the shale gas boom in recent years has been due to modern technology in hydraulic fracturing (fracking) to create extensive artificial fractures around well bores. Horizontal drilling is often used with shale gas wells, with lateral lengths up to within the shale, to create maximum borehole surface area in contact with the shale. Shales that host economic quantities of gas have a number of common properties. They are rich in organic material (0.5% to 25%), and are usually mature petroleum source rocks in the thermogenic gas window, where high heat and pressure have converted petroleum to natural gas. They are sufficiently brittle and rigid enough to maintain open fractures. Some of the gas produced is held in natural fractures, some in pore spaces, and some is adsorbed onto the shale matrix. Further, the adsorption of gas is a process of physisorption, exothermic and spontaneous. The gas in the fractures is produced immediately; the gas adsorbed onto organic material is released as the formation pressure is drawn down by the well. Shale gas by country Although the shale gas potential of many nations is being studied, as of 2013, only the US, Canada, and China produce shale gas in commercial quantities, and only the US and Canada have significant shale gas production. While China has ambitious plans to dramatically increase its shale gas production, these efforts have been checked by inadequate access to technology, water, and land.China's Coming Decade of Natural Gas?, Damien Ma in Asia's Uncertain LNG Future, The National Bureau of Asian Research, November 2013. The table below is based on data collected by the Energy Information Administration agency of the United States Department of Energy. Numbers for the estimated amount of "technically recoverable" shale gas resources are provided alongside numbers for proven natural gas reserves. The US EIA had made an earlier estimate of total recoverable shale gas in various countries in 2011, which for some countries differed significantly from the 2013 estimates. The total recoverable shale gas in the United States, which was estimated at 862 trillion cubic feet in 2011, was revised downward to 665 trillion cubic feet in 2013. Recoverable shale gas in Canada, which was estimated to be 388 TCF in 2011, was revised upward to 573 TCF in 2013. For the United States, EIA estimated (2013) a total "wet natural gas" resource of 2,431 tcf, including both shale and conventional gas. Shale gas