Thermal cracking of kerogens and bitumens is widely accepted as the major source of natural gas (thermal gas). Decomposition is believed to occur at high temperatures, between 100 and 200°C in the subsurface and generally above 300°C in the laboratory. Although there are examples of gas deposits possibly generated at lower temperatures, and reports of gas generation over long periods of time at 100°C, robust gas generation below 100°C under ordinary laboratory conditions is unprecedented. Here we report gas generation under anoxic helium flow at temperatures 300° below thermal cracking temperatures. Gas is generated discontinuously, in distinct aperiodic episodes of near equal intensity. In one three-hour episode at 50°C, six percent of the hydrocarbons (kerogen & bitumen) in a Mississippian marine shale decomposed to gas (C1-C5). The same shale generated 72% less gas with helium flow containing 10 ppm O2 and the two gases were compositionally distinct. In sequential isothermal heating cycles (∼1 hour), nearly five times more gas was generated at 50°C (57.4 μg C1-C5/g rock) than at 350°C by thermal cracking (12 μg C1-C5/g rock). The position that natural gas forms only at high temperatures over geologic time is based largely on pyrolysis experiments under oxic conditions and temperatures where low-temperature gas generation could be suppressed. Our results indicate two paths to gas, a high-temperature thermal path, and a low-temperature catalytic path proceeding 300° below the thermal path. It redefines the time-temperature dimensions of gas habitats and opens the possibility of gas generation at subsurface temperatures previously thought impossible. © 2009 Mango and Jarvie; licensee BioMed Central Ltd.
CITATION STYLE
Mango, F. D., & Jarvie, D. M. (2009). Low-temperature gas from marine shales. Geochemical Transactions, 10. https://doi.org/10.1186/1467-4866-10-3
Mendeley helps you to discover research relevant for your work.