Dissolved methane and propane concentrations were measured aboard Gulf's exploration vessels, the R/V Hollis Headberg and the M/V Gulfrex, on the continental margins of North America. Propane concentrations of less than 0.5 nL/L were observed in a majority of the samples in many of the areas studied. This observation is in agreement with open ocean concentrations reported by Swinnerton and Lamontagne. However, in a highly petroliferous area such as the Louisiana offshore, higher propane concentrations are more common with one-third of the samples exceeding 2 nL/L. In the Louisiana offshore, three-fourths of the samples contained

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over 250 nL/L of methane, whereas the reported open-ocean concentrations show a majority of the samples contained less than 50 nL/L.

Generally, on the continental shelf a local contribution is more common for methane than for propane.

The seawater hydrocarbon concentrations have been compared with the U.S. Geological Survey estimates of undiscovered oil and gas resources in each of the offshore geologic provinces where data were collected.

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