--> Abstract: Evaluation of Sampling Methods and Various Preservation Techniques for Offshore Surface Geochemical Surveys, by Malvin Bjorøy and Ian L. Ferriday; #90078 (2008)

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Evaluation of Sampling Methods and Various Preservation Techniques for Offshore Surface Geochemical Surveys

Malvin Bjorøy and Ian L. Ferriday
Geolab Nor AS, Trondheim, Norway

There has been significant discussion regarding which sampling techniques give the best quality samples, the costs of using one technique compared with others, and how to preserve the samples after collection to ensure that there is no degrading bacterial activity. In order to get scientifically reliable answers to these questions, two different surveys have been undertaken.

The first is from the Barents Sea, where samples from 100 cores were quartered and treated to four different combinations of preservation methods, including presence/absence of bactericide, storage at room temperature or freezing to -20 or -80oC. The headspace and occluded gases were analysed for molecular composition and carbon isotope composition, with quite revealing results. There was no indication of bacterial activity in samples frozen to -80oC, but significant bacterial activity in the other aliquots, even for the samples with bactericide and freezing to -20oC. The conclusion from this is that the only preservation technique that will stop any bacterial activity in surface geochemical samples is freezing at very low temperatures, preferably -65oC or lower.

In the second survey, offshore Faeroes, for each of 10 stations, one gravity core and one piston core were collected close together. Samples were collected at 0.5 m depth intervals over the 4.8 to 5.5 m cores and preserved by canning / freezing to -80oC. All the samples were analysed both for gaseous and liquid hydrocarbons. There are no significant differences in the results from the parallel samples. The conclusion from this is that there is no difference in the quality of the samples from gravity or piston corers. However, there are significant differences in sampling time, the piston corer requiring 2.4 times as long as the gravity corer.

 

AAPG Search and Discovery Article #90078©2008 AAPG Annual Convention, San Antonio, Texas