Testing marine shales' ability to generate catalytic gas at low temperature

Hydrocarbon gases are generally thought to originatevia low-temperature microbial or high-temperature thermogenicpathways (Whiticar, 1996) that can be distinguished by compound-specific hydrogen and carbon stable isotope ratios. An alternative low-temperature catalytic pathway for hydrocarbon generation from sedimentary organic matter has been proposed to be active at temperatures as low as 50oC (e.g.,Mango and Jarvie,2009,2010; Mango et al., 2010; Bartholomew et al., 1999). This hypothesis, however, still requires rigoroustesting by independent laboratory experiments.The possibility of catalytic generation of hydrocarbons in some source rocks (most likely in relatively impermeable and organic-rich shales where reduced catalytic centers can be best preserved) would offer an explanation for the finding of gas of non-microbial origin in formations that lack the thermal maturity for generating thermogenic gas.It is unknown whether catalytically generated methane would be isotopically different from thermogenicmethane (δ13CCH4>-50‰, δ2HCH4from -275‰ to -100‰) ormicrobially generated methane (δ13CCH4from -40‰ to -110‰, δ2HCH4from -400‰to -150‰) (Whiticar, 1998). In order to test for catalytic gas generationin water-wet shales and coals, we are conductinglaboratory experiments at three temperatures (60°C, 100°C, 200°C)and three pressures (ambient pressure, 107 Pa, 3x107 Pa)over periods of six months to several years. So far, our longest running experiments have reached one year. We sealed different types of thermally immature, pre-evacuatedshales (Mowry, New Albany, and Mahoganyshales) and coals (SpringfieldCoal and Wilcoxlignite)with isotopically defined waters in gold cells in the absence of elemental oxygen.Preliminary results show that these samples, depending on conditions, can generate light hydrocarbon gases (methane, ethane and propane) and CO2. Methane, CO2, and traces of H2havebeen generated at 60°C, whereas experiments at 100°C and 200°C also yielded ethane and propane. As of July 2013, our preliminary dataindicate that the amount of light hydrocarbon gasesincreases with temperature and decreases with increasing pressure.Gas yields after 1 year are typically about twice as large as after 6 months. Decreasing the pressure from 3x107 Pato 107 Payields 3 to 6 times more gases, all other factors being equal. A temperature increase from 60°C to 200°C increases gas yields by a factor of ~40. For example, at 60°C over 6 months at 107 Pa, MowryShale can generate 0.4 μmol methane per gram TOC, and the yield decreases to 0.06 μmolat3x107 Pa.δ2HCH4 from Springfield Coal and Mowry Shale decreases with time and temperature, whereas pressure's influence on δ2HCH4value seems to varyamong source rocks.δ2HCH4from Mowry, Wilcox and Mahogany shales andδ2HH2Oare not correlated, which stands in contrast tothe results of traditional hydrous pyrolysis experimentsat temperatures above 300°C (Schimmelmann et al., 1999). If the methane from gold cells was indeed generated via catalysis, the hydrogen isotope data would suggest that the mechanism of catalytic gas generation is different from thermogenesis.