| Sampling, preservation and analysis methods for offshore geochemical surveys, with data from the Barents sea |
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BJORNY M., FERRIDAY I.L. In surface geochemical surveys, 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 –80°C.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 –80 oC, but significant bacterial activity in the other aliquots, even for the samples with bactericide and freezing to –20 oC. 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 –65 oC 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 –80 oC. 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 over twice as much time as the gravity corer. Various analytical techniques have been tested both for gases and liquid fractions. Not surprisingly, headspace gas GC analysis have been found to give low yields when the samples have been preserved by freezing the samples to – 65 oC or lower temperatures, while occluded gas GC gave good results in showing what was present in the pores of the sample when collected. Adsorbed gas GC gave very good results, giving a picture of what gases had seeped through the sediment over geologic time. Regarding liquid hydrocarbons, tests showed that if the whole sample was analysed, i.e. if coarse reworked rock material was not removed before the analysis, the results would be wrong, i.e. the analysis showed “seeped” oil when there was no seepage. It was also found the Total Scanning Fluorescence (TSF) analysis would give completely wrong results in some instances, i.e. showing seepage of oil when there was no seepage and no seepage when other more reliable analyses showed clearly seepage of oil, i.e. one of the most used analytical technique for testing of seepage of liquid hydrocarbons failed in various instances while in other cases worked very well for all the samples, indicating an inherent unreliability. To find macro seeps, i.e. where large amounts of gases or liquid hydrocarbons come to the surface, it is necessary to have conduits that lead close to the surface. This has been found to be the case various places around the world, also in some locations in the Barents Sea. However, in the Barents Sea it is more common to find micro seeps, i.e. where the conduit is terminated well below the surface and the seeping hydrocarbons will then spread upwards over a larger area in the form of an inverted cone. A survey with 300 cores was undertaken over parts of the Norwegian sector of the Barents Sea in 2005. Micro seepage of gases, condensates and oils were found for a number of samples, while macro seeps were not detected. The results from the survey will be discussed showing where micro seeps were detected. Selected References |