The US oil economist M.K. Hubbert issued a chilling warning in the late 1960s that foretold the eventual decline of the single most important physical resource of the global economy. His simple approach was to consider petroleum, and by implication a great many other commodities, as having a fixed abundance that was not added to naturally at a rate that could keep pace with its exploitation. Oil and natural gas are non-renewable, as far as human society is concerned; they are “wasting” resources built slowly and episodically over tens of million years. Hubbert matched the exponential rate at which petroleum is extracted with various notions of how much is in the ground and how the easiest to find and pump out inevitably will give way to more tenacious reserves. His model for the future of the petroleum economy centred on a theoretical bell-shaped curve relating production to time, and we are now entering his predicted period of increasing difficulties. Estimates of reserves have increased considerably in the last 35 years, and so has the efficiency of getting out the fluids. Recent news leaking from the Shell oil giant that there has been a certain fiddling of the books about how much remains in its licence areas (a 20% overestimate) is perhaps a sign of just how difficult it is to keep pace with growing demand. Oil companies hope for the best as regards how quickly new discoveries add to their assets, yet they can never voice their fears of the worst for the sake of investor confidence and the volatility of the oil futures market. The history of petroleum discovery is indeed a bell-shaped curve, and it has been on the slippery downward slope for about 30 years, with a few cheering but brief upswings. On average, annual discovery has decreased from about 50 to 10 billion barrels each year, noting that the size of the discoveries is always an estimate of what might eventually be extracted to be tempered by the fact that it rarely if ever is. A great deal of the petroleum products now being used emerge from massive discoveries in the late 30s and 40s and the mid 1960s. Nothing like the huge Arabian and Iraqi fields has been found since then. Many commentators, as usual, consider the present upsurge in oil prices to stem from political issues, but there are deeper economic and technical issues that suggest that it is an irreversible trend while ever demand is insatiable and supply more difficult to achieve. Standing above the generally quoted reserves that can reasonably be expected to flow using current methods, are several categories of petroleum in the ground that require new extraction methods and a higher price to implement them. They are considerably larger, though much more fuzzily defined, and range from the dregs that are not easily pumped, through viscous oils, tars sands to oil shales, the primary source rocks for conventional petroleum fields when geological processes free their organic content to move. So the future is likely to depend increasingly on new extraction technologies, that Jim Giles of Nature recently reviewed (Giles, J. 2004. Every last drop. Nature, v. 429, p. 694-695). There are several problems to solve in boosting production: decreasing the viscosity of oil, freeing oil that remains in sediment pore spaces, and driving the oil out under pressure. One interesting possibility is setting fire to oil in the reservoir rock, by pumping air into it. That would create gas pressure as well as lower viscosity, and has been tried before after Russian engineers accidentally set fire to a deposit by trying pressurised air to drive oil out. Following their surprise (and no doubt a ticking off by top political management), oil did flow more freely from nearby wells, but later experiments have had mixed success. Bacteria that metabolise oil are increasingly used to clean up spills. Since they break it down to lighter and less viscous molecules, and generate various gases, they have a role to play underground. However, all kinds of secondary recovery methods that are deployed today do not add a great deal to production – about 3 to 4% – and are unlikely to stave off eventual decline without further massive increases in price.
Structural control over hydrothermal gold mineralisation
One of the world’s richest gold provinces is centred on the town of Kalgoorlie in Western Australia, site of the “Golden Mile” whose production and reserves exceed 2500 tonnes of gold. The geological control is a 200 km long shear zone trending SSE that cuts Archaean greenstone associations of mafic-ultramafic and felsic lavas, and volcanoclastic rocks of the 2700 Ma Yilgarn Province. Exploration along the trend has revealed a number of other world-class gold deposits, and the Boulder-Lefroy Shear Zone has come to typify syn-tectonic hydrothermal mineralisation. Detailed work has long demonstrated that smaller shear zones slightly oblique to the main trend focus the mineralisation. That is because the main line of movement was probably in compression, having a strike-slip sense of motion. Depending on the local orientation of lesser shear zones, some have trends likely to have encouraged dilatation in transtensional environments. Fluids are more likely to favour such opening zones, thereby concentrating their flow and deposition of minerals from them. Much of the research in the area has focussed on detail, in an attempt to discover a means of predicting new deposits, and exploration is dominated by systematic drilling in what is not a particularly well-exposed terrain, and one where standard methods of stream sediment analysis are thwarted by low rainfall. Robert Weinberg of Monash University, Paul Hodkiewicz and David Groves of the University of Western Australia have taken a broader view of the structural setting (Weinberg, R.F. et al. 2004. What controls gold distribution in Archean terranes? Geology, v. 32, p. 545-548). So intensively explored is the gold province that it is unlikely that any large deposits remain to be discovered, but very similar shear zones affect most of the world’s Archaean granite-greenstone terranes, where exploration is at an earlier stage of progress. A model of regional controls over gold is therefore pretty valuable. Weinberg et al. divide the Boulder-Lefroy Shear Zone into boxes along its length, each centred on 8 gold “camps”. They plotted the deviation in trend of local segments of the shear zone in each box from its overall trend against the box’s known gold “endowment”. What emerged was a clear confirmation of the regional association of mineralisation with likely zones of regional transtension, trend deviation matching closely the estimated gold endowment. The abundance of structural data also enabled the authors to analyse the fractal dimension of all shears and fractures, thereby assessing the variation in overall geological complexity of the province. The results are odd. The least well-endowed parts of the gold province are more complex than those containing the most gold. The Golden Mile itself occurs where complexity changes from low to high. The ideas await testing on less mature shear zones cutting Archaean greenstones elsewhere in the world, such as in South India and East Africa.