National and Global Petroleum Assessment

Assessment of Undiscovered Oil and Gas Resources in the Akita Basin 

Province, Japan, 2018

Introduction

The U.S. Geological Survey (USGS) quantitatively assessed the 

potential for undiscovered, technically recoverable conventional and 

continuous (unconventional) oil and gas resources in the Akita Basin 

Province of Japan (fig. 1). The tectonic evolution of the Japan arc and 

the Sea of Japan is well known and forms the basis for the development 

of the Miocene petroleum system assessed here (Okamura and others, 

1995; Wakita, 2013; Van Horne and others, 2017). The Akita Basin is 

part of a back-arc basin along the western margin of the volcanic Japan 

arc. Prior to about 30 mega-annum (Ma), the Japan arc was located 

on the eastern margin of the Eurasian terrane. Regional extension in 

the Oligocene began about 30 Ma, perhaps driven by trench roll-back 

forces, creating a series of grabens and horsts in the back arc, effectively 

separating the Japan arc from Eurasia. Following a regional transgression in the Miocene, deep-water conditions prevailed, and many of 

the grabens were filled with siliceous, organic-rich shales; tuffaceous 

sandstones; and volcanic rocks. Extension ceased by about 15 Ma, and 

post-rift thermal sag led to further deposition and thermal maturation 

of source rocks. Beginning in the Pliocene, a major phase of compression resulted in the inversion of many grabens, and petroleum generated 

from siliceous shales migrated into these structures. As inversion may 

have caused loss of seal integrity, some of the oil may have been lost or 

degraded (Okui and others, 2008). The model underlying the assessment 

is for some oil to have been retained within conventional reservoirs and 

for some oil to have been retained within the shales as a self-sourced, 

continuous (unconventional) shale-oil reservoir.

Total Petroleum System and Assessment Units

For undiscovered oil resources, the USGS defined a Miocene 

Onnagawa Total Petroleum System (TPS). Within this TPS is the 

USGS-defined Miocene Onnagawa Shale Oil Assessment Unit (AU), 

Miocene Onnagawa Shale Gas AU, and Miocene Onnagawa Conventional Oil AU. The Miocene Onnagawa Formation is similar in age 

and lithology to the Monterey Formation in California, with siliceous, 

organic-rich, deep-water shales intercalated with sandstones and 

volcanic rocks. The Onnagawa Formation and equivalent source rocks 

have total organic carbon values of as much as 5 weight percent and 

hydrogen index values as much as 650 milligrams of hydrocarbon 

per gram organic carbon; the source rocks are as much as 500 meters 

thick (Aoyagi and Iijima, 1983; Ujiie, 1995; Okui and others, 2008; 

Yokoi and others, 2012; Yokoi and Tsuji, 2015; Suzuki, 2017). Thermal 

maturation is generally modeled to have begun in the Pliocene, but 

Onnagawa shales, like the similar-age shales of the Monterey, show 

vitrinite suppression (Ujiie and others, 2004), which makes mapping the 

thermally mature source rocks problematic. Because of this, the boundaries of the shale-oil and shale-gas AUs are uncertain.

The geologic model for undiscovered conventional resources is for oil 

and gas to have been generated from thermally mature siliceous source rocks 

in the Pliocene; oil and gas migrated updip into traps within inversion structures and diagenetic traps and accumulated in tuffaceous sandstone, porous 

volcanic rocks, and fractured shale reservoirs (Aoyagi and Iijima, 1983; 

Tsuji and others, 2011; Tsuji and others, 2012). Although reservoirs generally exhibit conventional porosity and permeability, some of the potential 

reservoirs within conventional traps are low permeability and are described 

as tight reservoirs (Yokoi and others, 2012; Yokoi and Tsuji, 2015). Tight 

reservoirs within conventional fields are not included in the shale-oil or 

shale-gas AU areas.

The geologic model for the assessment of shale oil and shale gas is 

for some portion of the oil or gas to have been retained within thermally 

mature Onnagawa Formation source rocks. Only the Miocene Onnagawa 

Shale Oil AU was defined and shale-oil resources assessed. Shale-gas 

resources were not assessed given the lack of information on the extent 

of Onnagawa shale within the gas-generation window, such as in the 

Mogami Trough (Okamura and others, 1995), or the lack of tests that 

would reflect potential shale-gas resources. Thermal gas has been generated from Onnagawa source rocks, as gas is known from several conventional fields, but the downdip extent of the gas window is unknown.

EXPLANATION

Miocene Onnagawa Conventional Oil AU

Miocene Onnagawa Shale Oil AU

Miocene Onnagawa Shale Gas AU

0 50 KILOMETERS 25

0 25 50 MILES

SEA

OF

JAPAN

PACIFIC

OCEAN

Mogami Trough

139° E 140° E 141° E 142° E

41° N

39° N

40° N

JAPAN

Base map from U.S. Department of the Interior National Park Service

SEA OF

 JAPAN

PACIFIC

OCEAN

RUS RUS

CHN

PRK

KOR JPN

Map

area

Figure 1. Map showing the location of three assessment units 

(AUs) defined in the Akita Basin Province of Japan.

Fact Sheet 2018–3047

September 2018

U.S. Department of the Interior

U.S. Geological Survey

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

resources of 111 million barrels of oil and 85 billion cubic feet of gas in the Akita Basin Province of Japan.
Table 1. Key input data for two assessment units (AUs) in the Akita Basin Province of Japan.

[AU, assessment unit; %, percent; EUR, estimated ultimate recovery per well; BCFG, billion cubic feet of 

gas; MMBO, million barrels of oil. Well drainage area, success ratio, and EUR are defined partly using U.S. 

shale-oil analogs. The average EUR input is the minimum, median, maximum, and calculated mean. Shading 

indicates not applicable]

Assessment input data—Continuous AU Miocene Onnagawa Shale Oil AU

Minimum Mode Maximum Calculated mean

Potential production area of AU (acres) 200 825,000 1,650,000 825,067

Average drainage area of wells (acres) 5 10 40 18.3

Success ratio (%) 10 30 50 30

Average EUR (BCFG) 0.003 0.005 0.02 0.006

AU probability 0.9

Assessment input data—Conventional AU Miocene Onnagawa Conventional Oil AU

Minimum Median Maximum Calculated mean

Number of oil fields 1 12 36 12.8

Size of oil fields (MMBO) 0.5 1 100 2.5

AU probability 1.0

Table 2. Results for two assessments units (AUs) in the Akita Basin Province of Japan.

[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. 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]

Total petroleum system

and assessment units (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

Miocene Onnagawa Total Petroleum System

Miocene Onnagawa Shale Oil AU 0.9 Oil 0 66 200 79 0 52 160 63 0 0 1 0

Miocene Onnagawa Shale Gas AU Gas Not quantitatively assessed

Total undiscovered continuous resources 0 66 200 79 0 52 160 63 0 0 1 0

Miocene Onnagawa Conventional Oil AU 1.0 Oil 9 26 77 32 6 18 54 22 0 0 0 0

Total undiscovered conventional resources 9 26 77 32 6 18 54 22 0 0 0 0

Total undiscovered resources 9 92 277 111 6 70 214 85 0 0 1 0

Assessment input data are summarized in table 1. Input data for 

potential drainage areas, success ratios, and estimated ultimate recoveries 

for the Miocene Onnagawa Shale Oil AU are modeled after analogs from 

the Monterey Formation in the United States.

Undiscovered Resources Summary

The USGS quantitatively assessed oil 

and associated gas resources in two assessment 

units (table 2) in the Akita Basin Province of 

Japan. For undiscovered, technically recoverable resources, the mean estimated totals are 

111 million barrels of oil (MMBO) with an 

F95–F5 fractile range from 9 to 277 MMBO 

and 85 billion cubic feet of gas (BCFG) with 

an F95–F5 fractile range from 6 to 214 BCFG. 

Of the mean total of 111 MMBO, 79 MMBO 

are estimated as mean continuous shale-oil 

resources with an F95–F5 fractile range from 

0 to 200 MMBO, and 32 MMBO are estimated 

to be mean conventional oil resources with an 

F95–F5 fractile range from 9 to 77 MMBO.

References Cited

Aoyagi, K., and Iijima, A., 1983, Reservoir characteristics and petroleum migration in 

the Miocene Onnagawa Formation of Akita, Japan, in Isaacs, C.M., Garrison, R.E., 

Graham, S.A., and Jensky, W.A., II, eds., Petroleum generation and occurrence 

in the Miocene Monterey Formation, California: Pacific Section, Society of 

Economic Paleontologists and Mineralogists [now the Society for Sedimentary 

Geology], Los Angeles, California, May 20–22, 1983, Proceedings, p. 75–84.

Okamura, Y., Watanabe, M., Morijiri, R., and Satoh, M., 1995, Rifting and basin inversion in the eastern margin of the Japan Sea: The Island Arc, v. 4, no. 3, p. 166–181.

Okui, A., Kaneko, M.I., Nakanishi, S., Monzawa, N., and Yamamoto, H., 2008, An 

integrated approach to understanding the petroleum system of a frontier deepwater area, offshore Japan: Petroleum Geoscience, v. 14, no. 3, p. 223–233.

Suzuki, Y., 2017, Overview of unconventional oil and gas potentials and exploration activities in Japan: National Institute of Advanced Industrial Science and 

Technology, accessed March 27, 2018, at http://ccop.asia/uc/data/43/docs/

Japan-SUZUKI-UCM7.pdf.

Tsuji, T., Masui, Y., and Yokoi, S., 2011, New hydrocarbon trap models for the diagenetic transformation of opal-CT to quartz in Neogene siliceous rocks: American Association of Petroleum Geologists Bulletin, v. 95, no. 3, p. 449–477.

Tsuji, T., Waseda, A., and Yokoi, S., 2012, Conventional traps in unconventional 

reservoirs rocks in northern Japan: American Association of Petroleum 

Geologists, Search and Discovery Article No. 20179, 3 p., accessed March 27,

 2018, at http://www.searchanddiscovery.com/pdfz/documents/2012/20179tsuji/

ndx_tsuji.pdf.html.

Ujiie, Y., 1995, Petroleum source rocks from the Neogene Tsugaru Basin, northern 

Honshu, Japan: American Association of Petroleum Geologists Bulletin, v. 79, 

no. 6, p. 880–894.

Ujiie, Y., Sherwood, N., Faiz, M., and Wilkins, R.W.T., 2004, Thermal maturity 

and suppressed vitrinite reflectance for Neogene petroleum source rocks of 

Japan: American Association of Petroleum Geologists Bulletin, v. 88, no. 10, 

p. 1335–1356.

Van Horne, A., Sato, H., and Ishiyama, T., 2017, Evolution of the Sea of Japan 

back-arc and some unresolved issues: Tectonophysics, v. 710–711, p. 6–20.

Wakita, K., 2013, Geology and tectonics of the Japanese islands—A review—The 

key to understanding the geology of Asia: Journal of Asian Earth Sciences, v. 72, 

p. 75–87.

Yokoi, S., and Tsuji, T., 2015, Tight oil potential in Neogene Monterey-like biosiliceous shale of Japan: American Association of Petroleum Geologists, Search 

and Discovery Article No. 51119, 4 p., accessed March 27, 2018, at http://www.

searchanddiscovery.com/pdfz/documents/2015/51119yokoi/ndx_yokoi.pdf.html.

Yokoi, S., Waseda, A., and Tsuji, T., 2012, Shale oil potential in Neogene siliceous shales of Japan: American Association of Petroleum Geologists, Search 

and Discovery Article No. 80256, 9 p., accessed March 27, 2018, at http://

www.searchanddiscovery.com/documents/2012/80256yokoi/ndx_yokoi.pdf.

Akita Basin Province Assessment Team

Christopher J. Schenk, Tracey J. Mercier, Marilyn E. Tennyson, Cheryl A. Woodall, Thomas M. Finn, Phuong A. Le, Kristen R. Marra, 

Stephanie B. Gaswirth, Heidi M. Leathers-Miller, and Ronald M. Drake II

For More Information

Assessment results are also 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/fs20183047