Surface Geochemistry as an Exploration Tool in Frontier Areas Case Study From Offshore Brunei

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 techniques, Since most rock types are not totally impervious to hydrocarbons, both light and heavy hydrocarbons<br>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 that the samples are 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. A surface geochemical study was undertaken over the deep water areas offshore Brunei in 2001 on behalf of the Petroleum Unit of Brunei. The selection of samples was partly based on a 3D survey undertaken over a part of the study area closest to shore and partly based on a regional 2D survey for the part of the area<br>furthest from shore. A total of 200 first priority sample locations and 13 second priority sample locations were selected, Figure 1. A total of 203 sample locations were sampled resulting in 189 recovered cores with enough material for geochemical analyses, i.e. there was good material from the anoxic part of the sedimentary sequence. A total of 10 locations were not sampled due to equipment problems. The average water depth was 1640 m and the average number of cores collected per day was 16. A 4 m core barrel was used for all the sample stations. The average core length was 3.2 m, including the cores that had very short length, or none due to hard seafloor/rocks. By using the USBL system, all the samples were collected within a radius of 25 m from the target. The average distance from the target was 6.4 m. Details regarding the sampling are shown in Table 1. Three of the cores contained gas hydrates, which were collected for analysis. One of the cores contained oil, which was running out of the core liner after the core had been brought up on deck. A strong smell of hydrocarbons was detected in approximately 20 cores. Samples from the anoxic part of the cores were collected, put in pre cleaned cans, flushed with nitrogen, sealed and frozen to -80 oC within minutes after<br>the cores were brought on deck. After the survey was completed the samples were packed into special cooler boxes with dry ice for transport to the laboratory. A complete geochemical analysis program was undertaken, i.e. headspace gas, occluded gas, adsorbed gas, TOC and TC of clay fraction, solvent extraction, GC analysis of EOM, TSF of EOM plus GC-IRMS of gases and GC-MS of EOM of the samples which showed signs of petrogenic hydrocarbons. The abundance of gases in the headspace gas, the occluded gas and the adsorbed gas are lower than what is normally found in other studied areas, e.g. the North Atlantic and the Barents Sea, (Bjorøy and Ferriday, 2002). The abundance of adsorbed gas is particularly low. Similar low abundance of adsorbed gas was also found in other areas of the South China Sea, Abrams, (1996). This was interpreted to signify that there were no active seeps in the area by this author. This is not correct for the area we have studied. Our study showed that the percentage of clay in the < 63 μm fraction is far lower for the offshore Brunei samples than was found for samples in other areas, i.e. the North Atlantic and the Barents Sea samples. The reason for the lower abundance of adsorbed gaseous hydrocarbons is the low clay content in these samples. A number of samples showed active seepage of gaseous hydrocarbons. Before the survey was undertaken, the general belief was that the deep water area offshore Brunei would be a gas province at best. None of the analyzed samples in this study contained dry petrogenic gas. The samples with petrogenic gas contained oil-associated gas based on the composition and isotope values of the gases. A number of hydrocarbons seeps were detected in the survey area, ranging from light oil- to<br>condensate-associated, with varying degrees of biodegradation, and including seeps where hydrates were observed in the shallow cores. A total of 40 samples contained seeped liquid hydrocarbons, biodegraded or partly biodegraded, based on the GC analyses. Some of the analyzed samples contained that large abundances of liquid hydrocarbons that they are classified as representing megaseeps, i.e. the samples are in the close vicinity of a conduit where hydrocarbons are actively seeping to the surface. Some of these samples were already described as containing live oil during the sampling. The analyses of the solvent extracts show the samples to contain seeped liquid hydrocarbons with maturities from the lower oil window to the condensate window. Selected gas chromatograms are shown in Figure 2. The gas chromatograms of another group of samples showed these to contain a large unresolved envelope, representing biodegraded hydrocarbons. The seeped hydrocarbons in these samples were partly masked in the gas chromatograms by hydrocarbons originating from recent organic material (ROM). This is interpreted to represent old seeps which are not active any more. From the variation in GC-MS data, the light oils are proposed to have been generated in a deltaic / terrestrial source rock similar to known Brunei oils, with the possibility of two facies / formation variations contributing in different parts of the area. A map showing seeped hydrocarbons is shown in Figure 3.