Bulletin of the Geological Society of Malaysia, Volume 49, April 2004, pp. 93 – 106
1Surface Geochemical Services AS, P.O.Box 5740, 7437 Trondheim, Norway.
2Geolab Nor AS. P.O. Box 5740, 7437 Trondheim, Norway.
Abstract: Surface geochemical prospecting involves the search for near-surface or surface anomalies of hydrocarbons, which could indicate the occurrence of petroleum accumulations in the sub-surface. The methodology, as applied in offshore basins, covers a range of techniques, from observation of visible oil seepage at the surface to detection of micro-seeps in near surface sediments using sensitive analytical technique. Since most rock types are not totally impervious to hydrocarbons, both light and heavy hydrocarbons will migrate upwards, from either mature source rocks or reservoirs, to near surface sediments. While the methodology for surface geochemical surveys is the subject of continuous development, the current, most favoured practice is to detect possible migration pathways from the deep to the near-surface with the aid of seismic data, often together with remote sensing data (satellite imaging etc.). The expression of such pathways at the surface is then the focus of surface geochemical prospecting grids. Most articles concentrate on the analysis of the samples and integration of the geochemical data with the geological framework. It is, however, important that the samples are collected properly and preserved in such a way that the original hydrocarbon assemblage present in the samples when they are brought onboard are preserved for analysis, i.e. care must be taken that there is no bacterial activity after the samples are collected and before they are analysed. Another important factor when undertaking surface geochemical studies is cost. In all such studies, sampling constitutes by far the greatest cost. It is therefore important that the methods used for sampling are streamlined for the purpose, i.e. that methods are not used merely because they give apparently impressive results without increasing the quality of the samples. It is very easy to double the sampling cost by using expensive techniques which do not enhance the quality of the samples. The authors have experience from a number of deep-water exploration areas. In this paper we will discuss sampling methods, preservation of samples and present data from three North Atlantic Margin studies and compare the geochemical data with drilling results where such are available.