Laumontization and Secondary Pores of Sandstones in the Paleogene and Upper Cretaceous Coal Measures, off Northeast Honshu, Japan |
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| Authors: | Yutaka Yanagimoto Azuma Iijima |
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| Institution: | JAPEXResearch Center, 1–2-1 Hamada, Mihama-Ku, Chiba 261–0025, Japan [e-mail: ];University of Tokyo, 7–3-1 Hongo, Tokyo 113–0033, Japan |
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| Abstract: | Abstract. Sandstones with high reservoir quality occur in the Paleogene and Upper Cretaceous coal measures off Sanriku and Sohma in the Pacific coast of northeast Honshu. The sandstone porosity was generally produced by the dissolution of calcite cement and clastic grains such as feldspar and glassy volcanics. The most probable cause of dissolution is the organic acids generated from the maturation of coal and coaly matter in the deeply subsiding source area prior to thermogenic hydrocarbon generation. The pore fluid including organic acids dissolved calcite and clastic silicates to form a small amount of laumontite and kaolinite at around 60 C. The acidic and not neutralized pore fluid was responsible for the formation of kaoli-nite. On the other hand, laumontite was formed when the acidic pore fluid was neutralized and then made alkaline after the reaction with minerals such as plagioclase, glassy volcanics and calcite cement. Therefore, laumontite and kaolinite generally occur separately. Laumontite is 0.6–4.6 % by volume, whereas kaolinite is 0.6–9.8 % and the sandstone porosity remains from 10 to 22 %. This type of laumontization after the secondary pore formation might not give a severe damage to the reservoir property of the Paleogene and Upper Cretaceous coal measures in the Pacific coast of northeast Honshu and indicates further exploration possibility. |
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| Keywords: | laumontite kaolinite secondary pore organic acid coal measures diagenesis |
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