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Isotopic Testing of Marine Shales’ Ability to Generate Catalytic Gas at Low Temperatures

Lin Wei1, Arndt Schimmelmann1, Agnieszka Drobniak2, Peter E. Sauer1, and Maria Mastalerz2
1Department of Geological Sciences, Indiana University, 1001 E 10th St, Bloomington IN, 47405
2Indiana Geological Survey, 611 N Walnut Grove Ave, Bloomington IN, 47405

Hydrocarbon gases are generally thought to be originated from two sources in sediments: low-temperature microbial and high-temperature thermogenic sources (Whiticar, 1996). The gases from these two sources can be distinguished by their isotopic compositions of carbon and hydrogen. Thermogenic gases have δ13CH4 values >-50‰ with δD values in a range between -275‰ to -100‰, while biogenic gases have δ13CH4 values between -40‰ to -110‰ and δD values between -400‰to -150‰ (Whiticar, 1998). Recent years, low-temperature sources of catalytic gases have been proposed. As opposed to the high-temperature thermogenic sources, shales and coals may generate hydrocarbon gases at a temperature as low as 500C (i.e. 300°C below the typical temperatures of pyrolysis experiment in laboratory) (e.g. Mango, 1992; Mango, 1996; Mango & Hightower, 1997; Bartholomew et al., 1999). This idea needs to be tested with more geochemical and isotopic evidence.

In order to examine this idea, we conduct experiments at three temperatures (60°C, 100°C, 200°C) and three pressures (ambient pressure, 107 Pa, 3x107 Pa) to test if and how water-wet shales and coals generate hydrocarbon gases over 6 months and one year. We sealed different types of immature marine shales and coals with isotopically defined waters in anaerobic gold cells. Our preliminary results show that these samples do generate light hydrocarbon gases (methane, ethane and propane) and CO2. When the temperature is at 60°C, only methane is generated. When the temperature is at 100°C with high pressure, ethane and propane are also generated. In general, the amount of light hydrocarbon gases increases with temperatures and peaks at 200°C. On the contrary, the mount of generated gases decreases with pressures. δD decreases with time and temperature but it could increase or decrease with pressure in different rocks. Water with lower hydrogen values generates gases with lower hydrogen values, suggesting that water involves in the process of the generation of the hydrocarbon gases.

AAPG Search and Discovery Article #90182©2013 AAPG/SEG Student Expo, Houston, Texas, September 16-17, 2013