Our Clarence-Moreton Basin and Middle-Earth

Over at the European Geophysical Union Blog Between a Rock and Hard Place there is a very interesting post. It seems that a researcher from the University of Bristol has released a paper comparing the climate of Earth with that of Middle-Earth (As in Lord of the Rings and the Hobbit). Yes, you read that right!

The climate model the researcher (Dr Lunt) created seems to demonstrate that the climate of Middle-Earth is best represented by the climate that we experienced in the Triassic period. That means that the Middle-Earth's climate was similar to ours while the rocks of the early Clarence-Moreton (and Ipswich) basin were being laid down. That is the time that the rocks of Evans Head, Nymboida, Chillingham and others were being formed.

For those that are interested in Tolkien's world here is a link to the paper (in English). For those obsessed with Tolkien's world here are links to the paper in Elvish and Dwarven Runic. I love the conclusions including the observation that "Mordor would have had an inhospitable climate, even ignoring the effects of Sauron"

Gas from shale

So far I have very quickly covered a couple of natural gas ‘types’. These types are ‘tight gas’ and in-situ ‘syngas’. Before I examine coal seam gas I need to discuss another major gas source called shale gas. Shale gas is not very common in Australia at the moment. It is mainly exploited in the Moomba gas fields in South Australia and Queensland (The Cooper Basin). This gas field is where ‘conventional’ gas has all but run out and the companies operating there are exploring new ways to keep gas supply going. Shale gas is a very well-known source of gas because of the film Gasland which discusses many shale gas basins in the United States. Economical shale gas resources are not currently known in the Northern Rivers. The conditions of formation are not suitable in the Northern Rivers when compared to other gas sources. So, geologically, what is it?

Once again, comparison between ‘conventional’ gas and ‘unconventional’ shale gas is probably the best way to demonstrate how and why shale gas forms and the methods needed for extraction. The first point to make is that chemically there is very little (if any) difference between ‘conventional’ and shale gas. The gas is composed mainly of methane with very small amounts of carbon dioxide and sometimes heavier compounds such as ethane. The second point is ‘conventional’ gas has migrated to a reservoir (a porous and permeable rock that is capped by impermeable rocks). Shale gas, however, remains where it is formed. It is trapped within the natural micro-pores and fractures, or adsorbed to organic matter and clay's in the shale. The gas is formed by the heating and compression of organic rich sediments as they are buried in a sedimentary basin.

Shale gas behaves very similar in ways to shale oil both in terms of formation (except temperature differences) and the way it is trapped in the rock. Interestingly, shale oil was produced in large quantities in the eastern states of Australia during the oil shortages of the Second World War. But, I digress.

Shale gas is trapped tightly on a very small scale it does not flow under natural conditions which is where it differs most from ‘conventional’ gas. Shale gas in Australia is typically very deep gas, rarely found less than a kilometre underground and usually found about twice that depth. Because of the natural conditions, stimulation to get the gas flowing is required. Stimulation in the case of shale gas is almost entirely hydraulic fracturing. In fact, numerous fraccs are required for shale gas exploitation, generally even more so than ‘tight gas’.

Another aspect used all the time is directional drilling. Directional drilling is used for other gas sources such as tight, syngas sometimes ‘conventional’, and rarely for coal seam gas (CSG). Directional drilling allows an increase in the well surface area and therefore increase the rate of gas extraction. The drilling of a directional well follows a ‘conventional’ vertical borehole. Then at the required depth the well is deflected to follow the target formation in a horizontal manner. The interesting thing about shale gas is that directional drilling and hydraulic fracturing make this gas viable. Without both of these technologies this gas type would be an obscure novelty.

Due to the depth of shale gas sources and the surrounding geology the techniques required to extract the gas are tricky. These challenges include the high pressure fraccing required combined with the relative difference in the rock strength of adjacent formations to the shale. This means there is an increased risk to fraccing causing fractures to extend into adjacent non-target rock and therefore connecting other porous or permeable systems with the gas formation. Repeated fraccing may increase the risk further. However, the depth of shale gas in Australia is such that if damage does occur it may mitigate some of the affect to adjacent formations.

A good summary of shale gas in Australia can be found on the CSIRO website here.

The next post I will do relating to hydrocarbon geology will be on coal seam gas (CSG). CSG is one of the most likely sources for gas in the Northern Rivers.

Previous posts on the topic of petroleum geology (includes gas resources)

Deeply a Fire Smoulders

In a previous post I discussed how tight gas is defined and how it behaves. I promised in that post I would expand on the theme of hydrocarbon geology and discuss other sources or techniques for obtaining natural gas. These include the aforementioned tight gas, but I will eventually cover all aspects including coal seam gas, basin gas and lastly “conventional” gas.

In this post I’ll quickly cover underground coal gasification (UCG) producing “syngas”. This is to make clear that it is different from coal seam gas. Presently, syngas is not produced in Australia and recent trials in Queensland and South Australia have ceased and moved off-shore to China. The companies cited a more conducive research and regulatory environment there than Australia. I only cover UCG because the geology of the southern Clarence-Moreton, Ipswich and maybe Lorne basins may be seen as sources for syngas in the distant future.

Syngas is produced through the process of underground coal gasification. This is a relatively new and novel way to turn coal into gas, though the concepts are in many ways similar to the older concepts of shale oil extraction and town gas production. these techniques having been used for more than a hundred years. Like most aspects of science, something new builds upon something old.

The first step in UCG is to find coal rich strata confined by a high pressure of natural water in the coal seam. A vertical drill hole is installed in one end of the coal seam and is terminated at the bottom of the target coal seam. A second drill hole is drilled at the other end of the gas field, possibly 2 or 3 kilometres away. This second hole is however, directionally drilled and follows the bottom of the target coal seam all the way until it intersects the first vertical drill hole. A well head is then set up at the first vertical drill hole and gasification infrastructure set up at the horizontal (directional drill hole). It is from the directionally drilled hole that all the interesting action takes place. The vertical one is just used for pumping the gas to the surface.

Gasification infrastructure is comprised of pumps for forcing air and guiding an ignition source into the ground. The the actual process of UCG occurs in-situ, that is, in the coal seam itself. The coal is first ignited underground at the point where the horizontal and vertical drill holes intersect. Air is pumped into the coal seam to displace some of the water which allows the process to continue. If air is not injected the water occurring in the rock extinguishes the gasification process. The coal is continually kept ‘burning’ underground and slowly moves along the directional drill hole as air and ignition is applied.

This process is essentially incomplete combustion. A process that was used to produce town gas in most major towns and cities in Australia up until the 1970’s. The incomplete combustion leads to production of CO and CH4. Adding too much air into the process simply produces more CO2 and so a balance of water pressure, air pressure and gas production is needed.

UCG differs from coal seam gas (CSG) in that water is only partially displaced from the coal seam. CSG requires as much water as possible to be removed to stimulate the natural flow of gas. Groundwater in CSG can be considered a waste product of the extraction process, a bit like overburden in a coal mine. UCG leaves the “overburden” water essentially intact.

UCG is an interesting, challenging and clever way to turn coal into a gas resource. It has been marketed as an alternative to digging a huge hole in the ground to extract the coal in a mine. The groundwater issues are regarded as less invasive than direct mining but there is added potential for incomplete burning residues to contaminate the groundwater. For example incomplete combustion can produce chemicals such as polycyclic aromatic hydrocarbons (PAH). Although generally poorly soluble, the presence of these chemicals is perceived as a concern by many people. Whether or not there is an avenue for these chemicals to become a risk to the environment is hotly debated. It is therefore now surrounded by a lot of controversy. Like underground coal mining there is also the possibility of ground subsidence. But regardless, it appears that in the short run this process will not be used in our region.

A non textbook example

Text books are wonderful. They always have excellent ‘text-book’ examples! These show how a scenario can be interpreted and what information is used in that interpretation. As you get to know the textbook you get a feel for most or all of the information you can obtain to give you an answer. However, in geology many of the techniques are rarely all applicable to every field situation; or if they are they are applicable, they are unreasonably difficult to use.I have recently experienced one such example in an area south-west of Byron Bay. There is very little information available to interpret and therefore the possibility of misinterpretation can be high.

Byron Shire Council recently did some road works along a section of road between the village of Newrybar and the coast. This work refreshed some small road cuttings (road cuttings are geological tourist attractions). I took a close look at one of the road cuttings on the very edge of the Alstonville Plateau. The rock in this cutting was clearly different from the overlying and dominant Cenozoic aged basaltic lavas that make up the plateau. The exposure was made up of conglomerate.

Conglomerate is a sedimentary rock most often associated with high energy river environments. In this case the conglomerate contained clasts made from other older rocks that occur elsewhere in the region. This included chert, quartzite and fine to medium grained sedimentary rocks such as sandstones and siltstones. The rock though had been quite weathered and the sedimentary clasts had become quite broken down even though they retained their shape insitu.

Conglomerate near Newrybar on the road to Broken Head and Byron Bay
note the different clast types and sizes - typical of the Laytons Range Conglomerate

There is nothing particularly special about this conglomerate. Here the mapping indicates that I was at the very edge of the Clarence-Moreton basin and therefore the oldest rocks of the basin would be likely to outcrop. Indeed, the oldest rock in the basin is known as the Laytons Range Conglomerate. This outcrop looks very much like it. But… further to the east (for example on Broken Head road) are rocks of the Ripley Road Sandstone. These are younger rocks of the Clarence-Moreton basin than the Laytons Range Conglomerate. The Ripley Road Sandstone contains small layers of pebble conglomerate but nothing compared to that exposed in the road cutting. Weirdly this means that the current mapping of the basin indicates that the Ripley Road Sandstone should be older than the road cutting rocks. This is the opposite of the known sequence of the area. To make the road cutting conglomerate fit there is several hypotheses:

1. The conglomerate in the cutting is actually not part of the Clarence-moreton basin but was deposited more recently and then covered by basalt. Maybe it was a pre-volcanic river system,
2. The conglomerate in the cutting is actually part of a younger Clarence-Moreton basin unit that has needs to be redefined to include this particular type of conglomerate.
3. The depositional structure of the Clarence-Moreton Basin is different in this area to the current model e.g. the road cutting is on the western side of a small sub basin.
4. Faulting or folding has up-thrown the conglomerate in this area giving the impression that it is stratigraphically higher
5. Other reasons I cannot think of at the moment

The only trouble is there seems to be inadequate information and field exposure to narrow down the possibilities. I’d love to get a drill rig and core a 200m interval but who has a spare hundred thousand dollars to do that?!

For the time being all I can do is assume the conglomerate was deposited sometime during the formation of the Clarence-Moreton Basin maybe as long as 250million years ago or deposited sometime before the Cenozoic basalts of the Alstonville Plateau possibly 40million years ago.

Alas, there is not enough information available to interpret this situation. But this is normal! We rarely are lucky enough to get a text-book example. In science the examples we are most confronted with are incomplete and generally frustrating. We can’t lie to ourselves that we can answer every question and know everything.

To the lady that stopped, looked at me curiously, and then asked me if I was “alright?” when I was examining the road cutting: Yes, I’m alright. But I still want to know the answer.

Hills of old sea floor muck

There has obviously been a bit of a lull in my blogging of late. I’ve been busy with family medical trips to Queensland and I’ve had less free time too. But some interesting things have happened with one formal presentation on coal seam gas and water and another presentation to be given in a couple of weeks. But on the aspects that interest me most (non-CSG geology), I’ve also been contacted by academics from a couple of different universities. It is nice to know that they feel I can help them with some research projects. I'll post more about that at a future date.

Best of all lookout - Springbrook National Park
Except for the hills on the horizon the rock in this photo is mainly
of the Neranleigh-Fernvale beds.

During the trip to Queensland I met up with family on the Gold Coast. We decided to have a day up in the popular Springbrook National Park area. In particular the views in this country are astonishing. The Best Of All Lookout certainly lives up to its name with incredible views of the valleys of the Tweed region. Mount Warning looks stunning and the rugged terrain of the volcanic shield remnants beautiful. And this was on a hazy day!

To get to Springbrook national park from the Gold Coast it is necessary to traverse the oldest rocks in the Tweed region. These are sediments of the Neranleigh-Fernvale beds. These are represented by the initially steep hilly terrain as you head westward up the range. Hinze Dam, for example, is located on this rock type. Time has weathered and eroded much of this rock away but still it remains as a significant landscape feature. These rocks and hills would probably be better known if the lavas associated with the Tweed Volcano had not erupted.

The Neranleigh-Fernvale beds are interesting rocks because of their mode of formation. They are essentially muds and debris flows that have been deposited in a trench during a period known as the Paleozoic. The trench was caused by the subduction of a continental plate under the then eastern Australian landmass. These sediments were then scrapped off and buckled into a large mountain range that has since been mostly eroded away. All of this occurred while Australia was part of the super-continent Pangaea which existed well before Gondwana.

Today, in the Northern Rivers the Neranleigh-Fernvale beds form the steep eroded terrain in the Tweed Valley (with the exception of some lavas and intrusions associated with the Tweed Volcano). They outcrop in a band at the very edge of the Alstonville Plateau to Byron Bay. They only occur as a band in the Ballina area because they are obscured by Jurassic sediments and the Cenozoic volcanic rocks. Like the Springbrook area, driving from Ballina to Alstonville or from Cabarita to Chillingham means traversing this formation. As soon as you get off the coastal plain and head up the hills you are passing the rocks of the Neranleigh-Fernvale beds. These beds are then obscured by the more recent sediments or volcanic rocks associated with the Tweed Volcano.

As for the Springbrook area, if you’d like to know more I recommend a book by Warwick Wilmott called Rocks and Landscapes of the Gold Coast Hinterland. The processes and timing of events in the Gold Coast area are very very similar to those processes that occurred in the Tweed valley area and so might be worth a read even if you don’t cross the border!

Warwicks book can be obtained from the Queensland Division of the Geological Society of Australia here.

Being tight with loose terminology?

There has been a lot of discussion recently about a local company resuming exploration for gas in our region. In particular the announcement by the company that they intend to drill a deep borehole next to the Lismore-Kyogle Road at Bentley has raised a great deal of heated debate. For example, this story in the Northern Star shows just how intense the feelings (one way or another) can be. One thing has been clear though is people are sometimes having trouble figuring out what gas companies are doing. The news release from Metgasco and their Review of Environmental Factors report state the proposed drill hole will be for "conventional gas". Critics of gas companies say since hydraulic fracturing (fraccing, fracking etc) may be carried out in the proposed drill hole the gas must be "unconventional" tight gas. Some people (including the local members of parliament) seem to think any drill hole in the area must involve coal seam gas. It is all a little confusing.

The first thing to note is gas should not be described as either "conventional" or "unconventional". There is essentially no difference in the gas (mainly comprised of methane). The difference is in how it is extracted.

The second thing to note is "tight gas" is only termed such by an arbitrary permeability value assigned by oil and gas engineers. In the real world there is a spectrum between traditionally sourced gas and tight gas. The tighter gas is gas occurring in a reservoir but does not flow as rapidly as in other locations. Tight gas is restricted from flowing by the fill in the cross connecting voids by a material formed after the gas migrated there (usually a natural cement such as calcite or quartz). The lack of cross connection between gas filled pore spaces is what reduces the permeability of the rock.

To clarify I use filter analogies. A new filter will let a substance flow through it easily but an old one is more clogged up and doesn't let the substance flow as rapidly. In the oil and gas industry an arbitrary permeability value is used as an indication of when it is called tight gas. It usually has a permeability of less than 0.1mD (millidarcy). It is also important to be clear that permeability is not the same as porosity because even tight gas reservoirs still have high porosity.

What is a millidarcy? I should do a blog post specifically on Darcy's Law but in the mean time it is good to visualise 1 millidarcy as the permeability of water in a fine sand filter. The way a millidarcy is calculated is through measurement of the velocity of fluid flow through the filter, viscosity of the fluid, cross sectional area of the filter and the pressure. In the case we are talking about the fluid is gas. The main difference being gas has a much lower viscosity and therefore can pass through a finer filter even easier. For 1 millidarcy our analogy for gas might be a fine paper filter instead of fine sand filter for water. It is interesting to note that the measurements needed to calculate the permeability of a gas reservoir are identical for hydrogeologists trying to understand groundwater flow or brewers filtering a favorite beer.

When the permeability of a gas reservoir decreases to 0.1mD the gas flows at a lower and lower rate. This means that it becomes less economical to let the gas migrate out of the geological formation on its own. Instead companies often look to reservoir stimulation which in its simplest form is introducing acid to dissolve minerals. Such a common mineral is calcite that may have clogged the formation, opening up the blockages between the pores. This is a very useful technique in natural calcium cement rich formations. Acidification has been used for hundreds (perhaps thousands) of years to increase the flow rate of groundwater sources for drinking, irrigation and other purposes. It is still commonly used in Australia today for groundwater purposes too. However, the most well known form of reservoir stimulation is the increasingly used hydraulic fracturing (fraccing/fracking). Fraccing involves the introduction of a fluid such as water (plus other ingredients) under pressure to propagate fractures through the formation. These fractures allow gases to escape much more easily. I don't want to go into details about fraccing here but I will suffice to say that the method is controversial.

As far as the terminology goes, tight gas is a very loose term. Tight gas is not an "unconventional" gas, it is a bog standard gas that sometimes require "unconventional" techniques to extract it. It is also important to note that reservoir stimulation is an "unconventional" method of extracting gas, but this does not in itself say much because the "conventional" method of extracting gas is just sticking a big hole in the ground.

I hope this blog post makes sense. While I was writing, it became obvious that several different posts are needed to explain the different areas of gas reservoirs. In the mean time I hope that this short post makes sense. I'll see what I can do over the coming months to further delve into the hidden world of petroleum geology (while steering as far away from controversy as possible).

The Woodburn sands of time

I’ve been spending some time working on a project in the lower reaches of the Richmond River Valley. This project got thinking about the stratigraphy and depositional history of that area. Particularly about a unit of unconsolidated sand called the Woodburn Sands (Drury 1982). In some ways this post follows on from a couple of posts that touched on the subject of sea level changes during the Quaternary.

To begin to understand this unconsolidated sediments of the Richmond River Valley we turn to the most recent mapping of the area. Troedson et al. (2004) comprehensively mapped the coastal Quaternary sediments of the whole east coast of NSW. Troedson et al. (2004) demonstrated that over large areas of the lower Richmond Valley there are two units of coastal sand which formed in barrier environments. The most obvious coastal sands are active dune and beach systems formed from a barrier by the action of present day long-shore drift. These active barrier systems occur in many places along the coast. Troedson et al. (2004) also mapped extensive areas of what an earlier researcher Thom (1965) first identified as an inner coastal barrier. This inner barrier is comprised of an old beach system that is no longer active.

Drury (1982) undertook a comprehensive study of the Quaternary sediments of the Richmond River Valley. He confirmed the view by Thom (1965) that there was an old inner barrier system. This system was formed during a higher period of sea level than today and caused regional changes to coastal sedimentation (e.g. I previously posted on the estuarine sediments of the Lismore area). The high sea level eroded away the pre-existing beaches and formed new beach systems a significant distance inland (sometimes 15km or more). Then as the sea level retreated, the new beaches were no longer subject to erosion from the sea and were left intact. The beach systems continued to form on the sea-ward side of the old beaches and eventually built up a very large area of sand. These old beach systems are what made the Woodburn Sands.

The Woodburn Sands occur in a discontinuous zone from Broken Head National Park to the Evans River and the lowest reaches of the Richmond River (Swan Bay). The maximum thickness intersected is about 35 metres, so the sand layers can be very thick.

Like many places in eastern Australia, the action of coastal wave and wind processes can lead to concentrations of heavy mineral sand.  These deposits are called mineral placers. The Woodburn Sands is another of these areas where placers are common. Indeed, a lot of sand mining took place on the north coast to exploit the high concentrations of zircon, ilmenite and even gold. Presently, the Woodburn Sands is not mined for minerals but is used as an important source of good quality groundwater, this includes the regional town water supply authority.

Drury (1982) also included an unusual feature within the Woodburn Sands. This feature was named the Broadwater Sandrock by Mcgarity (1956). McGarity (1956) demonstrated that the Broadwater Sandrock was formed by the cementation of sand by organic rich material probably formed by changes occurring in a peat swamp environment. This sandrock is a common feature up and down the east coast of Australia. Another common feature is the diversity of names given to this material which include ‘indurated sand’, ‘coffee rock’, ‘coastal sandrock’, ‘painted rock’, ‘beach rock’, ‘humate’ and ‘B-horizon of the humus podzol’ (Drury (1982), Mcgarity (1956), Thom (1965) and Den Exter (1974)). Take your pick! I follow the terminology proposed by Drury (1982) who included the Broadwater Sandrock as a member of the Woodburn Sands, i.e. the Broadwater Sandrock member.

Postscript:
Since doing the above post an anonymous commenter has rightly corrected and provided further information. You can see the full comment below, the comment much more accurately describes 'coffee rock' formation but I reproduce this section specifically:

...humicrete (coffee rock) forms as the B-horizon of a fossil soil on sand (a podsol). It is NOT a sedimentary layer itself ie NOT a stratigraphic unit, so should not have been referred to as a "member" ...

As such, I have now changed my mind! The Broadwater Sandrock member is not the best name after all. It seems that 'B-horizon of the humus podzol' is indeed one of the best ones. Humicrete is another good one. Well, it seems that the diversity of names will probably continue, but we can remove the one I thought the simplest (Broadwater Sandrock member) from the list!

References/bibliography:

*Den Exter, P.M. 1974. The Coastal Morphology and & Late Quaternary Evolution of the Camden Haven District, NSW. Australia. PhD thesis. University of New England, Armidale.
*Drury, L.W. 1982. Hydrogeology and Quaternary Stratigraphy of the Richmond River Valley, NSW. PhD thesis. University of New South Wales, Kensington.
*McGarity, J.W. 1956. Coastal sandrock formation at Evans head, NSW. Proceedings of the Linnean Society of New South Wales. V81 p52-58.
*Thom, B.G. (1965). Late Quaternary morphology of the Port Stephens-Myall Lakes area, NSW. Journal of the Royal Society of New South Wales V98 p23-36.
*Troedson, A., Hashimoto, T.R., Jaworska, J., Malloch, K., Cain, L., 2004. New South Wales Coastal
Quaternary Geology. In NSW Coastal Quaternary Geology Data Package, Troedson, A., Hashimoto, T.R. (eds), New South Wales Department of Primary Industries, Mineral Resources, Geological Survey of New South Wales, Maitland.