Carbon Stable Isotopes

^{NBS = National Bureau of Standards, USA}

Carbon stable-isotope determinations are the most commonly used and are carried out on:

• Bulk samples - whole-oil, kerogen and individual oil/extracted bitumen fractions (saturates, aromatics, NSOs and asphaltenes).
• Individual gases - C₁-C₄ hydrocarbons and sometimes CO_2.
• Individual oil components - single compounds, particularly n-alkanes (C₅ to C₃₅).

The analysis of bulk samples requires controlled combustion of the sample and transfer of the evolved carbon dioxide into the isotope-ratio mass spectrometer. For individual hydrocarbons, the individual constituents of a bulk sample are first separated by gas chromatography prior to combustion and isotopic analysis. Naturally occurring CO₂ is purified and passed directly into the inlet of the isotope-ratio mass spectrometer. View a mass spectrometer.

The ratio of ¹³C to ¹²C in a sample is usually measured relative to that in a standard, and expressed in parts-per-thousand (per mil):

δ¹³C (‰) = [{(¹³C/¹²C sample)/(¹³C/¹²C standard)} -1] x 1000

A variety of standards can be used, but data are usually quoted relative to the PDB standard. This results in the need for some useful conversion factors:

• PDB to NBS-22: δ¹³C_NBS-22 = 1.03073 (δ¹³C_PDB) + 30.73 
• NBS-22 to PDB: δ¹³C_PDB = 1.9702 (δ¹³C_NBS-22) - 29.81

Isotopic ratios can be used as the basis for correlation of oils with their source-rocks. They can similarly be used to determine the source of methane or carbon dioxide in wells.

While the analytical procedures are robust and the analyses precise, our ability to interpret data from oils and source rocks is still poor due to the great complexity of the mixtures present and the effects of subtle changes in those mixtures. The advent of compound-specific isotope-ratio analysis coupled to capillary gas chromatography (GC-IR-MS) is improving this position. For further information see 'Isotope interpretation'.

Given the abundance of carbon and carbon species within the biosphere, great care must taken during sampling, particularly of gases.

Carbon isotope data for gases may be reported in the following way:

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