<P> Some gas fields yield sour gas containing hydrogen sulfide (H S). This untreated gas is toxic . Amine gas treating, an industrial scale process which removes acidic gaseous components, is often used to remove hydrogen sulfide from natural gas . </P> <P> Extraction of natural gas (or oil) leads to decrease in pressure in the reservoir . Such decrease in pressure in turn may result in subsidence, sinking of the ground above . Subsidence may affect ecosystems, waterways, sewer and water supply systems, foundations, and so on . </P> <P> Releasing natural gas from subsurface porous rock formations may be accomplished by a process called hydraulic fracturing or "fracking". It's estimated that hydraulic fracturing will eventually account for nearly 70% of natural gas development in North America . Since the first commercial hydraulic fracturing operation in 1949, approximately one million wells have been hydraulically fractured in the United States . The production of natural gas from hydraulically fractured wells has utilized the technological developments of directional and horizontal drilling, which improved access to natural gas in tight rock formations . Strong growth in the production of unconventional gas from hydraulically fractured wells occurred between 2000 - 2012 . </P> <P> In hydraulic fracturing, well operators force water mixed with a variety of chemicals through the wellbore casing into the rock . The high pressure water breaks up or "fracks" the rock, which releases gas from the rock formation . Sand and other particles are added to the water as a proppant to keep the fractures in the rock open, thus enabling the gas to flow into the casing and then to the surface . Chemicals are added to the fluid to perform such functions as reducing friction and inhibiting corrosion . After the "frack," oil or gas is extracted and nearly 30 percent to 70 percent of the frack fluid, i.e. the mixture of water, chemicals, sand, etc., flows back to the surface . Many gas - bearing formations also contain water, which will flow up the wellbore to the surface along with the gas, in both hydraulically fractured and non-hydraulically fractured wells . This produced water often has a high content of salt and other dissolved minerals that occur in the formation . </P>

Where does oil and natural gas come from