San Joaquin Basin

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History of the Basin

Fig. 1 Location of the San Joaquin Basin on a map of North America

[1] The San Joaquin Basin is situated in the central region of western California, where sedimentary rocks began depositing over sixty-five million years ago during the Mesozoic era (shown in fig.1). The formation is known for its profound petroleum production, which accounts for roughly three-quarters of the state crude oil total. [2] Shale and sandstone are represented in the lithology of the Great Valley Sequence. This fascinating geologic formation has sediment thickness of up to forty-thousand feet in some areas and is said to have formed via an ancient seaway of the Pacific Ocean!

Primary Geologic Risks

[3] Primary risks associated with the San Joaquin Basin include seismic and geologic hazards. Effects from seismic hazards are directly associated with earthquakes. This seismic activity, which California is notorious for, occurs along faults and fractures where rocks are displaced on the surface. The San Joaquin Basin (located in an active region) contains over ten fault systems that pose an imminent danger to individuals, the overall landscape, and the structure of the geologic formation below the surface. Unfortunately, the probability of a massive earthquake (magnitude of seven on the Richter scale) occurring in this area in the next thirty years sits right around fifty percent. Geologic risks of the basin include subsidence, slope instability, and expansive soils. The sinking of the Earth's surface in this region is an inevitable result of overdrafting groundwater. Instability is reflected in the normal geologic processes of mudslides, landslides, and rockfalls. However, with the intense seismic activity of the region, structures are far more susceptible to failure and flooding. Finally, expansive soils and erosion within the county pose a risk to foundations, which can harm the quality of water and create health hazards.

Petroleum Elements

Source Rock & Reservoir

[4] Two necessary geologic elements for oil and gas accumulation in amounts worth producing include organically rich source rocks (generation) and a porous reservoir rock to store the new petroleum. Geochemists have used biomarkers (organic compounds) and carbon isotopes to determine that the majority of oil in California is generated from Monterey shale. When kerogen is present in these rocks, they're considered organic-rich. This source rock is also high in sulfur content and creates oil at lower temperatures. [5] Reservoir rocks have characteristics of ample porosity and permeability so oil and gas can flow through the pore space. For the San Joaquin Basin, the Sierra Nevada Mountain Range provides reservoir rocks in the form of sandstone, where oil can accumulate with an observed porosity greater than eighteen percent.

Traps and Seals

Fig. 2 Diagram of an anticline that is representative of the traps and seals crude oil is extracted from in the San Joaquin Basin

[6] The trap element of a petroleum reservoir serves the purpose of structurally preventing oil and gas from escaping the concentrated storage area. [4] The top and side portions of the trap consist of a seal (impermeable shale in the case of this basin) that allows oil to accumulate without leaking or being exposed to contaminating factors. These formations generally exist in the shape of an anticline (structural trap, shown in fig.2) for optimal drilling and extraction. However, they can also form around faults, pitchouts, and unconformities.


[7] In the San Joaquin Basin, geologists and other researchers have found that shallow bitumen-rich samples have a sharp maturity increase, implying that mature hydrocarbons have migrated from origins of source rocks at a greater depth. This hypothesis is logical when the heat-absorbing source rocks of the depocenter (basin floor syncline formation) are examined. The thickest accumulation of sediment is found here, which displays evidence of upward migration to arches and valley margins throughout the region.

Future Petroleum Potential

[8] Putting the longevity of the San Joaquin Basin into perspective, the first big oil gusher in the valley was drilled in 1896. Over a century later, the basin is still producing a tremendous amount of oil, with nearly fifty-thousand wells in use. Historically, more than half of the crude oil produced in the state of California has come out of the San Joaquin Valley. This formation also contributes five percent of the national production total on a yearly basis. Although the San Joaquin Basin ranks behind other formations in states such as Texas and Louisiana, it is considered the fourth largest producing region in the Continental United States. [1] This claim is supported by surveys conducted in the early 2000s that showed fields containing up to four-hundred million barrels of crude oil production capacity (which have been surpassed by older entities). With the giant oil fields present and a positive cumulative annual oil production trend, the ability to extract heavy crude oil from the San Joaquin Basin will continue into the future with little to no doubt.

Engineering Aspects

Fig. 3 Illustration of the steam flooding process that produces crude oil in the San Joaquin Basin region

[8] Steam injection is an enhanced recovery method for extracting crude oil that is steadily increasing in popularity. This EOR method is most commonly used in the oil fields of the San Joaquin Valley in California. During the process of steam flooding, a portion of wells are used for injection while the others serve as producers of oil. These mechanisms work simultaneously to increase overall production. The main goal of the process is to heat the crude oil, which decreases viscosity and allows more effortless flow through the formation to the producing wells (shown in fig.3). At this point, the thick and heavy crude is in a more accessible state for recovery and is lifted to the surface by the well-producing system consisting of a pumping jack, wellhead, casing, tubing, sucker rod, etc.

Further Readings

[1] [2] [3]


  1. 1.0 1.1 San Joaquin Basin. (2021). Public Policy on Energy. Ballotpedia. Retrieved May 12, 2021, from
  2. Great Valley Sequence. (2021, January 14). Various contributors. Wikipedia. Retrieved May 12, 2021, from
  3. Seismic and Geological Hazards. (1992). San Joaquin County Public Health and Safety. Retrieved May 12, 2021, from
  4. 4.0 4.1 How to Find Oil & Gas. (2021, January 16). San Joaquin Valley Geology. Retrieved May 12, 2021, from
  5. Intro to Geology of the San Joaquin Valley. (2021, January 16). San Joaquin Valley Geology. Retrieved May 12, 2021, from
  6. San Joaquin Basin. (2021). California Resources Corporation. Retrieved May 12, 2021, from
  7. Kruge, M. A. (1986). Biomarker Geochemistry of the Miocene Monterey Formation, West San Joaquin Basin, California: Implications for Petroleum Generation. Montclair State University Digital Commons. Retrieved May 12, 2021, from
  8. 8.0 8.1 The San Joaquin Valley Oil Industry. (2021, January 16). San Joaquin Valley Geology. Retrieved May 12, 2021, from