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Reports: DNI254474-DNI2: Experimental Study of Carbon and Hydrogen Isotope Equilibria among CO2 and Hydrocarbons in Sedimentary Basins

Qi Fu, University of Houston

This project is designed to investigate the key controlling factors of carbon (C) and hydrogen (H) isotope fractionations between light hydrocarbons (including CO2) following their generation and/or expulsion from source rocks in sedimentary basins, by conducting a series of laboratory isotope exchange experiments. As proposed, the studies in the second year of this project mainly involved completion of setting up experimental apparatus, calibration of compound specific hydrogen isotope analysis of light alkanes, laboratory experiments under different chemical and physical conditions, and chemical and isotopic composition analyses of experimental products. Under the PI’s supervision, the graduate students have gained knowledge on setting up experimental apparatus and performing chemical and isotopic analyses of experimental products.

The details of research work that have been completed in the 2nd year and corresponding scientific findings are summarized as follows:

1) The setup of the experimental apparatus has been completed, following installation of a sample holder inside the reactor that allows addition of solid mineral catalysts in the reactor. The graduate student has written a computer program that can control the 2-position gas injection valve and corresponding abundances of dissolved multicomponent gases flowing into the reactor. A series of test runs under the range of proposed temperature (100 ~ 300 °C) and pressure (50 ~ 300 bars) conditions were conducted.

2) The gas chromatography - isotope ratio mass spectrometry (GC-IRMS) system was calibrated for compound specific hydrogen (H) isotope analyses. The calibration included analyses of natural gas mixes with known isotopic values. Repeated analyses have shown that the reproducibility is in the range of 3.0 to 5.1 ‰.

As a major objective of this project, reaction pathways and organic intermediates while attaining isotope equilibrium will be identified. It will be accomplished using an Agilent 6460 triple quadrupole liquid chromatography - mass spectrometer (LC-MS/MS). The analytical protocol for identification of aqueous organic compounds (carboxylic acids, alcohols, etc.) has been developed and currently under test.

3) A series of hydrothermal experiments have been conducted using CO2 and CH4 with the δ13C value of -40.5‰ and -32.2‰, respectively, whereas no mineral catalyst is present. The experiments are performed under 2 different temperature conditions: 150 and 250 °C, with pressure being 200 bars. The duration of those experiments is 500 hours.

Analytical results have shown that no other gaseous product is observed, and isotopic compositions of CO2 and CH4 are constant throughout all the experiments. Theoretical isotope equilibrium prediction indicated that the fractionation between CO2 and CH4, εc (CO2-CH4), at 150 and 250 °C is about 41.0‰ and 29.3‰, respectively. It is clear that CO2 and CH4 are not in 13C equilibrium without the presence of mineral catalyst. Experiments with different CO2/CH4 ratios (3:1 and 1:1) are also conducted under similar conditions. No attainment of carbon isotope equilibrium between CO2 and CH4 is observed.

This project has been approved for a no-cost extension for the 3rd year. The studies in the next year will primarily be devoted to experimentation along with chemical and isotope analysis. Experiments under a wide range of conditions that is encountered in sedimentary basins will be performed. All factors that might cause differences in carbon and hydrogen isotope fractionation, including temperature, pressure, and mineral compositions, will be systematically identified.