<P> The volume of water used to hydraulically fracture wells varies according to the hydraulic fracturing technique . In the United States, the average volume of water used per hydraulic fracture has been reported as nearly 7,375 gallons for vertical oil and gas wells prior to 1953, nearly 197,000 gallons for vertical oil and gas wells between 2000 - 2010, and nearly 3 million gallons for horizontal gas wells between 2000 - 2010 . </P> <P> Determining which fracking technique is appropriate for well productivity depends largely on the properties of the reservoir rock from which to extract oil or gas . If the rock is characterized by low - permeability--which refers to its ability to let substances, i.e. gas, pass through it, then the rock may be considered a source of tight gas . Fracking for shale gas, which is currently also known as a source of unconventional gas, involves drilling a borehole vertically until it reaches a lateral shale rock formation, at which point the drill turns to follow the rock for hundreds or thousands of feet horizontally . In contrast, conventional oil and gas sources are characterized by higher rock permeability, which naturally enables the flow of oil or gas into the wellbore with less intensive hydraulic fracturing techniques than the production of tight gas has required . The decades in development of drilling technology for conventional and unconventional oil and gas production has not only improved access to natural gas in low - permeability reservoir rocks, but also posed significant adverse impacts on environmental and public health . </P> <P> The US EPA has acknowledged that toxic, carcinogenic chemicals, i.e. benzene and ethylbenzene, have been used as gelling agents in water and chemical mixtures for high volume horizontal fracturing (HVHF). Following the hydraulic fracture in HVHF, the water, chemicals, and frack fluid that return to the well's surface, called flowback or produced water, may contain radioactive materials, heavy metals, natural salts, and hydrocarbons which exist naturally in shale rock formations . Fracking chemicals, radioactive materials, heavy metals, and salts that are removed from the HVHF well by well operators are so difficult to remove from the water they're mixed with, and would so heavily pollute the water cycle, that most of the flowback is either recycled into other fracking operations or injected into deep underground wells, eliminating the water that HVHF required from the hydrologic cycle . </P> <P> In order to assist in detecting leaks, an odorizer is added to the otherwise colorless and almost odorless gas used by consumers . The odor has been compared to the smell of rotten eggs, due to the added tert - Butylthiol (t - butyl mercaptan). Sometimes a related compound, thiophane, may be used in the mixture . Situations in which an odorant that is added to natural gas can be detected by analytical instrumentation, but cannot be properly detected by an observer with a normal sense of smell, have occurred in the natural gas industry . This is caused by odor masking, when one odorant overpowers the sensation of another . As of 2011, the industry is conducting research on the causes of odor masking . </P>

Where can oil and natural gas be found