Shale gas, accessed from scale rock using a process known as hydraulic fracturing, aka fracking, is just one of a number of non-conventional forms of natural gas such as deep natural gas, tight natural gas, and coal-bed methane.
It's not so much the gas that's the problem with shale, but the process of extracting it from the rock. Hydraulic fracturing is the process of forcing powerful jets of water into the rock to break or fracture it and release the gas. The issue is that this uses large amounts of chemical adulterated water that can re-enter the water table. There may only be 0.5% of chemical additives (friction reducer, agents countering rust, agents killing microorganism) added to the water which means that of the millions of liters of water used, hundreds of thousands of liters of the chemicals are put into the subsurface. The next issue is what happens to the water, approximately 50 to 70 percent of the contaminated wastewater is recovered and sent to storage tanks, however the other 30 to 50 percent remains in the subsurface where it can contaminate the water table. To avoid this problem, GasFrac of Canada invented a method of extracting the gas using liquefied propane gas. Other issues such as earthquakes have been discounted as being of minimal impact.
Deep natural gas, as the name suggest, is natural gas that is found much deeper than conventional gas. In deposits greater than 15,000 feet below conventional gas deposits it has been traditionally harder to extract. However, due to improved extraction techniques, it is becoming easier to extract and the costs of doing so are coming down.
Tight gas, is gas that is trapped in unusually impermeable, hard rock, or in a sandstone or limestone formation that is unusually impermeable and non-porous (tight sand). Due to the nature of the rocks, tight gas is often expensive to extract as it uses methods such as fracturing and acidizing, this in turn puts up the price for the consumer.
Coal-bed methane has in the past been responsible for explosions in coalmines and has therefore had to be vented and released into the atmosphere, but now the methane is being extracted and injected into natural gas pipelines for resale, sued as an industrial feedstock, or used for heating and electricity generation. The gas exists in the coal seams and is exposed when mining operations begin and since high concentrations of methane are a safety risk.
It's thanks to these discoveries and improvements in techniques that we still are able to get a regular gas supply and that the lifetime of fossil fuels is being extended beyond anything that was expected thirty years ago.