The liquid hydrocarbons present in source rocks or stained reservoir rocks contain similar compounds and so can be analysed in the same way as oil, and provide similar information about:
- Major contributors to kerogen
- Depositional environment
- Oil-oil and oil-source correlation
- Alteration of oil
- Inferring source age from oils alone
- Identification and characterization of oil (including palaeo-oil legs) in reservoirs
- Vertical and lateral heterogeneities in reservoirs and possible causes
- Influence of oil-based drilling mud
These liquids can all be described as bitumen, and before their analysis can be performed, those within a rock matrix must first be extracted. Whole oils and extracts, which are complex mixtures, can be fractionated to facilitate further analysis (for example, into saturates, aromatics, resins and asphaltenes, also known by the acronym SARA). These basic sample preparation techniques, and the information they can yield, are described in Extraction and Fractionation. However, some very useful information can be gained directly from whole oils.
Oils are usually subjected to a comprehensive range of analyses to characterize them fully. This permits oil-oil and oil-source correlation and, in the absence of the relevant source rock, to infer characteristics of an oil’s source. The main types of analysis cover:
- general physical properties
- isotopic composition
- detailed molecular composition
Physical properties of oil
Elemental and isotopic composition
Major element composition (CHNOS) can be performed on oils, extracted bitumen and kerogen. These can aid determination of oil properties and the type of kerogen present.
Trace element analysis of oils, for Ni and Vcan provide information on the depositional environment of the associated source rock.
Stable isotopic compositions of major elements provide more information, allowing correlation of oils and source rocks and further information about types of kerogen and depositional environment. C and H isotopic composition can be obtained for isolated kerogen, whole oil/bitumen and SARA fractions. More detailed information can be obtained from C isotopic ratios of individual, major n-alkanes and acyclic isoprenoids (compound specific isotope analysis, CSIA), particularly with regard to correlation and reservoir segregation.
Sulphur isotopic ratios of organic-S in oils can be worthwhile if information on depositional environment related to anoxicity and sulphate-reduction is required, and for additional correlation purposes.
The detailed analysis of hydrocarbon fluids at the molecular level requires a powerful separation technique because of the complex mixture of components present: gas chromatography (GC). The older but less frequently used term, gas-liquid chromatography, gives a more accurate description of the separation technique, because the individual compounds in a sample partition between a mobile gas phase and a stationary liquid phase – which is coated to the inside of a long (usually 25–50 m) capillary column – to different extents.
So separation is achieved on the basis of volatility and degree of affinity which each compound demonstrates towards the stationary phase. The temperature of the column is raised gradually, so that a relatively wide range of compounds can be determined (from methane to C60+, with varying modifications for different C-number ranges).
The detection system varies, depending upon the concentrations of the components of interest. A general purpose, flame ionization detector (FID) is used for the major components, such as n-alkanes, whereas a mass spectrometer (MS) is used for trace components, such as biomarkers, or where compounds cannot be adequately resolved and specificity is required.
When a simple FID is used, the abbreviation of GC is usually used, whereas when a mass spectrometer is used the term frequently employed is GC-MS. The most specific analyses involve a pair of mass selective detectors, represented by the term GC-MS-MS.
All of these GC analyses provide extremely detailed compositional analysis that can provide answers to all of the questions at the top of this page. The various analyses offered by APT include:
- GC of whole oils (GC-WO) and extracted bitumen (GC-EOM)
- High temperature GC (HT-GC)
- GC of saturates (GCsat) and aromatic (GCaro) fractions
- GC-MS by selected ion recording (GC-MS-SIR)