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3. Profile of Gas Hydrate Sample Data

3.7  Origin: Biogenic vs. Thermogenic

The origin of the encaged gas may be indicative of the distance of upward migration of the gas before it became a part of a clathrate hydrate. Biogenic hydrocarbon gases (which are overwhelmingly (≥ 99%) methane) are those produced as a direct consequence of bacterial activity and are usually generated a few ten's of meters below the seabed, although Parkes and others (Parkes and others, 1990) discovered such bacterial activity exists in sediment several hundred meters beneath the seabed. Typically, generation of biogenic gas is associated with fine-grained sediment due to its characteristically higher initial organic content. In contrast, the manufacture of thermogenic hydrocarbon gases usually occurs at subbottom depths exceeding 1,000 m (Floodgate and Judd, 1992) These hydrocarbon gases are produced under conditions of high temperature and great pressure from kerogens (which are derived from organic matter). With respect to methane as a thermogenic [⁷] gas, it is dominant only in the last stage (post mature or metagenesis stage) of hydrocarbon production. If identified in a hydrate, thermogenic methane may indicate considerable upward migration of the gas.

Of the 43 samples in the database for which there were determinations of gas origin, eight were either classified as or may be part thermogenic. Each of these eight was from a sea floor environment (of the total of 20 sea floor samples). Moreover, of the 15 regions listed in the database, 12 have gas that is predominantly of biogenic origin, two have, or likely have, both biogenic and thermogenic gases (Gulf of Mexico, Nigeria), and one has only thermogenic gas (Caspian Sea). The three thermogenic or mixed thermogenic gas sites are among the shallowest described in the database and, although they all plot above the methane hydrate phase boundary, it is noteworthy that some are a mixture of ethane, propane and other light hydrocarbons as well as methane. Ethane and propane are stable under less pressure at equivalent temperatures than is methane hydrate. Each site where thermogenic gas was identified is in an area characterized by two or more of the following features: faults, vents and seeps, diapirs, mud volcanoes.

^[7] Thermogenic is synonymous with petrogenic.

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