Introduction:

Natural Gas is a very important component of the world's supply of energy. This is one of the safest, cleanest and most helpful of all energy sources. In spite of its significance, though, there are numerous misconceptions regarding natural gas.

The Formation of Natural Gas:

Natural gas is a fossil fuel such as oil and coal. Fossil fuels are, fundamentally, the remains of animals and plants and microorganisms which lived millions of years ago. Thus, how do such once living organisms become a lifeless mixture of gases?

There are lots of various theories as to the origins of fossil fuels. The most broadly accepted theory states that fossil fuels are made when organic matter (like the remains of a plant or animal) is compressed under the earth, at extremely high pressure for a very long period of time. This is termed to as thermogenic methane. Like the formation of oil, thermogenic methane is made from organic particles which are covered in mud and other sediment. Over time, more mud and sediment and other debris are mounded on top of the organic matter. This sediment and debris place a great deal of pressure on the organic matter that compresses it.  This compression, joined by high temperatures found deep below the earth, break down the carbon bonds in the organic matter. The deeper beneath the earth's crust, the higher the temperature. At low temperatures (that is, shallower deposits), more oil is generated relative to the natural gas. At higher temperatures though, more natural gas is formed, as opposed to oil. That is why natural gas is generally related by oil in deposits that are 1 to 2 miles beneath the earth's crust. Deeper deposits, generally have primarily natural gas, and in most of the cases, pure methane. 

Natural gas can as well be made via the transformation of organic matter via microorganisms.  This kind of methane is termed to as the biogenic methane.  Methanogens, methane generating microorganisms, chemically break down the organic matter to form methane. These microorganisms are generally found in areas close to the surface of the earth which are void of oxygen. Such microorganisms as well live in the intestines of most animals, comprising humans. Formation of methane in this way generally occurs close to the surface of the earth, and the methane generated is generally lost to the atmosphere. In some circumstances, though, this methane can be trapped underground, recoverable as the natural gas. An illustration of biogenic methane is landfill gas. Waste having landfills generate a relatively large quantity of natural gas, from the decomposition of the waste materials which they have. New technologies let this gas to be harvested and added to the supply of natural gas.

The other manner in which methane (and natural gas) is assumed to be made is via abiogenic methods. Extremely deep beneath the earth's crust, there exist hydrogen-rich gases and carbon molecules. As such gases gradually increase towards the surface of earth; they might interact with minerals which as well exist underground, in the absence of oxygen. This   interaction  might  outcome  in  a  reaction,  making  elements and compounds which are found in the  atmosphere  (comprising  nitrogen, oxygen, carbon-dioxide, argon and water). If such gases are under extremely high pressure as they move towards the surface of the earth, they are probable to make methane deposits, identical to thermogenic methane.

Natural Gas under the Earth:

However there are some ways that methane, and therefore natural gas, might be formed, it is generally found underneath the surface of the earth. As natural gas consists of a low density, once formed it will increase towards the surface of the earth via loose, shale kind rock and other material.

Most of this methane will simply mount to the surface and dissipate to the atmosphere. Though, a great deal of this methane will rise up to geological formations which 'trap' the gas beneath the ground. These formations are formed of layers of porous, sedimentary rock (identical to a sponge, which absorbs the gas), having a denser, impermeable layer of rock on top. This impermeable rock traps the natural gas beneath the ground. If such formations are large adequate, they can trap a huge deal of natural gas underground, in what is termed as a reservoir. There are a number of various kinds of these formations; however the most general is formed if the impermeable sedimentary rock makes a 'dome' shape, such as an umbrella which catches all of the natural gas which is floating to the surface. There are a number of manners that this kind of 'dome' might be formed. For illustration, faults are a general location for oil and natural gas deposits to exist. A fault takes place whenever the normal sedimentary layers sort of 'split' vertically, in such a way that impermeable rock shifts down to trap the natural gas in more  permeable limestone or sandstone layers. Necessarily, the geological formation layers of impermeable rock over more porous, oil and gas rich sediment, consists of the potential to make a reservoir. The figure below exhibits how natural gas and oil can be trapped beneath impermeable sedimentary rock, in what is termed as an anticlinal formation. To successfully bring such fossil fuels to the surface, a hole should be drilled via the impermeable rock to discharge the fossil fuels under pressure. Note that in reservoirs that have oil and gas, the gas, being the least dense, is found closest to the surface, having the oil beneath it, generally followed via a certain quantity of water.

Fig: Schematic diagram of a Petroleum trap

Having natural gas trapped beneath the earth in this fashion, it can be recovered via drilling a hole via the impermeable rock. Gas in such reservoirs is generally under pressure, allowing it to escape from the reservoir on its own. Moreover to being found in the traditional reservoir like the one illustrated above, natural gas might as well be found in other 'unconventional' formations.

Historically, conventional natural gas deposits have been the most practical, and simplest, deposits to mine. Though, as technology and geological knowledge advances, unconventional natural gas deposits are starting to make up an increasingly larger percent of the supply picture. Therefore, what exactly is unconventional gas? A precise answer to that question is hard to get.  What was unconventional yesterday, might via some technological advance, or ingenious new process, become conventional tomorrow. In the widest sense, unconventional natural gas is gas that is more hard and less economically sound, to extract, generally due to the technology to reach it has not been developed completely, or is too costly.

For illustration, prior to the year 1978, natural gas that had been discovered buried deep underground in the Anadarko basin was virtually untouched. It simply was not economical, or possible, to extract this natural gas. It was unconventional natural gas. Though, deregulation of the area (and specifically the passage of the Natural Gas Policy Act, that provided incentives towards searching and extracting unconventional natural gas), spurred investment to deep exploration and growth drilling, making much of the deep gas in the basin usually extractable.

Thus, what is in reality considered unconventional natural gas changes over time, and from deposit to deposit? The economics of extraction play a significant role in finding out whether  or  not a specific deposit might be unconventional, or simply too expensive to extract. Basically, though, there are six major classes of unconventional natural gas. These are deep gas, tight gas, gas-having shale, coal bed methane, geo-pressurized zones, and arctic and sub-sea hydrates.

Deep Natural Gas:

Deep natural gas is precisely what it sounds like; natural gas which exists in deposits very far underground, beyond 'conventional' drilling depths. This gas is generally 15,000 feet or deeper underground, quite a bit deeper than the conventional gas deposits that are traditionally merely a few thousand feet deep, at most.

Deep gas has, in recent years, become more conventional. Deep drilling, exploration and extraction methods have substantially enhanced, making drilling for deep gas economical. Though, deep gas is still more costly to produce as compare to conventional natural gas, and as such, economic conditions have to be such that it is lucrative for the industry to extract from such sources.

Tight Natural Gas:

The other form of unconventional natural gas is termed to as tight gas. This is gas which is stuck in a very tight formation underground, trapped in uncommonly impermeable, hard rock or in a sandstone or limestone formation which is unusually impermeable and non-porous (that is, tight sand). In a conventional natural gas deposit, once drilled, the gas can generally be extracted quite readily and without problems. A great deal more effort has to be put to the extracting gas from a tight formation. Some methods exist which let natural gas to be extracted, comprising fracturing and acidizing. Though, these methods are as well very costly. Similar to all unconventional natural gas, the economic incentive should be there to incite companies to extract this expensive gas rather than more simply obtainable, conventional natural gas.

Shale Gas:

Natural gas can as well exist in the shale deposits. Devonian shales are made up from the mud of shallow seas which existed around 350 million years ago (throughout the Devonian period of the Paleozoic era). Shale is an extremely fine-grained sedimentary rock that is easily breakable to thin, parallel layers. It is an extremely soft rock, however doesn't disintegrate when it becomes wet. These shales can have natural gas, generally whenever too thick, black shale deposits 'sandwich' a thinner area of shale. Due to some of the properties  of  such  shales,  the  extraction of  natural  gas  from  shale formations is very difficult (and therefore expensive!) than extraction of the conventional natural gas. Most of the natural gas having Devonian shale in the U.S. is placed around the Appalachian Basin. However estimates of the amount of natural gas contained in such shales are high, it is expected that only around 10% of the gas is recoverable. Though, its potential as a natural gas supply is still extremely promising, given a sufficient technological and economic atmosphere.

Coal Bed Methane:

Coal, the other fossil fuel, is made underground under identical geologic conditions as natural gas and oil. Such coal deposits are generally found as seams that run underground and are mined via digging to the seam and eliminating the coal. Most of the coal seams as well have natural gas, either in the seam itself or the surrounding rock. This coal bed methane is trapped underground, and is usually not discharged to the atmosphere till coal mining activities unleash it.

Historically, coal bed methane has been considered as a trouble in the coal mining industry. Once a mine is built, and coal is extracted, the methane contained in the seam generally leaks out to the coal mine itself. This causes a safety threat, as too high a concentration of methane in the well make dangerous situations for coal miners. In the past, the methane which accumulated in a coal mine was intentionally vented to the atmosphere. Today, though, coal bed methane has become a popular unconventional form of natural gas. This methane can be extracted and injected to natural gas pipelines for resale, employed as an industrial feedstock, or employed for heating and electricity generation.

Geopressurised Zones:

Geopressurised zones are natural underground formations which are under uncommonly high pressure for their depth. These areas are made by layers of clay which are deposited and compacted very quickly on top of more porous, absorbent material like sand or silt. Water and natural gas which is present in this clay is squeezed out via the rapid compression of the clay, and enters the more porous sand or silt deposits. This natural gas, because of the compression of the clay, is deposited in this sand or silt under extremely high pressure (therefore the word 'geo-pressure'). Moreover to having such properties, geo-pressurized zones are generally positioned at great depths, generally 10,000-25,000 feet beneath the surface of the earth. The combinations of all of such factors make the extraction of natural gas in geo-pressurized zones quite complex. Though, of all of the unconventional sources of natural gas, geo-pressurized zones are estimated to hold the highest amount of gas. Most of the geopressurised natural gas in the U.S. is positioned in the Gulf Coast region. The quantity of natural gas in these geopressurised zones is uncertain.

Methane Hydrates:

Methane hydrates are the latest form of unconventional natural gas to be discovered and researched. Such interesting formations are formed of a lattice of frozen water that forms a sort of 'cage' around the molecules of methane. Such hydrates look similar to melting snow and were first introduced in permafrost regions of the Arctic. Research to methane hydrates has revealed that they might be much more abundant than first expected. Though, research into methane hydrates is still in its infancy. The type of effects the extraction of methane hydrates might have on the natural carbon cycle is not known.

Unconventional natural gas comprises a big proportion of the natural gas which is left to be extracted in the North America, and is playing an ever-increasing role in supplementing the nation's natural gas supply.

Offshore Gas Fields:

Similar to oil, natural gas is frequently found underwater in offshore gas fields like the North Sea, Corrib Gas Field off Ireland and the Scotian Shelf next to Sable Island. The technology used to extract and transport offshore natural gas is dissimilar from land-based fields in that a few, very large rigs are generally utilized, because of the cost and logistical difficulties in working over water. Rising gas prices have encouraged drillers to revisit fields which, till now, were not considered inexpensively viable.

1) Stranded Gas Reserve:

A stranded gas reserve is the natural gas field which has been discovered, however remains unusable for either physical or financial reasons. Gas which is found in oil wells is conventionally considered as associated gas and consists of historically been flared. This is at times re-circulated back to oil wells in order to maintain extraction pressure or transformed to electricity by using gas powered engines.

2) Economically Stranded Gas:

The reserve of gas can be economically stranded for one or two reasons: 

a) The reserve might be too remote from the market for natural gas, therefore making the construction of pipelines prohibitively costly.

b) The reserve might be in a region where demand for the gas is saturated, and the cost of exporting gas beyond this region is too great. These are most probable to be tapped in the future whenever existing sources start to run out.

3) Physically Stranded Gas:

A gas field which is too deep to drill for, or is beneath an obstruction, might be considered   physically stranded in spite of access being desirable. Continual evolution of drilling technology has progressively unlocked access to numerous hard fields.

Illustrations of Stranded Gas:

Alaska has huge reserves of natural gas stranded in its Prudhoe Bay oil field. The largest gas plant in the United States exists there for the sole principle of re-injecting the associated gas back to the oil fields. Marketing of the gas awaits the completion of the Alaska gas pipeline to carry it to the lower 48 states. Building of the pipeline has been delayed via the availability of low-cost natural gas in Canada and growth of non-conventional gas fields in the lower 48 states, and also political considerations. 

Canada has huge amounts of stranded gas in its Arctic Islands, Beaufort Sea and Mackenzie Delta. Marketing of this gas would need completion of the Mackenzie Valley Pipeline to bring it south all along the Mackenzie River. Some of the companies would like to join it by Alaska gas via building a pipeline offshore in the Arctic Ocean from Alaska to the Mackenzie Delta. The Government of Alaska is resisting this theory as it would prefer to bring the gas first to southern Alaska, and then transport it across the Yukon all along the Alaska Highway. 

Russia, which consists of the world's biggest natural gas reserves, has much of it stranded in Siberia. In certain cases, the simplest way to bring it to market would be to pipeline it across the Bering Strait, and then feed it to the proposed Alaska gas pipeline. The other options comprise moving it south to China, or west to Europe. The other option would be to build liquefied natural gas (or LNG) terminals at Siberian ports, in which case it could be shipped to any port in the world having an LNG re-gasification terminal.

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