The rapid detection of light hydrocarbons

GeoVOC represents a new approach to near-surface geochemical petroleum prospecting. Developed in conjunction with petroleum prospecting companies and verified as more accurate than existing GC-based geochem techniques, GeoVOC uses SIFT-MS to reduce sample analysis time from up to thirty minutes to less than thirty seconds. Large scale geochem prospecting surveys are now practical and economic.

Near-surface hydrocarbon analysis (the detection and analysis of light hydrocarbon seeps) has traditionally played an important role in the exploration of new petroleum fields. In the past this analysis has relied on various GC-based geochemical techniques.

Seeking to overcome the slow analysis times and poor sensitivity levels associated with existing GC-based methods, Syft Technologies has developed a rapid, high sensitivity geochemical test for soil, water and seabed samples. Called GeoVOC, the test applies advanced selected ion flow tube mass spectrometry (SIFT-MS) to the identification and quantification of volatile light petroleum hydrocarbons (C1-C4). GeoVOC, through the application of advanced SIFT-MS technology, claims three key advantages over existing analytical techniques:

1. Speed - each sample is analysed in approximately 15 seconds;
2. High sensitivity - hydrocarbons are detected at levels of a few ppb;
3. Simplicity - no chemical preparation of samples or calibration of columns is required.

Background

Near-surface hydrocarbon exploration techniques exploit the assumption that hydrocarbons are generated and trapped at depth, but often seep in varying quantities to the surface. This means that an anomalous hydrocarbon accumulation at the surface is a relatively reliable indicator to a deeper petroleum field.

Combined with sub-surface geochemical analysis, near-surface hydrocarbon surveys can greatly reduce the area to be searched by more expensive exploration techniques, such as 2D and 3D seismic surveys, thereby significantly reducing exploration costs and risk. In addition to indicating the location of petroleum fields, detailed analysis of the distribution and relative concentrations of surface-collected light hydrocarbon gases can provide other valuable information, such as the likelihood of oil versus gas yields and the field's thermal maturity.

Surface and marine hydrocarbon surveys can therefore:

• Identify areas with high potential for petroleum reserves;
• Maximise the effectiveness of exploration efforts at least cost;
• Predict the oil versus gas nature and thermal maturity of potential prospective targets.

Though labour and time consuming, the extensive use of light hydrocarbon surveys in the Gulf of Mexico was 88% effective in finding new commercial production fields and delivered the economic advantage of predicting whether a block was likely to produce oil or gas.

Surface hydrocarbon analysis

Nearly all the major oil-producing regions were first discovered through the investigation of surface oil and gas seeps. Seeps occur wherever a permeable pathway leads to the surface from a source strata or petroleum reservoir.

Of particular interest to petroleum prospecting is the occurrence of microseeps and their associated light hydrocarbons. Light hydrocarbon gases are mobile and can migrate rapidly to the surface, often reaching the surface relatively close to the subsurface petroleum sources. The composition of light hydrocarbons and the ratios of C1-C2, C1-C3 and C1-C4 indicate whether a sub-surface reservoir is more oil or gas prone.

Oil productive source-rock basins yield greater proportions of ethane, propane and butane relative to methane, whereas gas prone areas yield proportionally greater methane. The hydrocarbon ratios also indicate thermal maturity.

Marine geochemical surveys

In addition to detecting and measuring light hydrocarbons in soil vapour and soil headspace samples, marine petroleum prospecting tools include assaying the light hydrocarbon gases in seawater and seabed samples. Regional exploration, structural seepage trend evaluation, individual prospect evaluation and anomaly detailing studies are carried out to estimate petroleum source, maturity, composition and magnitude of fields.

Conventional GC-based light hydrocarbon detection technology

Soil samples are generally mixed with water, heated for an extended period, shaken and the resulting headspace gas is assayed for C1-C4 hydrocarbons by flame ionisation detector (FID) gas chromatography. Alternatively, soil vapour is collected from 4-8 BGL onto an adsorbent gas-sieve which is subsequently thermally desorbed and the resulting C1-C18 hydrocarbons are assayed by GC-FID and GC-MS.

A second GC-based application is the continuous analysis, for stripped methane (C1) through butane (C4) light hydrocarbons, of near-bottom seawater samples.

A third application is the analysis of sediment cores. For this application the cores are processed by adding sediment to degassed brine. Nitrogen headspace is added by displacement of the brine and the samples are heated for 12 hours and then shaken. Samples can then be displaced from the can into an evacuated serum bottle to ensure that gases are not altered during the time from sample collection to GC-FID analysis. Typical GC results are shown in Figure 1.

GeoVOC: applying advanced SIFT-MS to the detection of light hydrocarbons

In comparison to GC-based hydrocarbon analysis, the GeoVOC process is relatively simple. Samples are collected and held in sealed containers. Immediately prior to analysis, the containers are pierced and headspace samples are drawn through a heated capillary into the SIFT-MS instrument. A comprehensive hydrocarbon analysis, complete with detailed technical results and graphical summaries, takes approximately 15 seconds per sample.

GeoVOC has been developed in consultation with petroleum exploration companies and its accuracy has been verified through a dual prospecting program that applied parallel GeoVOC and FID-GC analysis methodologies.

Figure 2 shows typical raw data results for a GeoVOC analysis. Note that this analysis took 17 seconds and did not require a column.

Conclusion

Near-surface hydrocarbon analysis is a useful cost-saving tool that complements other petroleum prospecting techniques.

By replacing GC-based methods with the latest SIFT-MS techniques, GeoVOC represents a new generation of hydrocarbon analysis that, for the first time, makes large scale hydrocarbon surveys practical and economic. The key advantages of GeoVOC are its speed and simplified sample preparation procedures.