Methane Seeps, Public Hazards, and Seismicity: Hydrocarbon System Analysis of Coalbed Methane in the San Juan Basin as a Tool to Understanding

W. C. Riese¹, G. K. Arp², A. Sanford³, G. Snyder⁴, and W. L. Pelzmann^5
1 BP America Production Co, Houston, TX
² Booze Allen Hamilton, Edgewater, MD
³ New Mexico Institute of Mining and Technology, Socorro,
⁴ Rice University, Houston, TX
⁵ BP America Production Co, Houston,

Methane seeps at the outcrop of Fruitland Formation coals in the San Juan Basin were examined to determine their ages, activity histories, and causes. Seeps along the northern outcrop were found to exhibit an episodic history of activity spanning more than 100 years and to be preferentially located in north-south striking valleys which cut the east-west striking outcrop belt. These valleys are consequent geomorphic features related to fractures which reflect transmission of Rio Grande Rift extensional stresses across the San Juan Basin portion of the Colorado Plateau.

The northern flank of the San Juan Basin is underlain by northwest-southeast striking basement fractures and faults which have been intermittently active since the late Paleozoic. These basement faults remain seismically active today and exhibit significant increases in activity on approximately a 30-year cycle. Botanical mapping of the northern outcrop has determined that significant increases in methane seepage have also occurred on a 30-year cycle which corresponds closely to the cycle of seismic activity.

Geochemical characterization of fluids emanating from the seeps and collected from wells which penetrate the coals has determined that the gasses and waters present in the seeps are not sourced by the coals, but instead come from deeper formations, and in some cases from the basement. The intermittent seismic activity breaches deeper accumulations and allows the gas held there to escape to the surface.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005