Characterization of CO 2CH 4H 2O fluid inclusions by microthermometry and laser Raman microprobe spectroscopy: Inferences for clathrate and fluid equilibria

Abstract

Microthermometry (MT) and laser Raman microprobe (LRM) spectroscopy (at room temperature and at about 0°C) were done on 33 synthetically produced CO 2CH 4H 2O fluid inclusions in quartz (from R. Bodnar and M. Sterner). At room temperature, the inclusions consist of an aqueous liquid, a CO 2CH 4 supercritical carbonic fluid and (in most cases) graphite. In all these inclusions, the melting temperature for solid CO 2 is less than that for the homogenization of the vapor bubble in the carbonic fluid. A method is described whereby MT data for CO 2CH 4H 2O inclusions can be projected within the CO 2CH 4 binary phase diagram to infer CO 2:CH 4 ratios in the carbonic fluid (<2 to > 35 mole% CH 4 in the inclusions under study). This method takes into account the formation of CO 2CH 4 clathrate hydrate during MT analysis. Unless clathrate formation is properly considered, errors arise in the determination of the bulk CO 2CH 4 ratio. For the inclusions in our study, these errors are on the order of 5 to 8 mole% CH 4. Our interpretation of the MT data indicates that CH 4 is preferentially partitioned into the clathrate over the coexisting carbonic fluid, in contradiction to the prediction from Parrish and Prausnitz's (1972) model for clathrate equilibria. Comparison of LRM analyses on the bulk carbonic fluid (in the absence of clathrate) and the residual carbonic fluid (in the presence of clathrate) confirm the preferential partitioning of CH 4 into the clathrate. LRM analyses of the clathrate itself indicate that CH 4 occupies both types of cage sites in the clathrate structure, whereas CO 2 may only occupy one site. Two by-products of the combined LRM and MT analyses of the same inclusions are derivation of empirical ratios of Raman quantification factors for high-density CO 2CH 4 fluids and the ability to determine CO 2:CH 4 ratios of inclusions whose MT data lie near the critical region for CO 2CH 4. Thus, the joint use of LRM and MT techniques provides information that could not be obtained by either technique alone.