Forest Ring Related Methane: Distribution, Origin and Implications for Exploration of Shallow Methane Deposits in Boreal Forests

Kerstin Brauneder
University of Ottawa, Department of Earth Sciences Ontario, Canada; [email protected]

Large accumulations of shallow methane occur within “forest rings” in northern Ontario. Forest rings are large circular features (up to 1.5 km in diameter) commonly observed in boreal forests over carbonate-rich and recently deglaciated terrain. The circular impression reflects a topographic depression that formed over sources of negative charge. In Ontario, forest rings have been reported to form over bitumen, coal, and dissolved hydrogen sulphide accumulations. However, field studies using spectral absorbance lasers suggest that over 85% of the several thousand known rings are centred on natural gas.

This project aims at mapping the distribution of forest rings in northern Ontario and establishing the origin of the natural gas. The distribution of the rings suggests that marine sands underlie most rings and that no rings occur in permafrost regions. This, along with isotopic evidence, suggests a young, low temperature biogenic origin of the methane in most of the rings. Bacteria may also play a role in the formation of subtle positive spontaneous potential (SP) anomalies in the ring, associated with negative-inward redox gradients. Several methane-centered forest rings have been sampled to determine the role of bacteria in the generation of SP anomalies. Studies are currently underway to reproduce these electrical responses in vitro. The understanding of the origin of the SP anomalies has benefits for its use in mineral and energy exploration. Methane-sourced forest rings are potential targets for natural gas exploration because of their abundance, large size and visibility on aerial photographs, which allows targeting with minimal exploration expenditures.

AAPG Search and Discovery Article #90083 © 2008 AAPG Foundation Grants in Aid