GC analyses

Initial screening of source or reservoir rock extracts and oils is usually performed by GC (with FID detector), which can provide a broad picture of major component distributions and hence some information about:

• source of hydrocarbons
• maturity
• alteration
• contamination by organic components in drilling mud

It also serves as a basis for selection of samples for more detailed analysis by GC-MS (which is more costly and time-consuming, but also more informative). GC analysis can be undertaken on:

• Whole extracts/oils
• Saturates
• Aromatics

Oils/condensates are routinely analysed complete, in order that the most volatile components are not depleted by evaporation during isolation of SARA fractions. This analysis is termed whole oil GC (GC-WO), and provides extremely useful information from the distribution of light hydrocarbons.

The C₆–C₇ range is particularly informative, because virtually all components can be resolved and identified, allowing detailed comparison of samples in correlation studies, as well as evaluation of potential alteration processes upon entrapment, such as biodegradation, water washing, evaporative fractionation and thermal alteration (Thompson 1987, Halpern 1995).

GC analysis of extracted organic matter (GC-EOM) is effectively equivalent to GC-WO, although the most volatile components are inevitably lost during the extraction and concentration procedure. The C₁₀₊ range is usually little affected when care is taken not to evaporate extracts to dryness, as performed at APT (e.g. methylnaphthalene data is considered reliable).

The data can be useful for identifying presence of migrated oil in reservoir intervals and in characterising potential source rocks. The influence of organic drilling mud components (e.g. polyglycols and various oil distillation cuts) can also be usefully evaluated. As noted above, such data are valuable in informing selection of samples for further analyses.

A particular application is assessing reservoir segregation using ratios of pairs of neighbouring minor peaks in whole-oil or whole extract chromatograms. Pairs of reasonably resolved peaks from the large number eluting between the n-alkanes and that appear to differ from each other in relative abundance are chosen and ratios calculated for each sample:

Although useful information can be gained from C₁₀₊ components during GC-WO and GC-EOM, GC of the saturates fraction (GCsat) provides cleaner results because it eliminates co-elution problems from aromatics, enabling n-alkanes and major acyclic isoprenoid, such as pristine (Pr) and phytane (Ph) to be determined accurately. Any interference from polyglycols observed during GC-EOM is also removed.

Saturates gas chromatogram for NSO-1 oil standard

If an oil is suspected of being waxy, the distribution of alkanes above the normal range, i.e. >C₄₀, can be performed using high temperature GC (HT-GC).

Special GC conditions and column are required, enabling detection to >C₆₀. As with all chromatographic analyses, there are trade-offs to be made to optimize analyse for specific types of compounds or volatility ranges. In HT-GC, resolution of lighter hydrocarbons is sacrificed to enable determination of the heaviest components.