Diatomaceous rocks are widely distributed around the circum-Pacific region as Miocene to Lower Pleistocene deposits, including the Monterey Formation, California, USA (Ingle, 1981; Isaacs, 1987). The Middle to Upper Miocene Onnagawa Formation is a typical diatomaceous rock in northern Japan. The alternation of hard and soft layers is a characteristic feature, as shown in this picture. The hard layers consist of thick (< several tens of centimeters) porcelanite, while the soft layers consist of thin (< 10 cm) siliceous mudstone. The Onnagawa Formation is known as an excellent petroleum source rock in Japan. The total organic carbon (TOC) contents of porcelanite and siliceous mudstone are generally 0.1 to 3wt% and 2 to 6wt%, respectively. The Yurihara–Ayukawa oil field is located about 5 km west of this outcrop. The Onnagawa Formation is also a reservoir rock, in which oil and gas are reservoired in pores expanded by the diagenetic phase transformation of silica minerals. Moreover, the Onnagawa Formation draws attention as a shale oil resource.
(Photograph: Yuichi SAITO, Explanation: Takashi TSUJI, Amane WASEDA and Satoru YOKOI; Photographed on May 22, 1988)
The Monterey Formation is the primary source rock for oil in California. The light brown layers with fractures are Opal-CT chert. Because Opal-CT is still preserved in this rock, the rock was never heated to a high enough temperature to transform it into Quartz. Consequently, this outcrop probably did not reach the thermal maturity for oil generation. It is likely oil in fractures was generated at higher maturity areas in the basin center, and migrated updip into this lower-maturity fractured Opal-CT reservoir. Monterey shale has been producing oil for more than 100 years.
(Photograph & Explanation: Kazuyoshi HOSHI; Photographed on March, 1992)
