U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2017–3050

 October 2017

National and Global Petroleum Assessment

Assessment of Undiscovered Oil and Gas Resources in the Ventura Basin 

Province, California, 2016

Using a geology-based assessment methodology, the U.S. Geological Survey assessed mean undiscovered, technically recoverable

resources of 250 million barrels of oil and 519 billion cubic feet of gas in the Ventura Basin Province of California.

Introduction

The U.S. Geological Survey (USGS) completed a geology-based 

assessment of undiscovered, technically recoverable conventional 

and continuous oil and gas resources in the part of the Ventura Basin 

Province that lies onshore or within State waters (within 3 miles of the 

shoreline) of California (fig. 1). Conventional oil and gas resources are 

those that have migrated upward into structural or stratigraphic traps 

from deep zones where the oil and gas is generated; water is present below the oil or gas. Continuous accumulations, in contrast, are 

those in which oil or gas is pervasively present in essentially all wells 

that penetrate them, that may not be structurally or stratigraphically 

trapped, and that typically lack oil-water or gas-water contacts. They 

are commonly produced with well-stimulation technology, such 

as hydraulic fracturing, referred to as “unconventional.” The same 

stimulation technology, however, is also used in many conventionally 

trapped accumulations. We estimated both the likely range of oil and 

gas volumes remaining to be discovered in accumulations similar to 

existing conventional oil and gas fields in the Ventura Basin Province 

(previously assessed by Keller [1995] as 1,060 million barrels of oil 

[MMBO], 1,900 billion cubic feet of gas [BCFG], and 60 million 

barrels of natural gas liquids [MMBNGL]), and the potential for oil and 

gas that might be present in a continuous accumulation at extreme depth 

in the floor of the basin.

Geologic Summary

The modern Ventura Basin is an east–west trending trough that 

extends from north of Los Angeles to the coastline near Ventura and 

continues offshore as the Santa Barbara Channel. Until about 16 million 

years ago, however, the crustal block that includes the Ventura Basin, 

the Santa Barbara Channel, the northern Channel Islands, and the 

western Transverse Ranges lay along the coast of southern California 

adjacent to present-day Los Angeles, where for tens of millions of 

years, a sequence of Upper Cretaceous and Paleogene nonmarine, 

nearshore, and deep marine sediments accumulated. An episode of 

extensional faulting accompanied by volcanism began at the end of 

early Miocene time about 16 million years ago as the San Andreas Fault 

system formed, tearing the Ventura Basin-Transverse Ranges block 

from the continent and rotating it clockwise toward its present position. 

For the next 10 million years, a blanket of marine, organic-rich mud, 

the Monterey Formation, was deposited across the region, ending about 

5 million years ago when compression produced large folds and uplifted 

fault blocks including the western Transverse Ranges and the northern 

Channel Islands. The Ventura Basin is filled with more than 20,000 

feet of upper Miocene, Pliocene, and Quaternary sandstone, shale, and 

conglomerate deposited in a rapidly subsiding zone between two flanking faulted uplifts that shed great volumes of clastic sediments into the 

deepening trough. These sediments buried the Monterey Formation and 

older potential source rocks to depths of more than 18,000 feet, where 

temperatures are high enough to generate oil and gas from the organic 

matter. Oil and gas have migrated upward along sandstone beds and 

IP-082217_fig 01

–120° –119°

35°

34°

–118°

SANTA BARBARA CHANNEL

KERN

LOS ANGELES

VENTURA

SANTA BARBARA

SAN BERNARDINO

ORANGE

SAN LUIS OBISPO

RIVERSIDE

SAN

DIEGO

PACIFIC OCEAN

CALIFORNIA

EXPLANATION

Santa Clara Trough Monterey Continuous Oil AU

Ventura Basin Conventional Oil and Gas AU

Ventura Basin Province

Channel Islands

Channel Islands

Transverse Ranges

Los Angeles

Ventura

Source: U.S. Department of the Interior National Park Service

Santa Clara River

0 50 KILOMETERS 25

0 25 50 MILES

Map

area

CALIFORNIA

NEVADA

OREGON IDAHO

MEXICO PACIFIC

OCEAN

Figure 1. Map showing the location and boundaries of the Ventura 

Basin Province, California, and the two assessment units (AU) 

evaluated for this study. 

faults to fill traps such as the numerous large oil fields near Ventura and 

in the Santa Barbara Channel. Petroleum continues to be generated, and 

where it has been unable to escape, some may remain in the Monterey 

Formation as a potential continuous resource. This assessment addresses 

the potential resources that may remain to be discovered in both 

conventional traps and in a deeply buried continuous accumulation.

Total Petroleum Systems and Assessment Units

Two assessment units (AUs) were defined in the Ventura Basin 

Province: (1) the Ventura Basin Conventional Oil and Gas AU and 

(2) the Santa Clara Trough Monterey Continuous Oil AU. Boundaries for

the first AU outline the area where source rocks, either in the Monterey 

Formation or in older Cenozoic strata, have generated oil that has migrated

to conventional traps (Cenozoic Composite Total Petroleum System).

Boundaries of the continuous AU, where oil generated from the Monterey

Formation (Monterey Total Petroleum System) might remain, follow faults

that bound the deep structural trough parallel to the Santa Clara River on

the north and south. Westward, both AUs are truncated at the boundary that

separates California State waters from the Federal Outer Continental Shelf

(OCS), as assessed by the Bureau of Ocean Energy Management.
Ventura Basin Province 

Assessment Team

Marilyn E. Tennyson, Christopher 

J. Schenk, Janet K. Pitman, Paul G. 

Lillis, Timothy R. Klett, Michael 

E. Brownfield, Thomas M. Finn, 

Stephanie B. Gaswirth, Sarah J. 

Hawkins, Kristen R. Marra, Tracey J. 

Mercier, Phuong A. Le, and Heidi M. 

Leathers-Miller

For More Information

Assessment results are 

available at the USGS Energy 

Resources Program website at 

https://energy.usgs.gov.

ISSN 2327-6916 (print) ISSN 2327-6932 (online) https://doi.org/10.3133/fs20173050

Assessment input data—Conventional AU

Ventura Basin Conventional Oil and Gas AU

Minimum Median Maximum Calculated

mean

Number of oil fields 1 10 50 11.4

Number of gas fields 0 2 10 2.3

Sizes of oil fields (MMBO) 0.5 4 1,500 21.4

Sizes of gas fields (BCFG) 3 12 300 19.7

AU probability 1.0

Assessment input data—Continuous AU

Santa Clara Trough Monterey Continuous Oil AU

Minimum Mode Maximum Calculated

mean

Potential production area of AU (acres) 1,000 40,000 170,000 70,333

Average drainage area of wells (acres) 5 10 40 18

Success ratio (%) 5 8 40 17.7

Average EUR (MMBO) 0.003 0.005 0.02 0.006

AU probability 1.0

Table 1. Key assessment input data for two assessment units in the Ventura Basin 

Province, California.

[AU, assessment unit; MMBO, million barrels of oil; BCFG, billion cubic feet of gas; %, percent; EUR, 

estimated ultimate recovery per well. The average EUR input is the minimum, median, maximum, and 

calculated mean. Shading indicates not applicable]

Total petroleum systems

and assessment (AUs)

AU

probability

Accumulation

type

Total undiscovered resources

Oil (MMBO) Gas (BCFG) NGL (MMBNGL)

F95 F50 F5 Mean F95 F50 F5 Mean F95 F50 F5 Mean

Cenozoic Composite Total Petroleum System

Ventura Basin Conventional 

Oil and Gas AU 1.0 Oil 25 155 792 246 42 277 1,534 467 1 8 47 14

Gas 6 32 132 45 0 1 4 1

Total conventional resources 25 155 792 246 48 309 1,666 512 1 9 51 15

Monterey Total Petroleum System

Santa Clara Trough Monterey

Continuous Oil AU 1.0 Oil 1 3 12 4 1 5 18 7 0 0 1 0

Total continuous resources 1 3 12 4 1 5 18 7 0 0 1 0

Total undiscovered resources 26 158 804 250 49 314 1,684 519 1 9 52 15

Table 2. Assessment results for two assessment units in the Ventura Basin Province, California.

[MMBO, million barrels of oil; BCFG, billion cubic feet of gas; NGL, natural gas liquids; MMBNGL, million barrels of natural gas liquids. Results shown are fully 

risked estimates. For gas accumulations, all liquids are included in the NGL category. F95 represents a 95-percent chance of at least the amount tabulated; other fractiles 

are defined similarly. Fractiles are additive under the assumption of perfect positive correlation. Shading indicates not applicable]

Undiscovered Resources Summary

The Ventura Basin is well explored; no large conventional 

discoveries have been made for many decades except in the adjacent 

OCS. Potential for significant conventional oil and gas discoveries was 

judged to be highest in undrilled parts of the offshore State waters where 

major accumulations might exist. Onshore, most future conventional 

discoveries are predicted to be minor. 

Conventional AU assessment input data are shown in table 1. 

The estimated mean total resources of oil and gas in undiscovered 

conventional fields in the Ventura Basin Conventional Oil and Gas AU 

are 246 MMBO and 512 BCFG with F95–F5 ranges of 25 to 792 MMBO 

and 48 to 1,666 BCFG, respectively. The estimated mean total resources 

for natural gas liquids are 15 MMBNGL with an F95–F5 range from 1 to 

51 MMBNGL (table 2).

Continuous oil resources were estimated using success rates, 

estimated ultimate recoveries, and well drainage areas (table 1) similar 

to those used to assess the Monterey Formation in the San Joaquin and 

Los Angeles Basins (Tennyson and others, 2015, 2016). Volumes of 

undiscovered continuous oil in the deep basin are expected to be small 

because the large volume of oil that has rapidly migrated to shallow 

conventional traps indicates that oil is not likely to have been retained 

in the source rock. The estimated means of undiscovered recoverable 

continuous oil and gas in the Santa Clara Trough Monterey Continuous 

Oil AU are only 4 MMBO and 7 BCFG with F95–F5 ranges from 1 to 

12 MMBO and 1 to 18 BCFG, respectively. The estimated volume of 

natural gas liquids in any continuous accumulation is very small with 

a mean of 0 MMBNGL and only a 5 percent chance of 1 MMBNGL 

(table 2).

References Cited

Keller, M.A., 1995, Ventura Basin Province (013), in Gautier, D.L., Dolton, G.L., Takahashi, K.I., and Varnes, K.L., eds., 1996, 1995 National assessment of 

United States oil and gas resources—Results, methodology, and supporting data: U.S. Geological Survey Digital Data Series 30, release 2, 1 CD–ROM. 

[Also available at https://certmapper.cr.usgs.gov/data/noga95/prov13/text/prov13.pdf.]

Tennyson, M.E., Charpentier, R.R., Klett, T.R., Brownfield, M.E., Pitman, J.K., Gaswirth, S.B., Hawkins, S.J., Lillis, P.G., Marra, K.R., Mercier, T.J., Leathers, 

H.M., Schenk, C.J., and Whidden, K.J., 2015, Assessment of undiscovered continuous oil and gas resources in the Monterey Formation, San Joaquin Basin 

Province, California, 2015: U.S. Geological Survey Fact Sheet 2015–3058, 2 p. [Also available at https://doi.org/10.3133/fs20153058.]

Tennyson, M.E., Charpentier, R.R., Klett, T.R., Brownfield, M.E., Pitman, J.K., Gaswirth, S.B., Hawkins, S.J., Le, P.A., Lillis, P.G., Marra, K.R., Mercier, 

T.J., Leathers-Miller, H.M., and Schenk, C.J., 2016, Assessment of undiscovered continuous oil and gas resources in the Monterey Formation, Los Angeles 

Basin Province, California, 2015: U.S. Geological Survey Fact Sheet 2016–3036, 2 p. [Also available at 

https://doi.org/10.3133/fs20163036.]