The geology of natural gas resources

The growth of natural gas reserves and production from shale formations has sparked interest in the Nation's natural gas resources. The diagram shows the geologic nature of most major sources of natural gas in the United States in schematic form:

• Gas-rich shale is the source rock for many natural gas resources, but, until now, has not been a focus for production. Horizontal drilling and hydraulic fracturing have made shale gas an economically viable alternative to conventional gas resources.
• Conventional gas accumulations occur when gas migrates from gas-rich shale into an overlying sandstone formation, and then becomes trapped by an overlying impermeable formation, called the seal. Associated gas accumulates in conjunction with oil, while non-associated gas does not accumulate with oil.
• Tight sand gas accumulations occur in a variety of geologic settings where gas migrates from a source rock into a sandstone formation, but is limited in its ability to migrate upward due to reduced permeability in the sandstone.
• Coalbed methane does not migrate from shale, but is generated during the transformation of organic material to coal.