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ABSTRACT: Surface geochemistry as an exploration tool in frontier, deep water, areas. Case studies from West Africa and South East Asia

Bjoroy, Malvin1, and Geir Hansen2
(1) Geolab Nor AS, Trondheim, Norway
(2) Surface Geochemical Services AS, Oslo, Norway

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 seepages at the surface, to detection of micro-seeps, in near surface sediments, using sensitive analytical techniques,

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.

This methodology has been applied in several surveys in the relatively unexplored deep water basins of the western North Atlantic Margin, West Africa and South East Asia. In this paper we will present studies from West Africa and South East Asia, where we will discuss the planning, sampling and analytical results. The analyses include analysis of both gas and liquid hydrocarbons in sediment samples. The results vary significantly in the different basins, from showing only micro-seepage to showing a combination of micro-seepage and macro-seepage (with biodegradation of the seeped hydrocarbons in specific areas). Our studies clearly show that marine surface geochemical prospecting can be used to determine whether or not hydrocarbons have been generated in a basin, and whether these are oil or gas related. If oil related hydrocarbons are detected, then information on the types and maturities of source rocks which have generated these hydrocarbons can also be determined.

AAPG Search and Discovery Article #90913©2000 AAPG International Conference and Exhibition, Bali, Indonesia