Fluid inclusions – chemical analysis
Provided sufficient material containing petroleum inclusions can be harvested, analyses of the hydrocarbons present can be performed by GC and GC-MS analysis. The main purposes of these chemical analyses of HCFIs are:
- petroleum correlations
- PVT characterization
General characterization of oil composition can be undertaken, but so too can more oil-oil and oil-source correlations (e.g. biomarker analysis). To understand properties and movement of fluids during migration and within the trap, fluid densities and isochores are calculated from compositional data. This permits modelling of fluid characteristics (e.g. phase behavior and GOR) at various temperature and pressure conditions.
At present APT does not offer this type of analysis.
To obtain compositional information on petroleum inclusions at the molecular level they must be crushed and analyzed by standard geochemical techniques (GC, GC-MS, GC-IR-MS). This can be done ‘on-line’ or ‘off-line’.
On-line analysis involves crushing/decrepitation in the injector port of a GC, so relatively small amounts of sample are required (~20–40 mg). This method has the advantage of determining all major hydrocarbons, usually by GC-FID, and can permit composition to be determined suitable for PVT calculations using equations of state.
Off-line analysis involves crushing grains under solvent prior to routine injection on to GC or, more usually, GC-MS. A greater quantity of grains is required (~10 g) because of the dilution required to maintain a sufficient volume of solvent during the crushing.
Samples are selected on the basis of optical analysis, to ensure a sufficient density of HCFIs. If more than one population of HCFIs is present, ideally they should be subsampled. This usually involves manual picking, although density fractionation techniques applied to the disaggregated grains may help to varying extent.
Because of the small amounts of hydrocarbons likely to be harvested in relation to the bulk grain sample, residual contamination of grain exteriors by adsorbed hydrocarbons, whether migrated oil or oil based mud, is a serious concern.
Thorough cleaning is essential (Karlsen et al. 1993; George et al. 1997). The first step is to disaggregate grains as far as possible, with mud being washed off and the finest material discarded (which usually contains the lowest concentration of hydrocarbons). Repeated microscopic inspection is necessary to determine when suitably clean and disaggregated grains have been obtained.
Subsequently, several extractions are required, together with oxidation steps, to remove all organic material from grain surfaces. Only when the final extraction shows effectively zero residual contamination should crushing of grains to release hydrocarbons be undertaken. Standards may be added prior to the cleaning process to test for effective removal.
Burruss (1987) developed a method for on-line crushing by modifying a GC injector. The sample is crushed in the injector and the fluid is flushed into the column by the carrier gas flow. GC determination of C1–C40 is generally possible, so the entire gas, gasoline and C15+ hydrocarbon ranges are covered. With the necessary instrumentation, δ13C CSIA can also be performed on the major components.
Off-line analysis is not really suitable if gaseous components are of interest. It can be performed using a specialized crushing cell and gas syringe for sampling (Munz 2001). Liquid hydrocarbons are obtained by careful crushing of grains under a solvent suitable for injection into a GC (Karlsen et al. 1993). In a pestle and mortar, evaporation inevitably occurs, requiring the solvent to be topped up periodically in order to prevent the loss of the more volatile liquid hydrocarbons from the inclusions. Ideally, the extract should not be allowed to evaporate to dryness; it can be used for GC, GC-MS and GC-IR-MS analyses.
The main potential pit-fall with any fluid inclusion work is establishing the cleanliness and representability of HCFIs. The most likely problems are:
- insufficient abundance of HCFIs
- residual external contamination of grains
- difficulty subsampling individual HCFI populations
- influence of modified HCFIs (e.g. leakage losses)
Burruss R.C. (1987) Crushing cell, capillary column gas chromatography of petroleum inclusions: method and application in petroleum source beds, reservoirs and low hydrothermal ores. In (eds Roedder E., Kozlowski A.) Fluid Inclusion Research 20, 59 (abstract).
George S.C., Krieger F.W., Eadington P.J., Quezada R.A., Greenwood P.F., Eisenberg L.I., Hamilton P.J., Wilson M. (1997) geochemical comparison of oil-bearing fluid inclusions and produced oil from the Toro sandstone, Papua New Guinea. Organic Geochemisty 26, 155−173.
Karlsen D.A., Nedkvitne T., Larter S.R., Bjørlykke K. (1993) Hydrocarbon composition of authigenic inclusions: application to elucidation of petroleum reservoir filling history. Geochimica et Cosmochimica Acta 57, 3641−3659.
Munz I.A. (2001) Petroleum inclusions in sedimentary basins: systematics, analytical methods and applications. Lithos 55, 195−212.