POSITIVE EFFECTS OF GLACIALLY INDUCED ISOSTATIC REBOUND ON BELLINGHAM BASIN COAL/SHALE AND ITS IMPLICATION FOR ENHANCED NATURAL GAS PRODUCTION

Coal bed methane (CBM) and fractured shale (FS) natural gas exploration and production have evolved over the last two decades from novelties to a full fledged plays. CBM is now being produced from the San Juan, Raton, Powder, Black Warrior, and Cherokee Basins with FS producing gas from the Michigan, Illinois, and Appalachian Basins to name a few. Presently productive CBM and FS wells in the lower 48 fall outside of the known limits of the Pleistocene glaciations with the exception of the Antrim Shale of the Michigan Basin.

The next exception to this rule will be the Bellingham Basin (BB) of NW Washington state. The Puget Sound area and Whatcom County in particular was covered by ice sheets estimated to be up to 1.25 miles thick several times during the Pleistocene. The downward force exerted by a one mile thick column of ice averages 2100 lbs/sq in. The most recent rebound of the depressed sedimentary column in the Bellingham Basin is conservatively estimated to be 1000 feet and the majority of it occurred in only several hundred years.

It is theorized that this quick rebound due to the rapid decay of the ice sheet resulted in widespread fracturing of the sedimentary column down to measured depths of 2000’ similar to what has happened in the Michigan Basin (MB). The Antrim Shale of the northern MB is naturally fractured and gas prone down to depths of 2000’ due to isostatic rebound. The gas is of biogenic origin (95+ % methane) with the microbes transported downward through the sediments by the percolation of freshwater in the fracture systems. The same dynamics are in place in the BB helping explain the source of natural gas in water wells completed in its glacial drift deposits.

The biogenic gas is generated in the Eocene age coals and organic rich shale of the BB and collects in the fracture systems as free gas. Methane is also adsorbed to the shale and coal and can be released by lowering the pressure in the formations by the removal of water from the fractures. A significant hurdle that restricts gas production from coals and shale is low permeability. Thus, the enhanced fracturing caused by isostatic rebound increases the permeability improving the deliverability of gas through the formation to the wellbore.