What is CSG?

What is CSG? Very simply Coal Seam Gas (CSG) is natural gas obtained directly from coal seams. Another common name for CSG is Coal Bed Methane (CBM). Like most natural gases, the chemical components of CSG are dominated by methane. Though some higher end hydrocarbons such as ethane or propane may also be present.  Carbon dioxide and nitrogen are usually significant components of CSG too and the higher the proportion of these non-hydrocarbon gases the lower the quality of the gas. This simple summary does not tell us very much, so more detail is required.

CSG is an interesting gas when compared to ‘conventional’ gases. ‘Conventional’ gas has migrated away from coal and organic rich sedimentary rocks into other porous rocks. The gas is then held in place by impermeable layers. What makes CSG different is that the gas has only migrated very small distances (if at all) to natural pore spaces such as fractures (cleats) in the coal layers. These pore spaces usually contain natural water that was left in the coal when it was laid down or water that subsequently migrated into the coal seam. The water associated with the coal seam is very important because it is actually the pressure of the water in the coal seam that keeps the gas in place. It is the hydrostatic pressure that keeps the gas in place.

In open cut or underground coal mining, CSG is a curse. It is considered a waste product and an explosion hazard. It is therefore vented as much as possible to make the coal mines safe to work in. The recent Pike River Mine explosion in New Zealand is an example where the failure to vent enough CSG caused a tragedy. As water is removed from coal mines the chances for gas mobilisation increases due to the above mentioned effect of hydrostatic pressure. This further increases the risk of explosion in coal mines.

Idealised relationship between CSG and water production

Natural gas became more popular for domestic and industrial use over the last few decades and the means to transport it economically became available (e.g. LNG). This meant gas that was often a by-product of the oil and coal industries became important in its own right. This led to people searching for gas sources in their own right, CSG included. Petroleum engineers realised if you reduce the pressure of water in a coal seam and collected the gas you could actually use the gas as a resource leaving the coal in place. This means that drill holes can be placed into coal seams and the water drawn out. The water drawn out (called formation water) is actually a ‘by-product’ of the gas extraction process. The formation water is the nuisance that needs to be removed to allow the gas to escape. This means that a new gas well will produce very little gas at first and lots of ‘waste’ water. As the water is drawn down the gas production increases and the water production decreases. Interestingly this is the opposite of that which occurs for ‘conventional’ gas, where waste water is a problem in the later stages of production but not early on.

Many people are concerned about CSG in Australia, particularly in our northern rivers region. This concern is driven by the possible effect of CSG extraction on beneficial groundwater. The use of techniques such as hydraulic fracturing that may be used to increase or prolong gas production is also raised as a concern. To keep this post short I will cover both of these issues in future. However, I will suffice to say that there is evidence that groundwater can be affected during CSG extraction despite producers trying not to have any impact. These are particularly noted in certain geological formations. There are also situations where there is no impact on important aquifers too. This matter is clearly quite complex and a one size fits all understanding does not apply very well. Hopefully, my future posts will tease the details out a little bit more.

The Road to The Gorge

Note that since this post was written the Towgon Grange Granodiorite has been renamed the Towgon Grange Tonalite.

Many people in the region know about “The Gorge”. It is a remote, yet popular area on the Clarence River. The road to The Gorge is interesting because of the change in geology that is experienced. The main route to The Gorge is via Grafton and Copmanhurst. By travelling west from Copmanhurst along the Clarence Way you move from the sedimentary rocks of the Clarence-Moreton Basin. First,  the rugged cliffs made from the Kangaroo Creek Sandstone give way to the rolling hills of the Walloon Coal Measures then Koukandowie Formation. Some road cuttings show weathered examples of these rocks. Turning off the Clarence Way and passing over the camping ground, swimming hole and bridge at Lilydale leads you to The Gorge turn-off. The Lilydale and Newbold areas have some of the oldest rocks of the Clarence-Moreton particularly the Laytons Range Conglomerate. But on the day I was there, I was not so interested in those rocks… because I was getting into the New England Orogen.

It is rare opportunity for me to explore the foot hills of the New England region. I love the feeling of the place, the wonderful landscape, climate, history and even culture. The place just seems to have a feeling of connection with the people who live there. Luckily, I managed to visit the edges of the New England escarpment for a little while on the weekend. While there I managed to experience more of the rocks that are the foundations of the landscape of New England.

Towgon Grange Tonalite - on The Grange Road, Middle Clarence River area

Driving along The Gorge road the rocks of the Silverwood Group are passed by. These are slightly enigmatic rocks of the New England Orogen, interpreted as subduction complex rocks (Van Noord 1999). Mainly outcropping in streams the rock of the Silverwood Group in this area are none the less quite hard and old metamorphosed marine sedimentary and volcanic rocks. The Silverwood Group is interesting because it also occurs near Texas in Southern Queensland and it is only partially understood in our region. But more about the Silverwood Group in a future post. 

Round tors appear by the road side near Table Creek about 15km south of The Gorge. These tors are a classical shape formed by the weathering and erosion of granite type rocks. Here are rocks that make up part of the New England Batholith. The batholith is numerous masses of intrusive igneous rocks plutons that were molten well before Australia was separate from Gondwana. The ‘granite’ here is called the Towgon Grange Granodiorite. Like the Dumbudgery Creek Granodiorite that occurs about 20-30km further north the pluton is bisected by the path of the Clarence River. This helps to illustrate the unusual behaviour of the Clarence river as it travels backward and forward over soft and hard rocks. In fact the other side of the pluton can be easily found on the other side of the river just off the Clarence Way.

The Towgon Grange Granodiorite intrudes into the Silverwood Group meta-sediments. The rock sample at Table Creek (pictured) is actually not a granodiorite. It is notionally similar in appearance but contains much less potassium-feldspar. The main minerals are light coloured plagioclase feldspar, quartz and darker clinopyroxene and amphibole. The rock sample shows that much of the clinopyroxene is mantled (surrounded) by amphibole. The lack of potassium-feldspar means that this particular sample is probably a Tonalite according to the most popular rock classification (QAPF). In fact Bryant et al (1997) actually notes that the Towgon Grange Granodiorite only contains small amounts of Granodiorite, with most being Tonalite or Quartz Diorite. This is a good example how stratigraphic names may be misleading to first time geologists!

Bryant et al (1997) classifies the Towgon Grange Granodiorite as an I-type granite of the Clarence River Supersuite. This means that the Towgon Grange Granodiorite is derived from the melting of other igneous rocks. The Towgon Grange Granodiorite is also comparatively low on silica (quartz) in comparison to other Clarence-River suite intrusions. It still contains enough quartz that it is generally visible in hand specimens. The age of the Towgon Grange Granodiorite is about 248-249Ma old. The younger sedimentary rocks of the Clarence-Moreton Basin overlie parts of the Towgon Grange Granodiorite and Silverwood Group.

The Towgon Grange Granodiorite is one of those rocks that just about no one in the general public has heard of. But, it is a good example of rocks that illustrate many points about the landscape evolution of the New England Orogen and the Clarence River. It occurs in a scenic area and is also a very attractive rock in its own right.

References/bibliography: 
*Bryan, C.J., Arculus, R.J. & Chappell, B.W. 1997. Clarence River Supersuite: 250Ma Cordilleran Tonalitic I-Type Intrusions in Eastern Australia. Journal of Petrology V.38 No. 8.

*van Noord, K.A.A. 1999. Basin development, geological evolution and tectonic setting of the Silverwood Group IN Flood, P. G. (ed.) Regional Geology Tectonics and Metallogenesis: New England Orogen - NEO '99 Conference University of New England.

List of Natural Gas Posts

To try and bring some order to some subjects that have been dealt with in previous posts I think it would be useful to create some list posts. This first list is about a subject that is very topical at the moment, natural gas. Natural gas includes so called "unconventional" gas such as coal seam gas (CSG), shale gas, tight gas.

Posts on "conventional" gas

• Being tight with loose terminology?

Posts on coal seam gas

• Baseline CSG methane in groundwater
• Clarence-Moreton Basin CSG Bioregional Assessment
• What is CSG?
• Some musings on coal seam methane
• Coal seam gas gets a seismic thump!
• Why you wont find CSG here

Posts on shale gas

• Gas from Shale

Posts on tight gas

• Being tight with loose terminology?

Posts on other unconventional gas

• Fracking qualifies for aged pension
• Deeply a fire smoulders