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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 57 (1973)

Issue: 4. (April)

First Page: 773

Last Page: 773

Title: Generation of Light Hydrocarbon Gases in Deep-Sea Sediments: ABSTRACT

Author(s): G. E. Claypool, I. R. Kaplan, B. J. Presley

Article Type: Meeting abstract


Several hundred analyses have been made both on board the Glomar Challenger and in the laboratory on gas samples returned from the Deep-Sea Drilling Project (JOIDES). Methane was the dominant gas in all samples, commonly amounting to more than 99% of the total. Small quantities of ethane or propane were observed in areas of high heat flow, or over a possible petroleum reservoir.

Significant quantities (40 × 109 cu ft/cu km) of methane can be generated in the interstitial water of deep-ocean sediment where reducing conditions are initiated by rapid burial of organic matter. Comparison of carbon isotope (C13/C12) ratios of coexisting methane and dissolved carbonate indicates that the methane originates by bacterial CO2 reduction. This mechanism does not involve the formation of ethane or higher hydrocarbons, or require the rupture of carbon-carbon bonds. Therefore, bacterial methane is chemically and (usually) isotopically distinct from hydrocarbon gases derived from thermocatalytic maturation of organic matter. Bacterial methane production generally begins when all sulfate is reduced, and continues with increasi g depth of burial in the sediment, as long as symbiotic bacteria provide the required substrates, carbon dioxide, and hydrogen.

At some depth in the sediment column, depending on temperature and concentration, methane can exceed solubility in the interstitial water, migrate upward as a gas, and reach saturation at shallower depths. If the height of the overlying water column is greater than about 1.5 km, the gaseous methane may be converted to the solid clathrate hydrate within the uppermost (about 500 m thick) layer of sediment, where temperatures are below 20-25°C.

Stabilization of methane as a solid gas hydrate could be an important factor in the accumulation of natural gas deposits by (1) preventing loss of gaseous methane from the sediments; (2) allowing upward migration of gaseous methane at a pace controlled by the sedimentation rate; and (3) producing an enrichment of gaseous methane in the zone just below the lower limit of stability of the gas hydrate.

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Copyright 1997 American Association of Petroleum Geologists