Salt Lake oil field

The Salt Lake Oil Field is an oil field underneath the city of Los Angeles, California. Discovered in 1902, and developed quickly in the following years, the Salt Lake field was once the most productive in California; over 50 million barrels of oil have been extracted from it, mostly in the first part of the twentieth century, although modest drilling and extraction from the field using an urban "drilling island" resumed in 1962. As of 2009, the only operator on the field was Plains Exploration & Production (PXP). The field is also notable as being the source, by long-term seepage of crude oil to the ground surface along the 6th Street Fault, of the famous La Brea Tar Pits.

Gas explosions
Seepage of methane upwards along conduits, such as faults and old well boreholes, caused an explosion at a Ross Dress for Less store in 1985 on 3rd Street in the Fairfax District, which injured 23 people. The Ross Dress for Less store still stands in the 6200 block of 3rd Street, on the southeast corner of Fairfax Ave. and 3rd. Overnight on March 24, 1985, methane gas filled an auxiliary room at the store and ignited, causing a spectacular explosion which blew out the windows and tore the roof off of the building, injuring 23 people, and reducing the inside to rubble. In addition to blowing up the building, the methane explosion burst out portions of the adjacent parking lot and sidewalks, venting burning gas over a wide area, creating an eerie scene with pillars of flame lighting the night. Four blocks were cordoned off by emergency crews as officials scrambled to determine what had happened.

Since naturally occurring methane is odorless – utility companies add mercaptans to alert people to the presence of this flammable gas – no one had noticed the buildup of methane, so it may have accumulated to an explosive concentration slowly. The source of the methane gas was controversial; early theories involved a biogenic origin for the methane, in which it was seen as the product of decomposition of organic matter from an old swamp. In this scenario, a rising water table forced the gas from the pore spaces within the soil upwards to the surface.

A later theory, and the one now accepted, was that the gas originated in the oil field itself, and had migrated to the surface along a combination of the 3rd Street Fault and any number of improperly abandoned boreholes from the hundreds of now-lost wells drilled in the early years of the twentieth century. The reinjection of wastewater into the field to increase oil recovery increased the reservoir pressure to the point that gas was forced upwards along the paths of least resistance – newly formed fractures along the fault, as well as the old wellbores – until it reached the ground surface. Isotopic analysis of the near-surface methane supported this theory, as the specific isotope distributions did not match what would have been expected had the gas been recently produced by a recent biogenic mechanism, and they correlated strongly with isotopic analysis of oil field gas.

The finding had enormous implications for all of the urban development over old oil fields, and resulted in the construction of gas monitoring and venting wells in several locations in Los Angeles. The city of Los Angeles designated approximately 400 blocks overlying the old oil field as a "High Potential Methane Zone" as a result of the 1985 explosion and subsequent investigation, and later required all structures to have a methane detector, to give warning of accumulation of the gas before it could attain explosive concentrations.

1989 gas venting and evacuation
In 1989, a similar methane gas buildup occurred underneath 3rd Street and adjacent buildings, probably because of the accidental plugging of a gas-venting well built after the Ross incident. Since the venting well had become clogged with a buildup of debris, methane slowly collected under the street and adjacent impermeable surfaces, bursting out on the morning of Tuesday, February 7, 1989, in a fountain of mud, water, and methane gas; no explosion occurred, since there was no source of ignition, and city emergency crews quickly cordoned off the area. As a result of this incident, Los Angeles further upgraded their City Building Code to require new buildings to have adequate venting systems, and be underlain with an impermeable membrane to prevent methane from getting in beneath the foundation.

Related SourceWatch articles

 * California and fracking
 * PXP
 * United States and fracking

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