Los Angeles is famous for many reasons, although the enormous oil and gas deposits beneath the city are unfamiliar to many. According to researcher estimates, the Los Angeles Basin just might contain more oil per unit volume of rock than any other sedimentary basin on Earth. Impressive!
Signal Hill oil field, part of the Long Beach Oil Field, and in an area now completely surrounded by the City of Long Beach, Los Angeles County, circa 1923 (Wikimedia)
Diatoms, Deposition, and Death
The organic-rich Monterey Formation is the source rock for hydrocarbons beneath Los Angeles. Abundant diatoms, planktonic algae surrounded by a cell wall of silica, flourished in an ocean basin fed by nutrient-rich upwelling from the offshore California Current. Over millions of years, between about 18 and 5 million years ago (Miocene), layer after layer of diatoms fell through water depths that ranged from roughly 1,600 to 7,500 feet (500 to 2300 m). Eventually, thousands of feet of diatom layers were transformed into siliceous sedimentary rock that preserved the high organic carbon content of the algae.
Assorted diatoms as seen through a microscope (Wikimedia)
Tectonic activity set the stage for hydrocarbon production. Along the coast, the colliding North American continent and oceanic Pacific plate gradually evolved into the San Andreas fault (a change from subduction into a transform margin). The diatom-filled ocean basin deepened, while adjacent uplifted areas shed sand, silt and clay sediments that rapidly buried the silicious material. Then, the organic-rich sediments reached temperatures and pressures that became favorable for petroleum and natural gas production. Folding, or warping, of rock in the basin produced upfolded areas that effectively trapped the hydrocarbons deep underground. Fractures and faults allowed some of the petroleum to move upwards and leak out onto the land surface.
Diatomite (diatomaceous earth) from the Miocene-aged Monterey Formation in southern California (Wikimedia)
Focused in the center of Los Angeles, the La Brea Tar Pits are the most notable of the seeps. Beginning about 60,000 years ago, the “tar” entrapped animals and surrounding vegetation in asphalt, effectively fossilizing them. (Oil reaching the surface forms pools—when the lighter, viscous oil fractions biodegrade or evaporate, the sticky heavy fractions known as asphalt remain.) Saber-tooth cats, giant sloths, mammoths, mastodons, dire wolves, and many other animals were entombed, making the La Brea Tar Pits the richest Pleistocene fossil site on Earth. (My post, Did Snarling Saber-tooth Cats Have Kind Hearts? describes fossil bones from the La Brea site showing evidence that these animals cared for injured kin—fascinating!)
Tar sandstone from the Monterey Formation of Miocene age (10 to 12 million years old), southern California (Wikimedia)
A Valuable Resource
When indigenous people arrived in the region that is now Los Angeles, they found the asphalt to be useful as water-proof caulking for boats and containers. In the 18th century, Spanish explorers also noted the value of the material for caulking ships. By the 1890s, when William Doheny arrived in Los Angeles, he saw locals burning the asphalt as fuel. This inspired him to drill the first commercial oil well in 1892, at a location that is now beneath Dodger Stadium. Entrepreneurs drilled hundreds more in the next several years; by 1904 there were over 1,150 wells in the City of Los Angeles alone, and many more in surrounding areas. Fueled largely by the oil industry, Los Angeles grew rapidly, and wells on beaches and in the backyards of homes became common sights. Massive oil strikes in the 1920s and the extensive oil production that followed resulted in Los Angeles supplying a remarkable one quarter of the world’s oil. The black and gold vehicle license plates in California, in standard use until the 1960s, symbolized the state’s two renown natural resources: oil and gold.
The first oil district in Los Angeles, Toluca St., ca. 1895-1901. The The oil field is closely packed with about 20 derricks, reflected in the standing water or oil on the ground in the foreground. Oil pipes traverse the general area, which is now in Downtown Los Angeles, just west of where 101 and 110 cross, about 1 mile southwest of Dodger Stadium. (Wikimedia)
By the 1960s, the aerospace, manufacturing, and entertainment industries were also booming in Los Angeles. New city laws required drilling to be consolidated into small “islands” (typically less than half an acre) with an infrastructure compatible with the surrounding community. Urban drilling sites began hiding in plain sight, with shell buildings disguising equipment. At least one site, in a commercial district, appears to be a seven-story office building, but without windows. Many Los Angeles residents are unaware that they live above a giant oil field, and perhaps frequently pass by an active drilling site. Within the past few years, 40 oil fields in the basin containing 5,000 wells have produced around 18 million barrels of oil per year.
Times change. Public concerns about the health effects of production sites (disproportionally in less affluent neighborhoods), and the potential for industrial accidents, not to mention concerns about climate change, have turned the tide of public opinion against oil and gas production. New regulations to end these activities within the City are being proposed or adopted, with many now in active litigation.
Hydrocarbon production in Los Angeles is an economic bonanza and for decades has been a source of pride for residents, but now, sentiments are shifting toward ending production within the city.
Dozens of oil derricks in a Los Angeles oil field, Court St and Toluca St, 1904 (Wikimedia)
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Roseanne: Awesome post on LA’s geology. I grew up in Pomona – no oil there but learning about my home area’s geology is fascinating.
Thanks, Wayne! That region definitely has a very interesting history!