The Toyokawa field is a small oil field, covering an area of about six square kilometers, located in northeastern Japan’s Akita Prefecture.
The hillock area where the Toyokawa oil field is situated formerly was covered with natural asphalt (tar) deposits that had erupted from the subsurface. The current scenery resembles the famous La Brea Tar Pit in Los Angeles, Calif.
The tar pit in the Toyokawa oil field – Japan’s only tar pit – has an area of 0.5 kilometer by 1 kilometer. I named it the “Toyokawa Tar Pit.” Mammal fossils (teeth of Naumann elephants, head bones of boars, horns of deer, and more) have been excavated from the beds in the area.
It is estimated that the asphalt erupted about 20,000 years ago, during the Ice Age, considering the presence of Naumann elephant fossils. The asphalt layers are one meter thick in the Toyokawa hillock area, and five to 10 meters thick where they are intercalated with clay and sand layers in the valley floor and the plain.
The Asphalt Jungle
In 1868, Japan shifted from the feudal Tokugawa era to the modern Meiji era. Western types of industries and cultures aggressively flowed into Japan from Europe and the United States.
One of the new technologies was asphalt-paved roads – the first asphalt-paved road was laid in 1879 across the Syohei-bashi Bridge, an iron bridge spanning the Kanda River in Tokyo, using asphalt from the Toyokawa area.
Though this construction was successfully completed, it did not prevail throughout the rest of Japan because it was costly and the asphalt pavement technique was not yet mature.
By 1907, as development of asphalt-paved roads had accelerated in the whole country, the demand for natural asphalt increased, and the mining operation of natural asphalt in the Toyokawa area grew accordingly.
The asphalt mined amounted to more than 4,000 tons in 1912 alone.
As the asphalt was continuously mined, Chugai Asphalt Co., the main miner in the Toyokawa area, had serious concerns that the asphalt soon would be depleted.
The Toyokawa hillock consists mainly of shale of Neogene Tertiary age. Chugai Asphalt Co. confidently believed that oil would be present in the subsurface formations.
On Feb. 24, 1912, Chugai Asphalt Co. drilled the first well where they thought was the axis of an anticline, as was reported by the newspaper at that time, using a U.S.-made cable tool drilling rig. However, it seems that the drilling location actually was on the eastward-dipping flank, based on the geological map and the geological section published in 1903 (figure 2).
This well reached a depth of 390 meters on Feb. 26, 1913, finding several oil-bearing layers. These layers were tested and produced from 31 to 120 barrels per day. However, since the borehole collapsed because of the pressurized shale layers and the squeezed oil sand, it proved very difficult to control the conditions of this well.
These oil-bearing layers were named the “Toyokawa Oil Reservoir,” and the newspaper reported that this type of reservoir had never been found or recorded in the past in Japan.
The drilling was stopped at the depth of 442 meters, and the operation was completed on Oct. 28, 1913. It had taken one year and eight months to complete the well.
How to Succeed In Business
Several more wells were drilled concurrently with the No.1 well, and these resulted in rather poor production of oil – less than 6.3 barrels per well per day.
Chugai Asphalt Co. even changed its name to Chugai Oil Asphalt Co., though the company was commercially unsuccessful.
Then, to carry out more aggressive activities, the company increased its capital with the investment made by the Okura family (one of the zaibatsu families, which controlled various sectors of the Japanese economy) and other interested parties. In 1914 the company purchased an American-made rotary drilling rig and invited an American drilling engineer to the Toyokawa oil field.
One year later, a Mr. Youngring, who had drilling experience and a bachelor’s degree in petroleum engineering, was employed by Chugai Oil Asphalt. Unfortunately, the results of the drilling were disappointing and the production of oil was very poor; Youngring left the field one year later.
As the company needed further financing, it decided to transfer to another company, Ogura Oil Co., a part of the concession area where they thought the potential of oil was low.
In 1915, Ogura Oil drilled its first well, and at the depth of 486 meters it hit a spot that produced more than 906 barrels of oil for a time, and later flowed an average of 113 barrels per day.
Hearing of this success, Mr. Takakuwa, who was then responsible for technical engineering at Chugai Oil Asphalt, decided to change the well locations. The Toyokawa rotary No.12 well was drilled on the 10-degree southeast of the anticline plane (down-dip of the east side of the anticline).
In November 1915, the Toyokawa rotary No.12 well reached the depth of 430 meters, which successfully resulted in an oil production of more than 453 barrels per day.
Chugai Oil Asphalt maintained the same policy for the subsequent drilling locations. After those activities continued, the financial basis of the company became stable.
Gone With the Wind
A subsurface structure map of the Toyokawa oil field in the early stage (1919) is shown in figure 3. The contour (red) lines indicate the top of the Lower Oil-bearing Bed (“Toyokawa Oil Reservoir”).
It was believed that the structure of the Toyokawa oil field is divided into three blocks by east-west faults, and this contour map indicates almost an east-west trend. However, the trend of the successful wells seems to be aligned with the north-south trend line of the Ogura well No.1 and the Toyokawa well No.12.
Figure 4 depicts the scenery of the northern part of the Toyokawa oil field when the structure was being delineated.
Both figures also indicate that the well locations were evenly spaced at intervals of about 200 to 300 feet to avoid interference. Oil exploration and development technology was fully introduced from the United States by not only importing the drilling equipment, but also American operating know-how into Japan.
To the east of the Toyokawa oil field is the Kurokawa oil field, which is a typical anticlinal structure with four-way closures (figure 2). On May 26, 1914, the Kurokawa No. 5 well, operated by Nippon Oil Co., became a gusher flowing more than 12,000 barrels per day.
The news of this enormous success quickly ran throughout Japan as the culmination in the history of oil development in Japan.
The operation in the Toyokawa oil field was taken over by Nippon Oil from Chugai Oil Asphalt. In 1921 the annual production of the field reached the maximum of 547,000 barrels; after that oil production gradually decreased.
The production depths in the field were in the range of 300 to 450 meters. The ability of many production wells suddenly decreased as well. For an effective operation and increased oil production from the wells, pumping units were imported from United States starting in 1921.
Drilling in the Toyokawa oil field was finished by 1940. After that, only the production maintenance operations continued.
The total number of wells drilled at this field is 716. This number seems to be quite large compared with other oil fields of similar size.
This is largely because of the characteristics of this oil field. The reservoir zones consist mainly of broken breccia of hard shale and of mudstone containing less sandstone and tuff layers. This indicates that the reservoir is a fractured type.
The largest production came from the wells drilled in the lower part of the structure. The oil is asphaltic base and 13.4 to 18.7 API. The drive-mechanism of these reservoirs in the field seems to be an irregular edge-water system without a gas cap, which it is why it was necessary to drill so many wells to recover the production volumes.
Figure 5 indicates the geological cross section between the Toyokawa oil field and the Kurokawa oil field as reported by Ichizo Omura (1934).
Oil production operations at Toyokawa field, the first fractured-type reservoir discovered in Japan, were terminated in 2001, with a cumulative oil production of about eight million barrels.
At present, a small amount of natural gas is produced from this field, now more than 100 years old.
An interesting side note: modern Meiji-era Japanese were not the first users of Toyokawa asphalt.
In pre-historic times, about 5,000 years ago, the Jomon people lived around the Toyokawa area; for them asphalt was a very important material. They used it as an adhesive for broken earthenware and as a strengthener at the joint part of arrowheads or knives made of stone and obsidian.
Archeological surveys also indicate that Toyokawa asphalt was used in areas a few hundred kilometers beyond the Toyokawa Tar Pit in Akita, perhaps taken there by trade.
The Jomon people must thus rank among humanity’s earliest documented users – and presumably, traders – of oilfield products.