In this paper, the origin, maturity, migration, biodegradation and mixing of natural hydrocarbon gases in Japan have been interpreted using molecular and carbon isotope compositions. No indications of abiogenic gases have been found, the gases being classified as microbial, thermogenic or mixed microbial ⁄ thermogenic. However, secondary alteration (mixing, biodegradation, fractionation in migration processes) has had a major but variable impact on the composition of natural gases. Biodegradation, especially, has altered both molecular and isotopic signatures. Thus, the prime control of isotopic and molecular characteristics in gases is due to genetic phenomena, but secondary effects must be taken into account when attempting to understand the origin and distribution of gas. Where the extent of secondary alteration is small, carbon isotope compositions of thermogenic hydrocarbons are controlled largely by maturity. An isotope model developed by Berner and Faber was applied successfully to natural gases in northeast Japan. Besides maturity estimation, the application of the model enables detection of biodegradation, mixing between microbial and thermogenic gases, and mixing among gases with different maturities. The carbon isotope compositions of carbon dioxide also provide information for their genetic origins.

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Isotopic studies for 20 gas samples and 17 water samples from three water-dissolved gas fields, the Nichinan, Miyazaki and Sadowara fields, in Miyazaki Prefecture, were carried out. Their results are summarized as follows.1 Hydrocarbons are the major components in all gas samples. These hydrocarbons are chiefly composed of methane. The Kitago gas contains some carbon dioxide whose 13 C CO 2 value suggests a magmatic origin.2 The 13 C values of hydrocarbons indicate that the origin of the gases in the Nichinan and Miyazaki fields is thermogenic. Because their maturity of the gases is estimated to be about 0.8% Ro equivalent based on the 13 C 2 13 C 3 plot, they were probably generated in the Paleogene Nichinan Group.3 Most of the hydrocarbon gases in the Sadowara field are microbial and mixed with a small amount of low maturity thermogenic gases which were supposed to be generated within the late Miocene to Pliocene Miyazaki Group.4 Hydrocarbons in the Nichinan sample Kitago and one sample Oyodo of the Miyazaki field are biodegraded. 5 Formation waters in the Sadowara field are altered sea water trapped during deposition, whereas those in the Nichinan and Miyazaki fields are interpreted to be a mixture of meteoric water and fossil sea water.6 Formation waters in the Sadowara field have relatively homogeneous 87 Sr/ 86 Sr ratios which are slightly lower than the ratio of sea water in the period of deposition.

Carbon isotope compositions of methane, ethane and propane, and hydrocarbon ratios in gas samples provide information of their origin microbial vs. thermogenic , maturity of thermogenic component, compositional change due to migration, and extent of biodegradation. Mixing of gases with different origins or different maturities can also be evaluated using gas isotopic and molecular compositions. While these gas geochemical data have been used mainly for petroleum exploration, their applications for development, production and operation issues are also increasing. Headspace gas analyses can be used to delineate reservoir compartments and pay zones. Carbon isotope compositions in commingled production could be used to allocate contributions form individual production zones if isotopic differences exist between the gases from the contributing reservoirs. Origin of gas seepage in production sites could be investigated by the gas molecular and isotope compositions if enough reference data exist in the area.

Many surface oil and gas seepages including small mud volcanoes are found in the HigashiKubiki 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 origin 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