Chemical and isotopic analysis of hydrocarbon gas at trace levels: Methodology and results
Introduction
The demand for analysis of gaseous hydrocarbons at trace levels in natural samples has been increasing in the recent years. Many oil operators and companies have developed interest in surface geochemistry measurements during the exploration phase in order to better understand the genesis and history of hydrocarbons accumulated at depth (Pernaton et al., 1996, Prinzhofer and Pernaton, 1997). Also, during drilling, gas sampling is often done at different depths, appropriately identified by the mud logger system, in order to achieve a refined and complete description of drilling conditions. The characterization of trace hydrocarbon gas from surface and drilling samples provides useful data for unraveling the geological history of successive events in a hydrocarbon-bearing basin.
In the field of environmental protection, analyses of trace amounts of gaseous hydrocarbons in soils, surface water or underground water provides insights into the processes at work in polluted areas (abandoned or active industrial sites, gas stations, areas close to roads, oil slicks, etc). In paleoclimatology, analysis of trace methane found in air bubbles occluded in ice yields essential clues to the understanding of climatic evolutions in relation to the greenhouse effect (Chappellaz et al., 1993a, Chappellaz et al., 1993b). The characterization of hydrocarbon traces outgassed from pore water in low-permeability sedimentary rocks provides information on the degree of confinement in sites under investigation for waste repository.
These examples, among others, illustrate the importance of improving detection limits of techniques used to determine the isotopic and chemical composition of gaseous hydrocarbons in trace amounts. Except for systems dedicated to analysis of methane and CO2, current commercial spectrometers are not able to analyze trace hydrocarbons with sufficient accuracy. A pre-concentration step for the molecules of interest is therefore required. The difficulty is amplified by the fact that sampling is often done under low pressure conditions. In all cases, it is essential to know sampling pressure with a good precision (0.1 mbar) to be able to determine the final concentrations of the different gas compounds.
At the University of Grenoble, France, in the research group of the Laboratoire de Glaciologie et de Géophysique de l'Environnement (LGGE), J. Chappelaz and co-workers (Aballain, 2002, Bernard, 2004) developed a pre-concentration system for measuring methane in trace amounts from air bubbles in ice cores. Test was performed at the LGGE in a preliminary stage of our study in order to assess the applicability of the system to the analysis of trace hydrocarbons dissolved in argillites porewater (Girard et al., 2002). The results of these tests indicated that measurement was acceptable for methane, but not for heavier hydrocarbons. This leads us to modify and adapt the technique followed at the LGGE in order to develop the ability to make measurements of all major hydrocarbons, i.e., CH4 to C4H10, with sufficient accuracy (Huiban et al., 2004).
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Instrumentation and analytical procedures
A detailed diagram of the analytical system used at IFP is shown in Fig. 1. It consists of a stainless steel vacuum line equipped with single or multiple path valves, traps, pressure gauges and gas-flowmeters permitting control of key parameters at each of the six steps used for the process:
- step 1: prior standard/sample injection, operating conditions require the line to be evacuated down to a pressure of at least 5 ∙ 10− 4 mbar using a pumping system with a diaphragm primary pump (1) and a
Validation of the technique using laboratory standard gas
The pre-concentration system procedure was validated using a laboratory standard gas. Composition and concentration of the gas are certified by the manufacturer at the precision level indicated in Table 1. It is composed of about 50 ppm, diluted in nitrogen, of the following compounds: CH4, C2H6, C3H8, iC4H10, nC4H10 and CO2.
The carbon isotopic ratios of the different hydrocarbon compounds in this gas are indicated in Table 1. They are not provided by the supplier. This determination was
Implications and recommendations
The results reported here show the performances but also the limitations of the newly-developed pre-concentration system. Below a sample pressure threshold, it would not be very realistic to analyze these samples in acceptable conditions of accuracy and reliability. This limitation is more restrictive for CO2 and CH4 than for C2H6 and C3H8. To assess this threshold, the relative variation of measured concentrations recalculated after normalization has been plotted versus the pressure (Fig. 4).
First application study and further improvements
This methodology was developed as part of a project aiming at investigating porewater in the Callovo-Oxfordien argillites of Bure, eastern Paris Basin. GC-C-IRMS analysis was performed on gas naturally released from water-saturated argillite cores stored in specifically-designed outgassing cells shortly after drilling (Girard et al., 2005, Prinzhofer et al., 2009). It was possible to quantify the concentrations and δ13C of methane, ethane and propane. The measured concentration ranges are
Conclusions
We have developed and tested a pre-concentration system specifically designed for gaseous hydrocarbons which permits analysis, with good accuracy, of the chemical and carbon isotopic composition of alkanes in trace amounts under very low sampling pressure. The pre-concentration system is used online on a GC-C-IRMS. The method relies on the use of a cryogenic trap composed of Hayesep Q™ type cooled to − 115 °C. The analytical procedure was validated using commercial reference gas. The following
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