Compound specific isotope analysis (CSIA)
For oil accumulations, CSIA of n-alkanes can provide a useful means to investigate:
- sources of compounds
- mixing of oils
- reservoir segregation (when biodegradation has not been too severe)
In particular, mixing of charges may be more obvious via d13C values than from inspection of gas chromatograms from EOM or saturates. For the most accurate data on n-alkanes, these compounds can be isolated from a total saturates fraction using molecular sieve (or urea adduction), thereby removing interference from other compounds that elute closely during GC analysis (including any UCM in partially biodegraded or biodegraded and recharged accumulations).
Alternatively, a total saturates fraction can be analysed in order to obtain d13C values for acyclic isoprenoidal alkanes, which may be helpful when n-alkanes have been significantly depleted by biodegradation. However, the potential contribution of any hump of unresolved complex mixture (UCM) under the resolved alkanes must be taken into consideration.
Following isolation of a saturates fraction or an n-alkanes fraction, samples in solution are loaded on to a TriPlus RSH autosampler and introduced via a PTV (programmed temperature vaporizing) injector to a Trace 1310 GC connected to a Delta V plus IRMS (all supplied by Thermo Fisher Scientific) by a GC-Isolink II and Conflo IV.
Chromatographic separation of compounds is achieved using a Restek Rxi-1ms 60 m column with a 0.25 mm i.d. and 0.25 mm film thickness and a temperature programme of 50°C (2 min. hold) to 320°C (4 min. hold) at 4°C/min.
d13C values for nC10+ are reported relative to VPDB via the NBS-22 standard (–30.031‰) from the IAEA. More volatile compounds are usually not considered to provide sufficiently reliable results due to fractionation effects. Three replicate analyses are recommended for reliable results.
Light hydrocarbon CSIA, encompassing most of the C7 compounds, is performed on whole-oil samples by IsoLab, Netherlands.
- potential problems mostly relate to total saturates fractions, although mixing of charges (together with varying degrees of biodegradation) can affect the interpretation of genetic relationships
- base-line resolution of compounds is required because, for any compound, the 12C containing proportion elutes slightly earlier than the 13C, so partial co-elutions adversely affect d13C determinations
- the presence of unknown co-eluants may adversely affect d13C interpretation (e.g. crocetane and phytane)
- it may be difficult to allow for the contribution of UCM to individual acyclanes in biodegraded samples
- some isotopic fractionation can be associated with isolation of an n-alkanes fraction
Murray A.P., Summons R.E., Boreham C.J., Dowling L.M. (1994) Biomarker and n-alkane isotope profiles for Tertiary oils: relationship to source rock depositional setting. Organic Geochemistry 22, 521–542.
Rooney M.A., Vuletich A.K., Griffith C.E. (1998) Compound-specific isotopes analysis as a tool for characterizing mixed oils: an example from the West of Shetlands area. Organic Geochemistry 29, 241–254.
Whiticar M.J., Snowdon L.R. (1999) Geochemical characterization of selected Western Canada oils by C5–C8 Compound Specific Isotope Correlation. Organic Geochemistry 30, 1127–1161.