apparently migrating up the steeply dipping coal beds along the edge of the basin, and not vertically to the surface.

The characteristics of these seeps leads to implications for exploration and hazard soil gas surveys regarding surface pattern distrubution relationships to subsurface sources.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah