Introduction

The U.S. Geological Survey (USGS) assessed the potential for undiscovered, technically recoverable conventional and unconventional (continuous) oil and gas resources within the Los Angeles Basin Province of California (fig. 1). The tectonic evolution of the Los Angeles Basin Province is complex and related to the evolution of the continental margin from subduction to right-lateral transform motion of southern California (Wright, 1991; Nicholson and others, 1994; Ingersoll and Rumelhart, 1999; Sorlien and others, 2013). The structural configuration of the basin is due to two phases of extension in the Miocene and Pliocene that created the accommodation space for as much as 9 kilometers of sediment, in part consisting of world-class petroleum source and reservoir rocks (Biddle, 1991; Redin, 1991; Wright, 1991). Extension was followed by contraction in the late Pliocene to Pleistocene that resulted in the modification of many structures in the basin (Jung and others, 2015). This phase of deformation may have caused fracturing and loss of oil and gas from reservoirs, as shown by the numerous seeps throughout the basin (Biddle, 1991; Wright, 1991).

Total Petroleum System and Assessment Units

The USGS defined the Monterey Total Petroleum System (TPS) encompassing oil and gas generated from organic-rich Miocene Monterey Formation source rocks, and possibly from source rocks in the Miocene Puente and Modelo Formations. The Monterey Formation is characterized by Type II and Type IIS kerogen, total organic carbon (TOC) values as much as 18 weight percent, hydrogen index values as much as 600 milligrams of hydrocarbon per gram of TOC, thickness as much as 300 meters (Jeffrey and others, 1991; Jung and others, 2015), and a high silica content that readily fractures. The organic matter in the Monterey Formation is more sulfur rich (Type IIS) in the western side of the basin (Jeffrey and others, 1991; Tennyson and others, 2016). Onset of oil generation is modeled to have begun in the late Pliocene (Schultz and others, 2017).

The Central and Eastern Los Angeles Basin Monterey Shale Oil Assessment Unit (AU), the Central and Eastern Los Angeles Basin Monterey Shale Gas AU, and the Western Shelf Los Angeles Basin Monterey Shale Oil AU were defined to estimate the geologic uncertainty of, and potential for, shale-oil and shale-gas resources. The thermal window for oil generation from Miocene source rocks in the Central and Eastern Los Angeles Basin Monterey Shale Oil AU is generally placed at the −12,000-foot (ft) structure contour near the base of the Monterey Formation (Tennyson and others, 2016, from the map of Wright, 1991). The thermal window for gas generation estimated from one-dimensional modeling for the Central and Eastern Los Angeles Basin Monterey Shale Gas AU is generally placed at about the −21,000-ft structure contour near the base of the Monterey Formation. These depths for the beginning of oil and gas generation are uncertain and represent approximations only. The Western Shelf Los Angeles Basin Monterey Shale Oil AU was defined to encompass oil generated from Miocene shales below a depth of about 9,000 ft, due to the early generation oil from Type IIS kerogen (Peters and others, 2016; Tennyson and others, 2016) compared to Type II organic matter in the eastern part of the basin. The geologic model for the three AUs is for oil and gas to be partially retained within the shales after generation and migration. Siliceous shales of the Monterey Formation are commonly fractured, and oil and gas may have largely migrated along fractures and out of the shales and into conventional reservoirs or were lost to the surface.

The Los Angeles Basin Conventional Reservoirs AU encompasses most of the Los Angeles Basin Province (fig. 1). The AU was defined to include oil and gas generated from Monterey Formation and related organic-rich rocks to have migrated into conventional sandstone reservoirs within structural and stratigraphic traps. Reservoirs are deltaic and shallow marine sandstones, slope-channel sandstones, basin-floor sandstones, and possible sand injectites described from similar deepwater sandstone systems (Zvirtes and others, 2020). The assessment input data for four AUs are summarized in table 1 and in Schenk (2025).

Undiscovered Resources Summary

The USGS quantitatively assessed undiscovered oil and gas resources in three continuous AUs and one conventional AU in the Los Angeles Basin Province (table 2). The estimated total mean resources are 61 million barrels of oil (MMBO), with an F95–F5 range from 10 to 201 MMBO; 240 billion cubic feet of gas (BCFG), with an F95–F5 range from 33 to 741 BCFG; and 5 million barrels of natural gas liquids (MMBNGL), with an F95–F5 range from 1 to 20 MMBNGL. Approximately 80 percent of the undiscovered oil and gas resources are estimated to be in the Los Angeles Basin Conventional Reservoirs AU.

For More Information

Assessment results are also available at the USGS Energy Resources Program website, https://www.usgs.gov/programs/energy-resources-program.

Los Angeles Basin Province Assessment Team

Christopher J. Schenk, Marilyn E. Tennyson, Tracey J. Mercier, Phuong A. Le, Andrea D. Cicero, Ronald M. Drake II, Sarah E. Gelman, Jane S. Hearon, Benjamin G. Johnson, Jenny H. Lagesse, Heidi M. Leathers-Miller, and Kira K. Timm