Elsevier

Chemical Geology

Volume 265, Issues 3–4, 30 July 2009, Pages 363-368
Chemical Geology

Chemical and isotopic analysis of hydrocarbon gas at trace levels: Methodology and results

https://doi.org/10.1016/j.chemgeo.2009.04.013Get rights and content

Abstract

Isotopic mass spectrometry coupled online with gas chromatography (GC-C-IRMS) permits measurement of relative proportions of gaseous hydrocarbon (CH4 to C4H10) and CO2, and determination of carbon isotope ratio of hydrocarbon molecules. Access to these parameters provides valuable information about the source and the genesis of naturally-occurring gas, as well as on post-formation physico-chemical processes which might have taken place in the geological environment. In particular, it is possible to distinguish hydrocarbon gas of bacterial origin from that of thermogenic origin based on proportion and carbon isotope ratio of methane as measured by GC-C-IRMS. However, in samples containing very low amounts of hydrocarbons (from 1 ppm to 1000 ppm), accurate measurement of isotope ratios is often impossible due to the limitations of conventional GC-C-IRMS techniques using direct sample introduction. A technique was developed to overcome this limitation. It is based on a novel approach allowing pre-concentration of hydrocarbons prior to GC-C-IRMS analysis. The pre-concentration step consists in selective trapping of hydrocarbon molecules on a cold adsorbent phase, and removal of non-adsorbed gases (N2, O2, Ar,…). In a second step, pre-concentrated alkanes are desorbed, and released in an inert carrier gas, focused through a capillary and introduced into the GC-C-IRMS for chromatographic separation and measurement of concentration and carbon isotope composition of each individual carbon molecule. In order to achieve sufficient accuracy, several operating conditions are of prime importance, including sufficient signal intensity, well defined peak shape and low signal/noise ratio. Accurate measurements can be performed on samples as small as 10 cm3 of bulk gas in standard conditions, with concentrations as low as 1 ppm of methane, 0.5 ppm of ethane and 0.3 ppm of propane and butane. Total analytical uncertainty on δ13C measurements ranges from ± 0.2‰ to ± 1.5‰, depending on the hydrocarbon molecule.

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).

Section snippets

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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