Isotopic Analysis
All the commonly occurring elements encountered in petroleum and kerogen exist as a number of stable isotopes. The various isotopes of an element exhibit the same chemical behaviour because they possess the same number of protons and electrons, but they differ in the number of neutrons present in the nucleus, so they have different atomic masses. The common stable isotopes used in hydrocarbon exploration are:
Table listing the stable isotopes most commonly used in hydrocarbon exploration.| Element | Stable isotopes | Terrestrial abundance (%) | Common standards |
| Carbon | 12C and 13C | 98.888 and 1.112 | PDB (Pee-Dee Belemnite), NBS-18 (carbonatite), NBS-19 (marine limestone), NBS-22 (oil) |
| Hydrogen | 1H and 2H | 99.985 and 0.015 | SMOW (Standard Mean Oceanic Water, IAEA, Vienna) |
| Sulfur | 32S and 34S | 95.00 and 4.22 | CDT (Canyon Diablo Troilite) or relative to SMOW sulfate |
| Nitrogen | 14N and 15N | 99.63 and 0.37 | Atmospheric nitrogen |
| Oxygen | 16O and 18O | 99.763 and 0.200 | SMOW (or PDB for low temperature geothermometry) |
| Helium | 3He and 4He | Atmospheric helium | |
| CAUTION: Reported isotopic values for a single sample will differ depending on the standard used | |||
NBS = National Bureau of Standards, USA
Isotopic abundance is measured using a mass spectrometer. The simple ratio of the absolute amounts of two isotopes is rarely reported because the very small quantity of one isotope could lead to quite large variations in the ratio determined in different laboratories. To ameliorate such problems, the isotope ratio is calculated relative to that of a standard, which permits much greater precision (i.e. reproducibility). The standard notation is to report the abundance of the heavier of a pair of isotopes in a sample relative to the standard. For carbon the equation is:
δ13C (‰) = [{(13C/12C sample)/(13C/12C standard)} -1] x 1000
The relative amount of the heavier isotope is reported in parts per thousand (per mil) by the 'd' notation. Negative values are indicative of enrichment of 12C relative to the standard (i.e. the sample is isotopically lighter) and positive values indicate enrichment in 13C (the sample is isotopically heavier).
Due to the very small amounts of the heavier of the pairs of isotopes in the above table, the determination of accurate isotopic ratios required a large amount of sample when this technique was first developed. Consequently, the bulk isotopic composition of oils and kerogens were measured rather than that of individual compounds, unless they were particularly abundant and readily isolated, like methane. With improvements in the sensitivity of mass spectrometers and the resolving power of capillary gas chromatography, it is now possible to measure the carbon isotopic ratios of individual compounds, like n-alkanes.
Mass Spectrometric Measurements
For bulk isotope analysis, a single gaseous compound is produced from the many containing the atom of interest, CO2 in the case of carbon. The mass range required for isotopic analysis only needs to cover that represented by the noble (or inert) gases (e.g. helium), oxygen, hydrogen, nitrogen and the oxides of carbon and sulfur (up to m/z 72 for sulfur dioxide), compared with the need to resolve m/z values up to 500+ in biomarker analysis.
Sample and standard gases (the standard is commonly called the reference gas) need to be pure and typically pass through a number of preparation processes prior to passing into the inlet of the mass spectrometer:
A typical stable-isotope duel-inlet mass spectrometer with sample preparation and data correction.
As for all mass spectrometers, the material being analysed (a pure gas in this instance) is ionised by the impact of high-energy electrons. The positively charged ions are accelerated to the same velocity and pass into the mass analyser, where an electromagnetic field is applied, causing the ions to deviate from a straight path. The amount of deviation is a function of the mass:charge ratio of each ion. For singly charged ions of the same velocity, the deviation is simply a function of mass. By adjusting the electromagnetic field, ions of different mass:charge ratios (m/z) can be focussed on a detector ('Faraday cups'). For analysis of carbon as carbon dioxide, it is only necessary to determine m/z values of 44, 45 and 46 (the ion comprising one 13C and two 18O is of such low abundance it can be ignored).
A near-vacuum has to be maintained in the mass spectrometer in order that ionisation is efficient and that the ions produced have an unimpeded route through the mass analyser to the detector. Given that stable-isotope mass spectrometers measure small amounts of commonly occurring gases, it is also important that ambient concentrations of these gases are as low as possible. Generally, the system is maintained at a pressure of < 2.0 x 10-5 torr for active gases and at 10-8 torr for noble gases.
Mass spectrometer with attached gas chromatograph.
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