National and Global Petroleum Assessment

Assessment of Undiscovered Continuous Oil and Gas 

Resources in the Monterey Formation, San Joaquin Basin 

Province, California, 2015

U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2015–3058

September 2015

ISSN 2327-6916 (print) ISSN 2327-6932 (online)

http://dx.doi.org/10.3133/fs20153058

Abstract

Using a geology-based assessment 

methodology, the U.S. Geological Survey 

assessed mean volumes of 21 million 

barrels of oil (MMBO), 27 billion cubic 

feet of gas, and 1 million barrels of natural 

gas liquids in two assessment units (AUs) 

that may contain continuous oil resources. 

Mean volumes of oil for the individual 

assessment units are 14 MMBO in the 

Monterey Buttonwillow AU and 7 MMBO 

in the Monterey Maricopa AU.

Introduction

The U.S. Geological Survey (USGS) 

recently completed a geology-based assessment of technically recoverable continuous oil and gas resources in the Miocene 

Monterey Formation in the southern 

San Joaquin Basin of central California 

(fig. 1). The Monterey Formation is the 

source rock for most of the oil in structural 

and stratigraphic traps in the San Joaquin 

Basin, including several fields with multiple 

billions of barrels of recoverable oil. Recent 

drilling programs targeting oil remaining in 

the source rock, however, appear unsuccessful to date.

Gautier and others (2007) estimated 

the potential for undiscovered, conventionally trapped petroleum accumulations, 

and Tennyson and others (2012) estimated 

potential additions to reserves in discovered accumulations (including Monterey 

Formation reservoirs in conventionally 

trapped accumulations, such as Elk Hills 

and Lost Hills). The present assessment 

encompasses Monterey rocks that may contain a continuous accumulation in or near 

the zone of petroleum generation, where 

in-situ petroleum that has not migrated to 

shallower conventional traps could still be 

present. Two assessment units (AUs) were 

defined for these rocks: (1) the Monterey 

Buttonwillow Continuous Oil AU and 

(2) the Monterey Maricopa Continuous Oil 

AU. Conventionally trapped oil and gas 

fields with oil-water contacts were excluded 

from the areas of the AUs.

Geologic Summary

Strata in the San Joaquin Basin record 

120 million years of forearc deposition 

along a convergent continental margin, 

which was succeeded about 20 million 

years ago by a transition to transform 

motion along the San Andreas Fault system. 

The middle to upper Miocene Monterey 

Formation was deposited after this transition, from about 17 to 5 million years ago, 

while the coastline was well landward of 

its present position and relatively little 

coarse terrigenous sediment was transported offshore. The Monterey Formation 

comprises fine-grained, organic-rich strata 

with a substantial biogenic component, 

mainly diatoms and foraminifera, as well as 

interbedded deep marine sandstone in the 

southernmost part of the basin. Typically 

about 2,000 feet (ft) thick, it ranges up to 

about twice that thickness in the deepest 

parts of the basin. A structural high near 

Bakersfield separates the San Joaquin Basin 

into the Buttonwillow depocenter to the 

north and the Maricopa depocenter to the 

south. The top of the Monterey Formation 

reaches depths of over 14,000 ft in the 

Buttonwillow depocenter and 15,000 ft 

Figure 1. Map of the San Joaquin Basin Province. Assessment unit (AU) boundaries 

approximate the 10,000-foot structure contour on the top of the Monterey Formation, as revised 

from maps of Webb (1981) and Graham and Williams (1985) using well penetrations from data 

available online from the California Division of Oil, Gas, and Geothermal Resources.

Monterey

Buttonwillow

AU 

Monterey

Maricopa

AU

–122° –121° –120° –119° –118°

37°

36°

35°

San Joaquin

Basin Province

C A L I F O R N I A C O A S T R A N G E S

T E M B L O R R A N G E

SIERRA NEVADA

PACIFIC

OCEAN

San Francisco

Monterey

San Jose

Modesto

Fresno

Bakersfield

UTAH

NEVADA

ARIZONA

CALIFORNIA

Area of 

study

Source: U.S. National Park Service 0 25 50 MILES

0 50 KILOMETERS 25

San Joaquin Basin Province 

Monterey Buttonwillow AU 

Monterey Maricopa AU 

EXPLANATION

CALIFORNIA

NEVADA
in the southernmost part of the Maricopa 

depocenter. Generation of oil began as the 

Monterey reached burial depths of about 

12,000 ft within the last few million years. 

Assessment unit boundaries outline the 

areas where at least part of the Monterey 

is buried deeply enough to generate oil. 

Most oil readily migrates upward along 

natural fractures, faults, and interbedded 

sandstones to structural and stratigraphic 

traps. More than 80 wells have penetrated 

the Monterey Formation where it is deep 

enough to have generated oil, but none has 

returned more than small volumes of oil 

outside conventional traps.

Table 1. Key assessment input data for the two continuous assessment units in the San Joaquin 

Basin Province, California.

[EUR (estimated ultimate recovery per well); MMBO, million barrels of oil; AU, assessment unit; %, percent. 

The average EUR input is the minimum, median, maximum, and calculated mean]

Assessment input data–continuous AUs

Monterey Buttonwillow AU Minimum Mode Maximum Calculated mean

Potential production area of AU (acres) 5,000 182,000 343,000 176,667

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

Success ratio (%) 5 10 50 21.7

Average EUR (MMBO) 0.003 0.005 0.02 0.006

AU probability 1.0

Monterey Maricopa AU Minimum Mode Maximum Calculated mean

Potential production area of AU (acres) 5,000 113,000 223,000 113,667

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 2. Assessment results for continuous oil and gas resources in the San Joaquin Basin Province, California.

[MMBO, million barrels of oil; BCFG, billion cubic feet of gas; MMBNGL, million barrels of natural gas liquids (NGL); TPS, total petroleum system; AU, assessment unit. 

Results shown are fully risked estimates. F95 represents a 95 percent chance of at least the amount tabulated; other fractiles are defined similarly. Fractiles are additive under 

assumption of perfect positive correlation]

Total Petroleum System (TPS) 

and

Assessment Units (AU)

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

Miocene TPS

Monterey Buttonwillow AU 1.0 Oil 2 10 35 14 3 14 47 18 0 1 2 1

Monterey Maricopa AU 1.0 Oil 1 5 18 7 2 7 25 9 0 0 1 0

Total unconventional resources 3 15 53 21 5 21 72 27 0 1 3 1

Total undiscovered resources 3 15 53 21 5 21 72 27 0 1 3 1

This USGS analysis, based on 

drilling results to date, concluded that 

some recoverable oil probably remains 

in the source rock, but success rates 

(proportion of drilled wells that produce 

at least 2,000 barrels) will be low, and 

the estimated ultimate recovery will be 

low for even successful wells (table 1). 

Wells will be relatively tightly spaced, as 

they are in producing Monterey reservoirs in conventional traps such as those 

at the Buena Vista and Lost Hills fields. 

Anticipated extraction methods are mostly 

vertical (rather than horizontal) wells, acid 

stimulation, and hydraulic fracturing.

Resource Summary

The USGS assessed undiscovered, 

technically recoverable continuous 

(unconventional) oil and gas resources in 

two AUs in the Monterey Formation of 

the San Joaquin Basin Province (table 2). 

Assessed means are 14 MMBO, 18 billion 

cubic feet of gas (BCFG), and 1 million 

barrels of natural gas liquids (MMBNGL) 

in the Monterey Buttonwillow AU and 

7 MMBO, 9 BCFG, and 0 MMBNGL in 

the Monterey Maricopa AU.

References Cited

Gautier, D.L., Hosford Scheirer, Allegra, Tennyson, M.E., Peters, K.E., Magoon, L.B., Lillis, P.G., 

Charpentier, R.R., Cook, T.A., French, C.D., Klett, T.R., Pollastro, R.M., and Schenk, C.J., 2007, 

Executive summary—Assessment of undiscovered oil and gas resources of the San Joaquin Basin 

Province of California, 2003, chap. 1 of Hosford Scheirer, Allegra, ed., Petroleum systems and 

geologic assessment of oil and gas in the San Joaquin Basin Province, California: U.S. Geological 

Survey Professional Paper 1713. [Available at http://pubs.usgs.gov/pp/pp1713/01/pp1713_ch01.pdf.]

Graham, S.A., and Williams, L.A, 1985, Tectonic, depositional, and diagenetic history of Monterey 

Formation (Miocene), central San Joaquin Basin, California: American Association of Petroleum 

Geologists Bulletin, v. 69, no. 3, p. 385–411.

Tennyson, M.E., Cook, T.A., Charpentier, R.R., Gautier, D.L., Klett, T.R, Verma, M.K., Ryder, R.K., 

Attanasi, E.D., Freeman, P.A., and Le, P.A., 2012, Assessment of remaining recoverable oil in selected 

major oil fields of the San Joaquin Basin, California: U.S. Geological Survey Fact Sheet 2012–3050, 

2 p., accessed June 2013 at http://pubs.usgs.gov/fs/2012/3050/.

Webb, G.W., 1981, Stevens and earlier Miocene turbidite sandstones, southern San Joaquin Valley, 

California: American Association of Petroleum Geologists Bulletin, v. 65, no. 3, p. 438–465.

For Further Information:

Assessment results are available at the USGS Central Energy Resources 

Science Center Web site: http://energy.usgs.gov/OilGas/AssessmentsData/

NationalOilGasAssessment/Publications.aspx.

San Joaquin Monterey Assessment Team

Marilyn E. Tennyson, Ronald R. Charpentier, Timothy R. Klett, Michael E. 

Brownfield, Janet K. Pitman, Stephanie B. Gaswirth, Sarah J. Hawkins, Paul G. 

Lillis, Kristen R. Marra, Tracey J. Mercier, Heidi M. Leathers, Christopher J. 

Schenk, and Katherine J. Whidden