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Abstract

AAPG Bulletin, V. 88, No. 10 (October 2004), P. 1335-1356.

Copyright copy2004. The American Association of Petroleum Geologists. All rights reserved.

Thermal maturity and suppressed vitrinite reflectance for Neogene petroleum source rocks of Japan

Yoshihiro Ujiieacute,1 Neil Sherwood,2 Mohinudeen Faiz,3 Ron W. T. Wilkins4

1Department of Earth and Environmental Sciences, Hirosaki University, Hirosaki 036-8561, Japan; [email protected]
2Commonwealth Scientific and Industrial Research Organisation Petroleum, P.O. Box 136, North Ryde, NSW, 1670, Australia; [email protected]
3Commonwealth Scientific and Industrial Research Organisation Petroleum, P.O. Box 136, North Ryde, NSW, 1670, Australia; [email protected]
4Commonwealth Scientific and Industrial Research Organisation Petroleum, P.O. Box 136, North Ryde, NSW, 1670, Australia; [email protected]

AUTHORS

Yoshihiro Ujiieacute is a professor of earth and environmental sciences at Hirosaki University, Japan. He received a B.Sc. degree in geology from the Tokyo University of Education in 1972 and a Ph.D. in geology from Hokkaido University in 1979. His current research interests include diagenetic changes in pollen color and simulation experiments on hydrocarbon generation from living algae and pollen.

Neil Sherwood graduated with a B.Sc. honors degree in geology from the University of Manitoba, Canada, in 1977 and with a Ph.D. on organic petrology of oil shales in 1991 from the University of Wollongong, Australia. He has coauthored about 30 journal and conference papers, abstracts, and posters, as well as about 80 unpublished organic petrological reports for industry and research institutions.

Mohinudeen Faiz is a research scientist at Commonwealth Scientific and Industrial Research Organisation Petroleum, with his main research interests being coal seam gas and organic petrology. Faiz has been working as a geologist for groundwater exploration, coal seam gas studies, gas outburst studies for coal mines, and petroleum source rock studies. He holds an M.Sc. degree and a Ph.D. from the University of Wollongong, Australia.

Ron Wilkins holds a Ph.D. from Cambridge and a D.Sc. from Melbourne University. He has worked on the weathering of minerals, the infrared, Raman, Mossbauer, and fluorescence spectroscopy of minerals, fluid inclusions related to the origin of ore deposits, and most recently carried out research in coal technology and petroleum exploration. He has lectured and researched extensively in France and China.

ACKNOWLEDGMENTS

We acknowledge the efforts of Paul Marvig for his excellent sample preparation and Carol Buckingham for preparing many of the figures. We are grateful to the Ministry of Economy, Trade, and Industry of Japan and the Japan National Oil Corporation for providing samples and data and permission to publish this paper. We also acknowledge the constructive reviews of the paper by David Eby, Paul Green, and Jacqueline Huntoon, as well as the editorial advice of John Lorenz.

ABSTRACT

The northern part of Honshu Island contains the major petroleum resources of Japan. The source rocks mainly comprise a sequence of Miocene marine mudstones that are overlain in places by thick Pliocene and Pleistocene sediments. In the past, mainly vitrinite reflectance analyses have been used to evaluate thermal maturities of these rocks. However, vitrinite reflectance suppression caused by compositional variation of vitrinite is common in marine-deposited rocks, and therefore, modeled estimations of the extent of petroleum generation from the Japanese sequences could be in error.

Fluorescence alteration of multiple macerals (FAMM) analysis is a method that aids in solving the problem of vitrinite reflectance suppression and gives improved evaluations of thermal maturity. Combined vitrinite reflectance and FAMM analyses of potential source rock sequences intersected by the Ministry of International Trade and Industry (MITI) Shin-Takenomachi and MITI Nishi-Kubiki wells of the Niigata Basin and the MITI Honjo-Oki and MITI Yuri-Oki-Chubu wells of the Akita Basin show that vitrinite reflectance suppression is common in the Neogene source rocks. This leads to major differences between the depth profiles for vitrinite reflectance and for FAMM-derived, equivalent vitrinite reflectance. On the basis of vitrinite reflectance, the thermal maturity and hence, the petroleum source rock potential is underestimated for the Miocene Noudani Formation of the Niigata Basin and the Pliocene Funakawa and Miocene Onnakawa and Nishikurosawa formations of the Akita Basin. The new thermal maturity data indicate that these formations would have generated more oil than previously thought, such that petroleum prospectivity for areas including these sequences should be reassessed.

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