Identification of the presence of petroleum gas, together with indications of its source, maturity and how it may have been affected by a range of alteration processes can be assessed from a combination of molecular and isotopic compositional analyses.
Because there are few gaseous hydrocarbons, only limited differences in distribution are possible, so the isotopic composition of individual compounds is particularly helpful in providing more information.
Gas chromatography should be undertaken to resolve and quantify methane (C1), ethane (C2), propane (C3), isobutane (iC4), normal butane (nC4) and remaining higher hydrocarbons (C5+) as a minimum. It is also useful to determine the amount of CO2 and H2S present, which help identify microbial alteration, particularly with respect to gas souring.
Nitrogen is generally measured, which helps monitor air contributions and He can also be useful if a mantle contribution to methane is suspected. Abundance of H2 in annulus gas can provide information about steel corrosion. The stable C isotopic compositions of the C1–C4 components are extremely useful in determining genetic affinities, as is the stable H isotopic composition of methane, and all these isotopic measurements are recommended.
The potential sources of carbon dioxide can be revealed from its d13C value and aids assessment of the origins of methane, which can have a variety of sources.
Among other isotopic analyses, APT also offers d15N of N2 and d34S of H2S.
Usually, molecular composition of mud and/or headspace gas is monitored at regular intervals, but with greater sampling frequency around the reservoir zone. Isotopic composition is usually applied to a sub-set of samples, which can be selected at the start of a study or after molecular compositions have been determined. Occluded gas (i.e. that trapped within rock cuttings) can also be analysed after headspace gas has been sampled; it is typically much less abundant than headspace gas.
Test gases are often supplied in cylinders, mud gas samples in IsoTubes (preferred) or bags, and cuttings for headspace gas in sealed cans or IsoJars. Further information on these containers can be found at wells\gas samples.
In the laboratory, gas cylinders, IsoTubes and aluminium gas bags can be stored under ambient conditions in designated places as required by HSE regulations. Once the more routine gas composition analyses have been completed, selected samples can be resampled for isotopic analysis, as required.
If concentrations of hydrocarbons are low, some pre-concentration may be required prior to isotopic analysis.
Molecular composition of gas
Aliquots of gas samples are transferred to exetainers, from which 0.1-1 mL is sub-sampled using a Gerstel MPS2 autosampler and injected into an Agilent 7890 RGA GC equipped with Molsieve and Poraplot Q columns, a flame ionization detector (FID) and two thermal conductivity detectors (TCD).
Hydrocarbons, as individual C1–C5 compounds (including neopentane) together with C6+, are determined by FID, whereas H2, CO2, N2 and O2/Ar are measured by TCD. Sensitivities down to 1 ppm by vol. are possible under ideal conditions. Commercially available calibration gas is used as an external standard.
Molecular composition of occluded volatiles
Where occluded volatiles in canned cuttings are to be analysed, the samples are washed with cold water (without detergent), sieved and the 1–4 mm fraction collected and weighed. A wet aliquot is crushed in a ball mill fitted with a septum (5 min.) and the mill is then heated at 60°C for at least 30 min. 1 mL of the gas sampled through the septum and analysed.
Another aliquot is weighed wet and again after drying at 60°C to enable gas concentrations to be reported as mL/kg dry rock (the mill volume can also be recorded).
A HP5890 II instrument is used for the analysis, with a gas loop fitted to the injector end and a FID. The column is a 50 m x 0.32 mm i.d. HP-1 (film thickness 0.52 mm). The injected volatiles are trapped with liquid nitrogen for three minutes before being released into the GC column, whereupon the following temperature programme is used: 30°C (3 min.), 2°C/min to 70°C (10 min. isothermal), 2°C/min to 130°C (0 min.). A gas with known composition is used as a standard.
Carbon isotopic composition is determined by a GC-C-IRMS system, comprising a Thermo Fisher Scientific Trace 1310 GC with PTV injector and Thermo Fisher Scientific Delta V Advantage IR-MS. Samples are introduced by syringe via a Triplus RSH autosampler (one for vials and one for IsoTubes/IsoJars) to a Poraplot Q column.
The GC to MS interface comprises GC-Isolink II and Conflo IV units. The separated components are combusted to CO2 and H2O at 1000°C over a Cu/Ni/Pt catalyst, and the water removed by Nafion membrane separation. Replicate analyses of standards (6 or 7 are used) indicate that the reproducibility of δ13C values is better than 1 ‰ PDB (2 sigma).
If preconcentration is required because of low methane levels, a dedicated precon system coupled to a Delta plus XP IRMS is used. An aliquot of the gas is sampled with a GCPal autosampler and any CO2, CO and H2O removed in chemical traps.
Hydrocarbons other than CH4, together with any remaining traces of CO2, are removed by cryotrapping. The methane is then combusted to CO2 and H2O at 1000°C over Cu/Ni/Pt. The water is again removed by Nafion membrane separation, prior to δ13C analysis of the CO2.
The hydrogen isotopic composition of methane is determined using the same system, but the separated methane is decomposed to H2 and coke in a 1420°C furnace and δD measured on the former.
The international standard NGS-2 and an in-house standard are used for testing accuracy and precision, replicate analyses of which indicate that the reproducibility of δD values is better than 10 ‰ PDB (2 sigma).
- leakage of gas containers, leading to loss of gas sample and contamination with atmospheric gases
- diffusion via very small leaks may cause preferential loss of methane with possibly some isotopic fractionation
- samples used for PVT testing may no longer be representative of the original gas
- mud gas composition, and to an extent headspace samples, can be affected by drilling operations (e.g. lost circulation, bit changes)
Clayton C. (1991) Carbon isotope fractionation during natural gas generation from kerogen. Marine & Petroleum Geology 8, 232–240.
Galimov E.M. (1973) Carbon Isotopes in Oil-Gas Geology. Nedra, Moscow. NASA, Washington DC (1974 translation from Russian).
James A.T. (1983) Correlation of natural gas by use of carbon isotopic distribution between hydrocarbon components. AAPG Bulletin 67, 1176–1191.
Machel H.G., Krouse H.R., Sassen R. (1995) Products and distinguishing criteria of bacterial and thermochemical sulfate reduction. Applied Geochemistry 10, 373–389.
Schoell M. (1983) Genetic characterization of natural gases. AAPG Bulletin 67, 2225–2238.