Southern African Natural Gas Advisory Service Service

Where does it come from and types of natural gas?

There are different sources and types of natural gas and these are described in this section. The vast majority of commercially produced and supplied natural gas comes from deep under the earth's surface.

This type of gas is either associated with oil production or non-associated when produced from dedicated natural gas deposits. Then there are a range of other sources such as coal bed methane and shale gas. In addition there is Town Gas, Bio gas and Synthetic Natural Gas produced from coal. There is also a distinction between what is referred to as ' conventional gas ' and ' unconventional gas ' and these terms relate primarily to the types of subterranean reservoirs where these two types of gas are found.

In the case of conventional gas this is usually found in permeable sandstone underground reservoirs whereas unconventional gas refers to reservoirs that have only become commercially viable in recent years through the availability of new technology and extractive techniques. So the term ' unconventional gas ' is used to refer to shale gas found in shale deposits, tight gas situated in relatively impermeable underground formations; and, coal bed methane located in coal deposits.  A useful distinction is between subterranean gas deposits laid down millions of years ago (the single major source of natural gas used today in the world) and man-made natural gas such as Town Gas and Synthetic Natural Gas; and, Bio gas that results from the natural decay of organic matter (biomass).

Gas can also be found in carbonate reservoirs (fractured limestone) and through fractures and faults originating from deep sources, the origin of which is sometimes unknown. This gas is often referred to as Biogenic.

Subterranean Natural Gas

Natural gas is located in underground rock formations. It was created over time by either thermogenic or biogenic mechanisms. Deep in the earth thermogenic gas is created at great pressure and temperature over many years from buried organic material. Biogenic gas is created from methanogenic organisms in shallow

sediments in bogs and marsh areas.

Millions of years ago plant and animal remains decayed and were laid down in thick layers. This organic material was buried under sand and silt. Over time the sand and silt changed to rock and covered the organic material trapping and compressing it beneath the rock. Heat, pressure and time changed some of the compressed organic matter into coal, oil and natural gas.

The search for natural gas (as with oil and coal) is performed by geologists who study rock structures using a sophisticated array of modern exploration techniques. These include seismic surveys, three dimensional mapping of underground rock formations and the analysis of core samples from drilling into the earth. They explore and discover underground natural gas deposits  and are able to assess the quality of the gas deposits lying underground, how much  there is likely to be and how best to extract it in a cost effective and environmentally safe manner.   

Some natural gas fields are on the land but many others are out at sea (offshore) many miles from the coast.

Once a gas field is ' proven ' and it is decided to extract the natural gas, unobtrusive gathering well head sites are created with various wells drilled down into the underground gas reservoir.

In new gas fields it flows up through the well shafts to the surface. It is then transported by pipeline to a gas processing facility to be made into commercial natural gas ready to be supplied to users.  

A topical and controversial source of ' unconventional gas ' that has virtually revolutionized natural gas supply in the U.S. is shale gas.

This type of natural gas is located in shale beds deep under the earth's surface and it is liberated from the shale using a technique called hydraulic fracturing.

It is estimated that there are some 900 trillion cubic meters of ' unconventional ' gas such as shale gas of  which some 180 trillion cubic meters may be recoverable.

Town Gas

Town gas is a mixture of methane and other gases, often the highly toxic carbon monoxide. It is produced from coal and was usually a byproduct of coke ovens which heated bituminous coal in air-tight chambers.

The volatiles (or gases) contained in the coal are driven off and collected, stored and distributed via networks of pipes to users. Town Gas is rare today since it is usually uncompetitive with naturally occurring natural gas and is uneconomic. It is also a less benign fuel from an environmental and safety perspective than natural gas.

There used to be Town Gas manufacturing plants in Johannesburg, Cape Town and Port Elizabeth that used coal as a feedstock to produce the gas. These no longer exist although in Johannesburg the gas distribution network has been retained and modernized and is still in use supplying natural gas to local users that is supplied by Egoli the local natural gas distributor.

Egoli Gas is a wholesale customer buying natural gas from Sasol. See the section on Synthetic Natural Gas in this part of the web service.

Coal Bed Methane

For many years coal bed methane from coal seams was regarded as an uneconomic resource. One of huge potential, but low value due to poor recovery rates, high associated water production, safety and cost. However in recent years with a host of new technologies this vast resource is being increasingly unlocked and more and more coal bed methane (CBM) is becoming economically recoverable.

Some of these new technologies include underground gasification, hydraulic fracturing, compressed air stimulation, better de-watering techniques and cavitation completions.  These proven methods are now being adopted increasingly in the coal producing regions of the world to improve mine safety, while also optimising the recovery of two valuable and vital natural resources - natural gas and coal.   

Synthetic Natural Gas

With more plentiful supplies of naturally occurring natural gas being produced every day, the need to manufacture synthetic natural gas from coal is becoming rare.

One exception has been in South Africa, devoid of access to natural gas until a few years ago, Sasol has been a manufacturer of synthetic natural gas.

Historically when Town Gas was no longer manufactured in Gauteng (previously Johannesburg) this was replaced by synthetic natural gas that Sasol produced from coal. This methane enriched manufactured gas was supplied via the upgraded gas distribution network in and around Gauteng to residential, commercial and industrial customers. These customers are now supplied with natural gas that is imported to South Africa from Mozambique.

With the advent of recent large discoveries of natural gas offshore in Mozambique it is likely that synthetic natural gas will go into decline in South Africa. The one exception is where such gas is produced as a byproduct of heavy industry such as steel making and petrochemicals where various types of ' producer gas ' are available and deployed within the premises of these industrial complexes for heating, processing and electricity generation purposes.

Bio gas

Bio gas is produced by the anaerobic decay of organic matter, biomass.  Such gas is often referred to as natural bio gas that comes from landfill sites, marshlands, swamps, sewerage sludge and manure.

Landfill gas is created through the decomposition of waste in landfills and once water vapour is removed approximately 50% of land fill gas is methane with most of the rest being carbon dioxide. Both carbon dioxide and sulphur scrubbers are sometimes used to improve the quality of the gas. Land fill gas can be burned to produce heat or electricity.

Bio gas is also often produced using the waste materials such as unusable parts of plants or manure. Devices called digesters' can turn today’s organic waste material into natural gas relatively quickly that avoids waiting millions of years for the gas to form naturally under the earth's surface. It is important to note however that the quantities of bio gas that are available from these various sources is extremely limited in comparison with the vast quantities available and being produced from conventional underground gas fields.

Methane Hydrates

Massive amounts of natural gas with high levels of methane exist on the Earth in hydrate form in permafrost in arctic regions, under sediment in offshore continental shelves; and in the great lakes. However no economically viable technology currently exists to extract this natural gas.   

Major natural gas fields of the world

The world's largest gas field is currently the Qatar offshore North Field in the Persian Gulf that is currently estimated to have some 25 trillion cubic meters of natural gas. The second largest field is the South Pars Field also in the Persian Gulf in Iranian waters with an estimated reserve of 14 trillion cubic meters.

The Qatar North Field is estimated to contain many years of natural gas supply so the reserves are enormous and ever more natural gas is being discovered throughout the world.

In 2011 the total proved reserves of natural gas in the world stood at 6608.9 trillion cubic feet (tcf) or 187.1 trillion cubic metres (tcm) as follows:

As you can see from the above table there is more than enough naturally occurring natural gas that can be extracted and supplied in an environmentally safe way to fuel the growing global need for clean and efficient energy for many years to come.