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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 65 (1981)

Issue: 5. (May)

First Page: 997

Last Page: 997

Title: Geochemistry of Gaseous Hydrocarbons Adsorbed in Shallow Sediments Offshore Santa Barbara, California: ABSTRACT

Author(s): Wolfgang J. Stahl, Eckhardt Faber

Article Type: Meeting abstract


An offshore geochemical surface survey was conducted west of Santa Barbara, California. Two hundred sediment samples were taken in an area of approximately 400 sq km. The samples were degassed and the gases analyzed by gas chromatography and isotope mass spectrometry. GC-MS techniques were applied to the extracts of several samples to look for C28 and C29 steranes. In addition, grain size determinations and fluorescence analyses were carried out on selected samples. The analytical results, mainly the 13C/12C ratios of the methane, allow discrimination between (1) an area of exclusively bacterial methane and (2) an area where thermogenic (petroleum-related) hydrocarbons are present.

The area of biogenic gases is characterized by isotopically light methane in the range of ^dgr13C1 = 90 ppt. High methane yields occur up to 50,000 ppb.

The area of thermogenic gases is characterized by ^dgr13C1- values in the range of -44 ppt which correspond to methane originating from predominantly marine source rocks (type II kerogen) within the oil window. The methane yields are approximately 200 ppb. Three oil fields, not yet in production, are situated within the area exhibiting the shallow, thermogenic methane. This observation supports the conclusion that hydrocarbons can migrate to the surface from deeper source rocks or from deep gas and oil reservoirs where they can be identified by isotope geochemistry. The application of this methodology to the field of hydrocarbon exploration is obvious.

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