Oil wells creak in the Bakersfield twilight. Petroleum is not confined to Southern California and Central Valley; we have it in the Bay Area too. Photo by Andrew Alden.

Norma Desmond, the washed-up movie star of Sunset Boulevard, had some memorable lines. One of them was, "I've got oil wells in Bakersfield, pumping pumping pumping." We think of our petroleum as a Southern California thing, or at least no closer than the Great Valley. But petroleum—oil and gas, or both—is widespread around the state, including the Bay Area.

It wasn't just gold, water and agricultural soils that made California rich. Petroleum is one of our greatest natural assets, and the state still ranks fourth in oil production behind Louisiana, Texas and Alaska. While the forests of derricks that once dotted Los Angeles are now subdued and disguised, the Central Valley is still a proud petroleum region, with gas fields in the Sacramento Valley and oil fields in the San Joaquin Valley.

The oil and gas fields of the Central Valley intrude into the Bay Area from the Delta as far as Concord and the Suisun Bay to its north. Gas was produced from the hills north of Concord in the 1960s, and today the old Los Medanos gas field is used by PG&E for storage.

Photo (c) 2007 Andrew Alden

Before petroleum exploration became high-tech, the best way to find oil and gas was to look for natural seeps. An oily sheen on a stream, trickles of tar from a sunny sea cliff, and persistent odors like kerosene are typical signs. These are more common than most people think. Much rarer are actual tar flows like those at Carpinteria Beach near Santa Barbara.

Photo (c) 2010 Andrew Alden

Oil seeps were discovered near Half Moon Bay and in the Santa Cruz Mountains in the 1800s. Asphalt and tar sand were mined from these areas for the streets of San Francisco.

Natural asphalt from the McKittrick tar seep. Photo by Andrew Alden

Later, conventional exploration opened up four districts of producing wells in the San Mateo Peninsula: the Half Moon Bay, La Honda and Oil Creek fields still yield oil today. The Moody Gulch field, which started as a tar pit in 1878, was shut down in 1960 and is now under Route 17.

Photo by Les Magoon, U.S. Geological Survey

Elsewhere in the Bay Area, oil and gas have been produced in Petaluma, Pinole and Livermore. Oil was reported in the Caldecott Tunnel excavations, which is to be expected as the tunnel penetrates our local piece of the Monterey Formation, a petroleum source rock of great extent.

Oil and gas are called fossil fuels, but maybe a better concept is that they are geological compost. They are the bodies of dead plankton, trapped in the mud with no oxygen to consume them. Instead their living substance breaks down and is transformed into simpler hydrocarbon compounds.

The simplest and lightest of these is methane, and that's refined and sold to us as "natural gas." But actual natural gas, the stuff bubbling up in tar pits, is a mixture of compounds. It won't smell like the gas in your stove—that smell is the odorant butyl mercaptan. It's more like the smell of crude oil—a tantalizing, sweeter version of your local gas station. If you don't know how crude oil smells, take a side trip to the oil fields on your way south, like this one just off Interstate 5 at Lost Hills.

Lost Hills oil field. Photo (c) 2010 Andrew Alden

The U.S. Geological Survey has a wealth of material on California's oil and gas seeps at walrus.wr.usgs.gov/seeps.

You may listen to the "Chevron's Plans" Radio report online, as well as find additional links and resources.

Amy Standen is a Reporter for QUEST and Radio News at KQED-FM.