Abstract: The 1985 Ross Store Explosion and Other Gas Ventings, Fairfax District, Los Angeles
HAMILTON, DOUGLAS, Consulting Geologist, Atherton, CA; RICHARD MEEHAN, Consulting Civil Engineer, Menlo Park, CA
Methane gas ventings in the Fairfax District of Los Angeles resulted in the explosion of a Ross Dress for Less department store in 1985, and the evacuation of several buildings in 1989.
The Fairfax District overlies part of the old Salt Lake oil field and is about one-half mile from the LaBrea Tar Pits. The oil field, once developed by more than 400 wells, was largely abandoned prior to being redeveloped by slant drilling starting in 1962. Since then production of oil, salt water and gas has been continuous with the water being reinjected into the field since 1980. The disposal reinjection was into a block adjacent to the Third Street fault, which projects to the surface near the surface venting sites. Injection was at surface pressures of up to 770 psi giving rise to a gradient of about 0.7 psi/ft within the subsurface near the point of injection. We conclude that this resulted in episodic fracturing of the Third Street fault. The fault, acting as a valve structure, was temporarily jacked open by the local fluid overpressure and while open served as a conduit for escape of pressurized methane to the near surface. With depressurization the fault conduit would collapse and the venting cease.
Similar effects of fault activation by fluid pressure excursions have been demonstrated in connection with the 1963 failure of Baldwin Hills dam in the Inglewood field, with seismicity triggering elsewhere, and also in paleo effects preserved in vein structures.
Clearly the phenomenon of methane venting in the urban environment can be hazardous, especially if no provision has been made to control it at the surface. But we propose that adequate response to this hazard should include developing an integrated understanding of both surface and subsurface conditions, starting, where oil field activity is involved, with the geology and operations within the producing zone.
AAPG Search and Discovery Article #90911©2000 AAPG Pacific Section and Western Region Society of Petroleum Engineers, Long Beach, California