Walloon Coal Measures of the Southern Clarence-Morton Basin

In previous posts I’ve briefly discussed the upper most layers of the Clarence-Moreton Basin. The Grafton Formation which overlies the Kangaroo Creek Sandstone which in turn overlies the Woodenbong Beds/MacLean Sandstone Member. The MacLean Sandstone Member is a member of a larger unit called the Walloon Coal Measures and it is this unit that I will briefly comment on now.

I’ve often heard people mistakenly say that the Walloon Coal Measures is a coal seam. This is not correct because the balance of the unit is actually made up of mixed rocks. According to Wells & O’Brien (1994) the coal measures include sandstones (made from volcanic rock fragments), carbonaceous siltstone, shale, mudstone, coal and clayey siltstones. Also clayey ironstone and infrequently oil shale and limestone can be found. Apparently tree stumps remaining in their growth position have also been found, though these have become carbonised (coal). The coal layers themselves are thin (millimetre scale) to occasionally thick (30-40cm) in the Southern Basin but the whole unit of all the different rock types that make up the Walloon Coal Measures totals at least 200 metres of thickness and is variable from location to location.

The coal in the measures is formed from peat that grew in a moist but temperate environment during the early to middle Jurassic in this area (smack in the middle of the age of the dinosaurs). The depositional environment appears to have been mainly flood-plain and meandering stream environments. Boggy mires forming the peat were common, but layers of volcanic ash from occasional volcanic eruptions from close by are preserved. This makes some of the coal seams high in ash content which reduces the quality of the coal. The environment was thought to be reflective of a period of high sea level.

The Walloon Coal Measures in Bexhill Brick Pit at Bexhill

Interestingly, the Walloon Coal Measures are some of the most extensive and continuous sedimentary rock formations in eastern Australia. They are correlated with almost identical units in the Surat Basin and the Maryborough Basin making the potential spatial extent of the unit huge. The outcrop of the Walloon Coal Measures is fairly limited with much obscured by the Grafton Formation, Kangaroo Creek Sandstone and Woodenbong Beds as well as Cenozoic aged volcanic rock especially associated with the Focal Peak and Tweed Volcanic areas. In our region the best exposures are in the Nimbin area and further north but also at Coaldale where the Clarence-Moreton Basin has been deformed creating a bulge which has been eroded exposing the Walloon Coal Measures. Areas to the south of MacLean show some outcrop and on the other side of the Basin, the Kangaroo Creek and areas near Tabulam show good exposures. Other places have exposures of the Walloon Coal Measures because of local faulting and folding that has occurred in places like the Richmond Range.

I understand that coal mining was historically carried out near Tabulam, Kangaroo Creek and Nimbin but the size of the deposits was such that these were only small and fairly short lived enterprises, though Murwillimbah did have a power station earlier last century which was fueled on local coal transported from the area around Tyalgum. Of course now the Walloon Coal Measures has been frequently under discussion regarding its gas potential especially in the form of coal seam gas (CSG) also known as coal bed methane.

The presence of gas in the coal measures is a natural function of coal and the formation of coal when it was formed. As the rock is gently ‘cooked’ following its deposition as peat gases are given off. Peat is made from decayed plant and animal matter which when broken down into its elemental constituents is mainly hydrogen (H) and carbon (C) atoms. The hydrogen is bonded to the carbon in oxygen poor environments and forms methane (CH4) and sometimes more slightly moe complex organic molecules such as C2H6, C3H10 etc, or if conditions are right the molecules are big enough and complex enough to form oils. In the case of the Southern Clarence-Moreton Basin Walloon Coal Measures the conditions were too hot for oil to be stable so the smaller gas molecules are formed. Gas may be trapped in the layers of coal within voids and cracks (called cleats) or they may sometimes migrate to other layers where they can be trapped. This is actually the difference between ‘conventional’ gas and coal seam gas, i.e. all conventional gas was once coal seam gas. Oil shale and shale gas are also present in some areas of the Walloon Coal Measures but these are very rare and are small deposits (I might do a post on these in the future but given their insignificance I might not get there). Russell 1994 noted that the best quality gas, mature or 'dry gas' was likely to be found abundantly in the eastern portion of the basin, whereas wetter gas and oils were likely to be more prevalent in the west. Interestnigly it is thought that the maturity is a response to the thermal changes in the Earths crust during the formation of the Tasman Sea.

The Walloon Coal Measures contains both conventional and coal seam gas and very little oil. Indeed, I understand that substantial amounts of conventional gas was first discovered in the Hogarth Ranges about 40 years ago and that more recently Metgasco have discovered significant amounts at Kingfisher which I think is to the south of Casino. As far as coal seam gas goes, if Walloon Coal Measures are present there is coal and so there is also a chance that gas may also be present.

References/bibliography:

*O’Brien, P.E., Korsch, R.J., Wells, A.T., Sexton, M.J. Wake-Dyster, K. (1994) Structure and Tectonics of the Clarence-Morton Basin. In Wells, A.T. and O'Brien, P.E. (eds.) Geology and Petroleum Potential of the Clarence-Moreton Basin, New South Wales and Queensland. Australian Geological Survey Organisation. Bulletin 241.
*O'Brien, P.E., Powell, T.G. & Wells, A.T. (1994). Petroleum Potential of the Clarence-Moreton Basin in Wells, A.T. and O'Brien, P.E. (eds.) Geology and Petroleum Potential of the Clarence-Moreton Basin, New South Wales and Queensland. Australian Geological Survey Organisation. Bulletin 241.
*Russell, N.J. 1994. A Palaeogeothermal study of the Southern Clarence Moreton Basin in Wells, A.T. and O'Brien, P.E. (eds.) Geology and Petroleum Potential of the Clarence-Moreton Basin, New South Wales and Queensland. Australian Geological Survey Organisation. Bulletin 241.
*Wells, A.T. and O'Brien, P.E. 1994. Lithostratigraphic framework of the Clarence-Moreton Basin. In Wells, A.T. and O'Brien, P.E. (eds.) Geology and Petroleum Potential of the Clarence-Moreton Basin, New South Wales and Queensland. Australian Geological Survey Organisation. Bulletin 241.