The development of coalbed methane (CBM) has been limited to moderate- to high-permeability reservoirs (>1 md). However, a significant resource of natural gas exists within low-permeability coals (<1 md). Worldwide CBM resources are estimated to range from 3,500-7,000 Tcf, and EIA estimates >800 Tcf are technically recoverable. As of 2010, however, only 60-70 Tcf of CBM reserves were proved. Vast CBM resources are untapped.