Debug (line 260 of RequestHandler.php): Testing '$Action//$ID/$OtherID' with '' on DenshamTechnology\SilverStripe\IGI\Website\PageTypes\ArticlePageController

Debug (line 267 of RequestHandler.php): Rule '$Action//$ID/$OtherID' matched to action 'handleAction' on DenshamTechnology\SilverStripe\IGI\Website\PageTypes\ArticlePageController. Latest request params: array ( 'Action' => NULL, 'ID' => NULL, 'OtherID' => NULL, )

Debug (line 184 of RequestHandler.php): Action not set; using default action method name 'index'

Carbon Stable Isotopes - IGI

Carbon Stable Isotopes

Carbon stable isotopes.
Stable isotopes Terrestrial abundance (%) Common standards
12C and 13C 98.888 and 1.112 PDB (Pee Dee Belemnite), NBS-22 (oil), NBS-19 (marine limestone)
CAUTION: Reported isotopic values for a sample will differ depending on the standard used

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 CO2.
  • 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: δ13CNBS-22 = 1.03073 (δ13CPDB) + 30.73 
  • NBS-22 to PDB: δ13CPDB = 1.9702 (δ13CNBS-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:

Table 180.2: Gas carbon isotope data. 
Sample NoDepth (m)StratigraphyMethaneEthanePropaneiso-Butanen-ButaneUnits
abc/xyz 999.9 Brent -43.2 -32.1 -30.4 -26 -27.5 ‰ (PDB)

 

End of Example Content