TE-GC & Py-GC

Application

Thermal extraction GC (TE-GC) liberates bitumen present in milled rock samples, much like the S1 measurement of Rock-Eval, but allows detailed analysis of molecular distributions of the major components, particularly acyclic alkanes.

Even where TE-GC is not specifically required, it is beneficial because it removes bitumen and residual mud contamination more effectively than solvent extraction. So it is a useful additional ‘clean-up’ step after solvent extraction, and prior to the Py-GC step and also before any determination of kerogen kinetic parameters. Internal or external standards can be used for quantification.

For source rocks, the TE temperature is usually no greater than ~300°C (although it can be varied, depending upon maturity and kerogen type), in order to avoid thermal decomposition of kerogen.

The TE-GC analysis is usually followed by pyrolysis GC (Py-GC), at a temperature of typically 600−650°C, to degrade kerogen into fluid hydrocarbons. This mimics the S2 pyrolysis step of Rock-Eval, and again enables detailed examination of the types of major hydrocarbons generated. This allows more accurate evaluation of source-rock quality than Rock-Eval alone and can be used to predict the type of oil likely to be generated.

Sample requirements

The same preparation procedures and potential problems apply to samples for these analyses as for TOC and Rock-Eval.

The amount needed is very small, and can conveniently be obtained from the material prepared for TOC and Rock-Eval, provided the requirement is known prior to sample preparation.

Analytical procedure

Samples are placed in purpose designed, glass tubes (the same microscale sealed vessels – MSSVs – used for compositional kinetics), which are sealed by melting the tip. A HP5890 II gas chromatograph is used with an MSSV injector and an FID.

The column is a 50 m x 0.32 mm i.d. HP-1 (film thickness 0.52 mm). The sample tube is placed in the injector system and then broken when the injector pressure has stabilised (4 min). The released volatiles are cold trapped with liquid nitrogen for 10 min., during which time the injector/pyrolyser temperature remains at 300°C.

The GC oven temperature programme is: 40°C (for 13 min from breaking of sample tube) to 300°C (25 min) at 5°C/min and then to 320°C (10 min) at 5°C/min.

The PyGC step can be performed on the same sample using the same instrumentation. The pyrolyser temperature is increased to 600°C at a rate of 25°C/min and the pyrolysis products cold trapped and then analysed on the GC using exactly the same operating conditions as above.

If only PyGC is required, the pyrolyser is preheated to 300°C. An open sample tube is placed in the injector system and volatile compounds allowed to evaporate before the pyrolysis oven is closed. Thereafter the procedure is as above; the temperature is then increased to 600°C at 25°C/min, the pyrolysis products cold trapped and then analysed as the GC oven temperature increases.

Internal or external standards may be used for quantification if relative abundance and comparison with Rock-Eval S1 and S2 is insufficient.

Potential problems

Contamination of TE components by drilling mud additives, particularly OBM, can be a problem, although it is readily recognised from the appearance of chromatograms. Because of the very small amount of material required, it is important to ensure that a representative sub-sample is taken from an homogenised bulk sample (e.g. as used for TOC/Rock-Eval or extraction of bitumen).

References

Horsfield B. (1989) Practical criteria for classifying kerogens: Some observations from pyrolysis-gas chromatography. Geochimica et Cosmochimica Acta 53, 891–901.

Larter S.J., Senftle J.T. (1985) Improved kerogen typing for petroleum source rock analysis. Nature 318, 277−280.