Many surface oil and gas seepages including small mud volcanoes are found in the Higashi-Kubiki area, Niigata Prefecture. The purpose of this paper is to demonstrate the geochemical characteristics of these seepages and to discuss their source rocks and migration processes. The geochemical characteristics of oils from the Kamou mud volcano show they derive from mature source rocks in the Lower Teradomari Formation. The gases collected from seepages including mud volcanoes are of a thermogenic

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... igin and are slightly biodegraded based on their geochemical characteristics. The maturity of the gases inferred from their carbon isotopic compositions indicates that they were generated in the Lower Teradomari Formation which is more deeply buried than the source rocks of Kamou oil. Most of them probably migrated upward slowly in formations based on their high C1/ (C2 + C3) ratios, and were biodegraded near the surface. Our head-space gas analysis of two shallow boreholes shows that the head-space gas analysis is a useful tool for understanding the vertical distribution of absorbed gases, and the migration and alteration process of gases in a shallow subsurface. Key words: mud volcano, seepage, overpressured formation, source rock, Higashi-Kubiki area, head-space gas analysis, thermogenic gas, biodegradation : 泥火山 徴候 異常高圧層 根源岩 東頸城地域 ヘッドスペースガス分析 熱分解 ガス 微生物分解 * (株)地球科学総合研究所 ** 石油資源開発(株)技術研究所