THE ANALYSIS OF THE C1-C5 COMPONENTS OF NATURAL-GAS SAMPLES USING GAS-CHROMATOGRAPHY COMBUSTION ISOTOPE RATIO MASS-SPECTROMETRY

Recent developments in porous polymer fused silica capillary columns combined with high sensitivity GC-C-IRMS have enabled the routine analysis of the C1 to C5 compounds of natural gases without any prior sample preparation. The analysis time for the carbon isotope ratios of the individual components is reduced to that of routine GC-FID analysis. Selection of column type is critical to analytical performance. Analytical conditions have been determined which allow the separation and isotopic analysis of C1, C2, C3, iC4, nC4, iC5 and nC5 in an aliquot of <250 mul of natural gas. Good separation of components was achieved using a Poraplot Q GC column. Analytical precision of between 0.03 and 0.21 parts per thousand (+/- 1sigma) has been determined using a standard reference natural gas. Isotopic fractionation during sample injection and chromatographic analysis has been identified. Using this method we have been able to analyse samples on a VG Isochrom II system from natural gas samples previously too small for analysis (e.g. seeps, headspace). A study of headspace ps data, which would not have been possible previously, demonstrates the effects of sample leakage and biodegradation on deltaC-13 of samples from an actively leaking reservoir. Biodegradation appears to have caused an increase in deltaC-13 of methane and ethane with a lesser increase in deltaC-13 of propane. The carbon dioxide in the samples appears to be derived largely from oxidation of oil with a contribution from oxidation of gas.