Category Archives: Economic and applied geology

Fracking leaks

Cameron speaking in 2010.

David Cameron speaks (credit: Wikipedia)

The start of 2013 saw a massive puff from the British government for development of shale gas, Premier David Cameron crying ‘Britain must be at the heart of the shale gas revolution’. Fearful of the rapidly growing shift from Britain’s natural-gas self reliance to dependence on the Gulf, Russia and Norway the Conservative-Liberal  Democrat coalition gave the green light for ‘frack drilling’ to restart. This followed a pause following seismicity in the Blackpool area that attended Cuadrilla’s exploratory drilling into the gas-rich Carboniferous Bowland Shale thereabouts. There is also a nice sweetener for the new industry in the form of tax breaks.

English: Boris Johnson holding a model red dou...

Boris Johnson holds a model London red bus (Photo credit: Wikipedia)

London Mayor Boris Johnson, a possible contender for Tory leadership, seems pleased. And perhaps he should be, as the Lib-Con coalition will be tested because the junior partners depend electorally, to some extent, on ‘green’ credentials. The Lib-Dem Energy Minister, Ed Davey, seemingly favours an automatic halt to drilling should there be seismicity greater than 0.5 on the Richter scale; an energy level less than experienced every day in London from its Underground trains. Political commentators have forecast that green issues may exacerbate tensions within the coalition in the second half of its scheduled 5-year term, especially as the electorate seems set to reduce the Liberal Democrat partners to irrelevance in future elections.

Natural gas’s biggest ‘green’ plus is that being a hydrocarbon its burning releases considerably less CO2 than does its coal energy equivalent, the hydrogen content becoming water vapour. Yet the dominant gas is methane, which has a far larger greenhouse effect than the CO2 released by its burning. To avoid that presenting increased atmospheric warming, extracting natural gas needs to avoid leakage. Unfortunately for those bawling lustily about the economic potential of fracking source rocks such as the Bowland Shale, recent aerial surveys over US gas fields will come as a major shock. At the annual meeting of the American Geophysical Union in early December 2012 methane emissions from two large gas fields in the western US were released (Tollefson, J. 2013. Methane leaks erode green credentials of natural gas. Nature, v. 493, p. 12). They amount to 9% of total production, which would more than offset the climatic ‘benefit’ of using natural gas as a coal alternative.

A shift from coal to natural gas-fuelled power generation would slow down climatic warming, if leakage is kept below the modest level of 3.2% of production. So if the latest measurements are an unavoidable norm for gas fields then natural gas burning in fact increases global warming. Even more telling is that, until the shale ‘fracking revolution’, gas was produced by drilling into permeable reservoir rocks capped by a seal rock – usually a shale. The gas would not have leaked except from the well itself. Fracking, by design, increases the permeability of what would otherwise be a seal rock – hydrocarbon-rich shale – over a large area.

English: Schematic cross-section of the subsur...

Schematic cross-section illustrating types of natural gas deposits (credit: Wikipedia)

Aerial analyses to check emissions over oil and gas fields, let alone over shale-gas operations, are not widespread. However, the technology is not new. Where emissions are strictly enforced in populated areas, as over oil terminals and refineries, overflights to sample the air have been routine for several decades. Little mention is made of such precautionary measures in the promotion of fracking.

Another point is that as well as often being far from habitations, US shale-gas operations are generally into simple stratigraphy and structure. The Lower Carboniferous Bowland Shale now being touted as fuel for Britain’s escape from a descent into economic depression, with its estimated 200 trillion cubic feet of as potential, is intensely faulted and broadly folded, having experienced the Variscan orogeny at the end of the Palaeozoic Era. The complexity and pervasiveness of this brittle deformation is amply shown by geological maps of former coalfields that incorporate subsurface information from mine workings. The Bowland Shale lies below the Upper Carboniferous Coal Measures, many of the likely targets for fracking have never been subject to intensive underground mining simply because the Coal Measures were eroded away tens of million years ago. Consequently the degree to which many fracking targets may be riven by surface-breaking faults and fracture zones is not and possibly never will be known in the detail needed to assess widespread methane leakage.

Sometime in early 2013, the British Geological Survey is set to release estimates of the Bowland Shale gas reserves, in which its detailed mapping archives will have played the major role. That report will bear detailed scrutiny as regards the degree to which it also assesses potential leakage.

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Groundwater in Africa

English: Mwamanongu Village water source, Tanz...

Drinking water for many rural Africans often comes from open holes dug in the sand of dry riverbeds, and it is invariably contaminated. (Bob Metcalf on Wikipedia)

Sub-surface water supplies have rarely, if ever, figured in Earth Pages except in passing or in relation to the on-going crisis of arsenic pollution in drinking-water supplies. That is largely because of the paucity of groundwater publications that have a general interest. So it was welcome news to learn that hydrogeologists of the British Geological Survey and University College London have produced a continent-wide review of groundwater prospects for Africa, probably in most need of good news about water supplies (MacDonald, A.M. et al. 2012. Quantitative maps of groundwater in Africa. Environmental Research Letters, v. 7 doi:10.1088/1748-9326/7/2/024009. They used existing hydrogeological maps, publications and other publically available data to estimate total groundwater storage in a variety of aquifer types and the yield potentials of boreholes. Details can be seen at http://www.bgs.ac.uk/research/groundwater/international/africanGroundwater/maps.html

Dominated by the vast sedimentary aquifers of Libya, Algeria, Egypt and Sudan, such as the Nubian Sandstone, around 0.66 million km3 may lie below the continental surface: more than 100 times the annually renewable freshwater resources, including the flows in  three of the world’s largest rivers, the Nile, Congo and Niger. Though only a fraction of this subsurface potential may be available for extraction through wells, the arithmetic, or rather the statistics, suggest that small diameter boreholes and simple handpumps, as well as traditional wells, can sustainably satisfy the drinking water needs of the bulk of Africa’s rural populations with yields of individual wells between 0.1 to 1 l s-1. However, groundwater use in irrigation and for large urban supplies demands well productivities an order of magnitude higher from thick sedimentary sequences, which rarely coincide in Africa with areas suitable for large-scale agriculture or existing cities and large towns. Both the humid tropical lowlands with thick unconsolidated sediments and the deep sedimentary rock aquifers beneath the Sahara and other arid areas match great groundwater potential with either little need for groundwater or virtually no potential for agricultural development and very few people. Moreover, the truly vast reserves of North Africa that are an order of magnitude or more greater than in any other countries are at such depths and so remote that development needs commensurately huge investment, in the manner of oil-rich Libya’s Great Man Made River Project projected at more than US$25 billion investment. To say that reserves, convenience and yields are inequitably distributed in Africa would understate the hydrogeological difficulties of the continent.

Average well productivity predicted by MacDonald et al from Africa’s regional geology

Much of Africa has crystalline basement at the surface that has useful yields (>0.1  l s-1) only when deeply weathered, and even then rarely yields better than 1 l s1. An exception to this general rule is where basement has been shattered by large faults and fractures. Sedimentary cover is generally thin across the continent and with highly variable yield potential. The other issue is that of sustainability, for if extraction rates exceed those of recharge then groundwater effectively becomes a non-renewable resource. About half of the African surface, mainly in its western equatorial region, has sufficient rainfall and infiltration potential to outpace universally high evapotranspiration to give recharge rates of more than 2.5 cm of annual rainfall. For all the areas repeatedly hit by drought and famine, average recharge through the surface that escapes being literally blown away on the wind is less than half a centimetre.

To have synopses of all the important issues surrounding African groundwater – the best choice for safe domestic supplies in hot, poor areas – would seem to be very useful to those engaged in development and relief strategies; i.e. to governments, the UN ‘family’ and World Bank. But there are important caveats. An obvious one is the antiquity of many of the surveys drawn on by MacDonald et al. Some 23 out of 33 were published more than 20 years ago using data that may be a great deal older: such has been the snail-like pace of publication by all geological surveys, including BGS. That is compounded by the small scale of the maps (mainly smaller than 1:1 million) and the extremely sparse geophysical data concerning subsurface geology across most of Africa. ‘Quantitative’ is not the adjective to use here, for unlike in most of the developed world, groundwater reserves and locations in Africa have not been measured, but estimated from pretty meagre data. In fact to be brutally realistic, most of the source maps are based on educated guesswork by a few hard-pressed geoscientists once personally responsible for areas that would cripple most of their colleagues working in say Europe or North America.

If there is a truism about water exploration in Africa, outside the well-watered parts, it is this: sink a well at random, and it will probably be dry. The stats may well be encouraging, as MacDonald et al. clearly believe, but finding useful groundwater supplies relies on a great deal more. Outside cities, people survive as regards groundwater often as a result of traditional means of water exploration and well digging: they or at least some locals are experts at locating shallow sources. Yet to improve their access to decent water in the face of both rising populations and climate change demands sophisticated exploration techniques based on geological knowledge. Most important is to ensure supplies to existing communities, whose locations do not necessarily match deeper groundwater availability, bearing in mind that a universal problem for most African villagers is the sheer distance to wells with safe water. Rigs used to drill tube wells are expensive to hire, so the likelihood of success needs to be maximised. In the absence of large-scale (1:50 000) geological maps – rarities throughout Africa – only skilled hydrogeological interpretation of aerial or satellite images followed-up by geophysical ground traverses offer that vital confidence.

Geologically useful ASTER image of the Danakil Block in Eritrea/Ethiopia, showing Mesozoic and Recent sedimentary aquifers and crystalline basement (Steve Drury)

In fact, thanks to the joint US-Japan ASTER system carried in sun-synchronous orbit, geologically-oriented image data are available for the whole continent. Interpretation requires some skills but few if any beyond learning in a practical, field setting. Indeed, the African surface in its arid to semi-arid parts, most at risk of drought and famine, lends itself to rapid hydrogeological reconnaissance mapping using ASTER data. Given on-line training in image interpretation, a ‘crowd-source’ approach coordinating many interpreters could complete a truly life-giving and easily available map base for local people to focus their own well-construction programmes.

Britain to be comprehensively fracked?

Tower for drilling horizontally into the Marce...

Drill rig in Pennsylvania aimed at hydraulic fracturing of the hydrocarbon-rich Marcellus Shale of Devonian age. Image via Wikipedia

In ‘Fracking’ shale and US ‘peak gas’ (EPN of 1 July 2010) I drew attention to the relief being offered to dwindling US self-sufficiency in natural gas by new drilling and subsurface rock-fracturing technologies that opens access to extremely ‘tight’ carbonaceous shale and the gas it contains. The item also hinted at the down-side of shale-gas. The ‘fracking’ industry has grown at an alarming rate in the USA, now supplying more than 20% of US demand for gas. This side of the Atlantic the once vast reserves of North Sea gas fields are approaching exhaustion. This is at a time when commitments to reducing carbon emissions dramatically depend to a large extent on hydrocarbon gas supplanting coal to generate electricity, releasing much lower CO2  by burning hydrogen-rich gases such as methane (CH4) than by using coal that contains mainly carbon. Without alternative, indigenous supplies declining gas reserves in Western Europe also seem likely to enforce dependency on piped gas from Russia or shipment of liquefied petroleum gas from those major oil fields that produce it. The scene has been set in Europe in general and Britain in particular for a massive round of exploration aimed at alternative gas sources beneath dry land. Unlike the US and Canada, the British are not accustomed to on-shore drilling rigs, seismic exploration and production platforms, and nor are most Europeans. Least welcome are the potential environmental and social hazards that have been associated with the US fracking industry, which seem a greater threat in more densely populated Europe.

The offshore oil and gas of the North Sea fields formed by a process of slow geothermal heating of solid hydrocarbons or kerogen in source rocks at a variety of stratigraphic levels, escape into surrounding rocks of the gases and liquids produced by this maturation, and their eventual migration and accumulation in geological traps. By no means all products of maturation leave shale source rocks because of their very low permeability. That residue may be much more voluminous than petroleum liquids and gases in conventional reservoir rocks; hence the attraction of fracking carbonaceous shales. British on-shore geology is bulging with them, particularly Devonian and Carboniferous lacustrine mudstones, Carboniferous and Jurassic coals, and the marine black shales of the Jurassic (see http://www.bgs.ac.uk/research/energy/shaleGas.html and https://www.og.decc.gov.uk/upstream/licensing/shalegas.pdf), to the extent that areas of potential fracking cover around a third of England, Wales and southern Scotland.

News is breaking of a major shale-gas discovery beneath Blackpool, the seaside resort ‘noted for fresh air and fun, where Mr and Mrs Ramsbottom went with Young Albert their son…’ (Albert poked a stick at Wallace the lion and was eaten), said by energy firm Cuadrilla to have gas reserves of 5.7 trillion m3. The announcement followed 6 months of exploratory drilling, and drew attention to the burgeoning interest by entrepreneurs in the upcoming 14th Onshore Licensing Round for petroleum exploration in Britain. It isn’t just from major petroleum companies, but in some cases even what amount to family businesses finding sufficient venture capital to spud wells; similar in many respects to the US fracking boom that began a mere 10 years ago.

Hi-tech future may be saved by ocean floor sediments

Global rare earth element production (1 kt=106...

China's growing REE market share. Image via Wikipedia

Since the now far-off founding of the Club of Rome and the re-emergence of Malthusian ideology, time and again there have been warnings about the imminent running out of resources that are essential for modern life. The latest concern one of the formerly haunted wings of the Periodic Table, central to petrogenetic geochemistry, but little else; the rare-earth elements. From early beginnings as the source for phosphors in the screens of colour televisions all 15 REEs now have a growing commercial role in applications ranging from precision guided weapons, night-vision goggles and stealth technology in the military sphere, through the satiation of artificial appetites for electronic gaming and mobile phones, to applications of super-efficient magnets in medial scanners and ‘green’ power generation. The crisis being discussed currently is not so much a shortage – REEs are not so rare – but the cornering of their mining by the Peoples’ Republic of China, which produces more than 95%  of RREs used at present (~120 thousand tons). Yet world reserves are estimated at almost 100 million t, of which China has 36 million. Mining is often in only a few known, high-grade deposits; for instance most of the US reserves of 13 million t are locked in the Mountain Pass Mine, California that is currently on a ‘care-and-maintenance’ regime, i.e. shut. This one-sided economy sends shudders through capital’s strategy forums, i.e. in the US, Silicon Valley and the Pentagon.

Not surprisingly, geochemists and oceanographers from Japan, the world’s most hi-tech country, have bent their collective will to finding alternative sources, and may have revealed one in an unexpected location (Kato, Y. et al. 2011. Deep-sea mud in the Pacific Ocean as a potential resource for rare-earth elements. Nature Geoscience, v. 4, p. 535-539).  Their work stems from ‘mining’ existing geochemical data from deep-sea drilling projects on the floor of the Pacific Ocean, that reveal a wide range of REE concentrations in the ooze coating the seabed: from <250 to >2000 parts per million. The richest pickings seem to lie in a swath either side of the East Pacific Rise at around 15°S, where the group estimate that a 1 km2 plot could yield about one fifth of current world annual production, even though REE concentrations lie way below their on-shore economic cut-off grade. Apart from the need for dredging at depths around 3-5 km on the abyssal plains, and the inevitability of destroying a largely unknown ecosystem, the positive aspect of these metal-rich oozes is that the REE can be extracted simply by acid leaching of the goethite (FeOOH) in which the bulk of the elements reside. Goethite is something of a geochemical ‘mop’ with a capacity for adsorbing elements of all kinds on grain surfaces; so much so that it is being considered as a means of cleaning up heavy-metal pollution. Both the REEs and the iron probably arise from seabed hot springs where oxidising conditions result in dissolved ferrous iron combining in ferric form with oxygen to form goethite, which in turn scavenges other dissolved ions. Many of the on-shore REE deposits are carbonatites (intrusions of carbonate-rich magmas) that contain fluoro-carbonates and phosphates that host the REE, or beach sands in which wave swash concentrates the durable heavy phosphates in so-called black-sand deposits. Carbonatites are rare, most occurring in ancient ‘shields’, as in southern Africa, Canada and China, but being so unusual are not difficult to find.  One in the Canadian Shield known as the Big Spruce Lake deposit provides phosphorus- and potassium-rich soil that encourages the growth f conifers and so creates a geobotanical anomaly of large trees where local climate generally supports only stunted ones.

The rising demand and currently restricted supply of REEs is creating an exploration boom for carbonatites as the metal prices rise inexorably. Yet it may also produce a shift to what seems to be an alternative kind of source; iron-rich deep-sea sediments, though more likely those preserved on-shore in ophiolite complexes than at the huge depths of the abyssal plains. It is worth bearing in mind, however, that oceanographers and geochemists have pointed to untold metal riches before: manganese nodules that litter huge tracts of the seabed and contain sufficient copper, nickel and cobalt to maintain supplies for millennia. Despite a half-billion dollar investment in the 1960s and 70s, there is no nodule-dredging industry. There are however well-advanced plans for deep water mining of gold-rich hydrothermal sites, but miners will go just about anywhere to gloat over Marx’s ‘money commodity’

Gold, magma and groundwater in Nevada

Twin Creeks gold mine, Nevada, USA; Carlin-sty...

Twin Creeks gold mine, Nevada, USA; Carlin-style mineralisation. Image via Wikipedia

With the price of gold having climbed to above $1400 oz-1 while social revolutions develop in North Africa and the Middle East, articles on how gold deposits form will get a wider readership than they would have during its doldrum years in the late 20th century – the price has increased 7-fold since 2000. The bulk of gold nowadays can be mined profitably from ores in which it cannot be seen, except using a microscope, at grades well below 1 g t-1 (1 part per million) thanks to cheap heap-leaching with sodium cyanide of lightly milled ore. The epitome of low-grade gold is that produced at huge open-pit mines in Nevada from sedimentary host rocks. The gold is far too fine grained to have been deposited as placers, and also occurs dissolved in pyrite (Fe2S), so most experts regard it as having been introduced by hydrothermal fluids. Yet that covers two possibilities: by deep penetration of groundwater or from magmatic waters, and it is hard to decide which, again because the mineralisation is too fine grained to allow conclusive studies of fluid inclusions and stable isotopes. Also, such evidence as there is suggests low temperature fluids (~200° C) with low salinity; ambiguous data.

By using a synergy of ore-mineral chemistry, experimental data and ages of magmatism and mineralisation, Nevadan geologists have developed a convincing model for these ‘Carlin-type’ deposits (Muntean, J.L. et al. 2011. Magmatic-hydrothermal origin of Nevada’s Carlin-type gold deposits. Nature Geoscience, v. 4, p. 122-127). First, the mineralisation is of Eocene age and was introduced in Lower Palaeozoic sediments. The Eocene in the western USA saw the end of a period of compressional tectonics related to subduction since the Jurassic, fluids from which gave rise to partial melting of the overlying mantle wedge. This was succeeded by extensional tectonics and further intrusive magmatism dated between 40 to 36 Ma. This provided thermal energy and passageways for fluid migration. The second line of evidence is that hydrogen- and oxygen isotopes from fluid inclusions in hydrothermal gangue minerals show evidence that both mantle-derived and meteoric water mixed in the ore-forming fluids, and sulfur isotopes are similarly evidence of dual origin. Thirdly, the authors reasonably postulate from experimental data that basaltic back-arc magmas of Jurassic to Eocene age may have repeatedly added metals, including gold, to the mantle wedge that underpinned Nevada during subduction over a 175 Ma period. Thus later extension-related magmatism sourced in the wedge would itself have become metal enriched from this ‘fertile’ source. Moreover, conditions would have been ripe for highly oxidised conditions in the magmas and high concentrations of water in their fractionated descendants. Under such conditions gold and other metals favour entry into hydrothermal fluids. Given the extensional tectonic conditions such fluids could rise efficiently. Initially highly saline and very hot, rapid rise of the fluids would eventually result in them cooling adiabatically and separating into a dense salty liquid or brine and remaining vapour. That would force down the chlorine content in the vapour, favour some metals (Fe, Ag, Pb, Zn and Mn) ending up in the brine, while others (Au, Cu, As and Sb together with S) would remain in the vapour phase together with dissolved CO2 in large amounts, making the vapour acidic. Able to pass into the fractured Palaeozoic cover, the fluids widened fractures in the carbonate sediments and facilitated their own precipitation of minerals, the foremost being gold-bearing pyrite. Nevada is probably unique, but my goodness it is a big gold province; >6000 t of gold in tham thar hills.

Surprise in store for coal burning

A coal mine in Wyoming, United States. The Uni...

Image via Wikipedia

Of the fossil fuels coal has long been assumed to be the most plentiful, even the most pessimistic forecasters having acknowledged a global lifetime of centuries for known reserves. The determination of the emerging giant economies of China and India and of the USA to fuel themselves through coal-burning seems inevitable if highly risky for the climate. But that depends on coal remaining the cheapest fuel, largely because of the sheer abundance of supplies. A recent commentary on coal (Heinberg, R. & Fridley, D. 2010. The end of cheap coal. Nature, v. 268, p. 367-369) suggests that there is a growing tendency for reserve estimates to decrease as geologists factor in practical restrictions – place, depth, seam thickness and quality – on feasibility under current mining conditions, instead of just looking at known masses of coal. Astonishingly, the end-19th century estimate of five thousand years of US coal supplies dropped to about 400 years by 1974 and is currently judged to be 240 years. China and India look likely to have less than 60 years-worth left. On top of that, the widely publicized turn to carbon capture and storage (CCS)for ‘clean-coal’ future supplies will inevitably drive-up prices of coal-fired energy. The two main factors in this remarkable transformation of ‘King Coal’ are fundamental economic forces in capitalism and the increasing refusal of miners to accept dangerous working conditions. The second is especially the case for China, where most coal is deep-mined; in the late 1990s it saw a drive to close down unsafe mines that caused production to fall, although it has greatly accelerated this century – further driving down coal’s lifetime there. It seems from this analysis that any realistic hope for a CCS-based coal economy, especially in China and India, depends on declining safety and environmental standards in their largely underground mines, which in turn depends on the highly unlikely willingness of their workforces to accept worse conditions.

New clues to origin of porphyry-type ore deposits

The prominence of porphyry Cu-Au-Mo deposits above active subduction zones at continental margins, as in the Andes, has long encouraged ore geologists to suggest that they form as part of continental arc magmatism. Typically they occupy cupolas above large, intermediate to felsic, subvolcanic magma chambers that source the ore-forming fluid and most of the metals. Most show evidence of the influence of explosive fluid boiling that shatters the host porphyry mass during late stage hydrothermal activity thereby producing myriad cracks that become mineralised as a stockwork. One of the largest, among the longest worked and most investigated porphyry deposits is that at Bingham Canyon in Utah, USA. New isotope geochemistry bucks the accepted wisdom about porphyry-type mineralisation, in particular the source of the contained metals (Pettke, T. et al. 2010. The magma and metal source of giant porphyry-type ore deposits, based on lead isotope microanalysis of individual fluid inclusions. Earth and Planetary Science Letters, v. 296, p. 267–277).

The Bingham Canyon ores and host intrusion are Cenozoic in age (~38 Ma). However, isotopes of lead in fluid inclusions within the ore zone reveal a much more ancient metal endowment of the mantle underlying continental crust, around 1800 Ma ago, probably by metasomatism during the accretion of Palaeoproterozoic island arcs. Magmatism in the late Eocene, presaging the evolution of the Basin and Range extensional province drew in Cu and Au from the mantle and Mo from assimilated continental crust; i.e. Bingham Canyon and other huge porphyry deposits of the Western USA inherited metal enrichment from long beforehand, unlike those of active continental arcs. The intrinsic importance of the discovery is that given intermediate to felsic magmatism of any age, if it is sourced in relics of earlier arc-related igneous events then there is a chance that more recent activity may spawn rich porphyry deposits; more or less anywhere, given a metal endowed infrastructure. That opens up exploration possibilities to hitherto unexplored ground above ancient subduction zones.

New clues to origin of porphyry-type ore deposits

The prominence of porphyry Cu-Au-Mo deposits above active subduction zones at continental margins, as in the Andes, has long encouraged ore geologists to suggest that they form as part of continental arc magmatism. Typically they occupy cupolas above large, intermediate to felsic, subvolcanic magma chambers that source the ore-forming fluid and most of the metals. Most show evidence of the influence of explosive fluid boiling that shatters the host porphyry mass during late stage hydrothermal activity thereby producing myriad cracks that become mineralised as a stockwork. One of the largest, among the longest worked and most investigated porphyry deposits is that at Bingham Canyon in Utah, USA. New isotope geochemistry bucks the accepted wisdom about porphyry-type mineralisation, in particular the source of the contained metals (Pettke, T. et al. 2010. The magma and metal source of giant porphyry-type ore deposits, based on lead isotope microanalysis of individual fluid inclusions. Earth and Planetary Science Letters, v. 296, p. 267–277).

The Bingham Canyon ores and host intrusion are Cenozoic in age (~38 Ma). However, isotopes of lead in fluid inclusions within the ore zone reveal a much more ancient metal endowment of the mantle underlying continental crust, around 1800 Ma ago, probably by metasomatism during the accretion of Palaeoproterozoic island arcs. Magmatism in the late Eocene, presaging the evolution of the Basin and Range extensional province drew in Cu and Au from the mantle and Mo from assimilated continental crust; i.e. Bingham Canyon and other huge porphyry deposits of the Western USA inherited metal enrichment from long beforehand, unlike those of active continental arcs. The intrinsic importance of the discovery is that given intermediate to felsic magmatism of any age, if it is sourced in relics of earlier arc-related igneous events then there is a chance that more recent activity may spawn rich porphyry deposits; more or less anywhere, given a metal endowed infrastructure. That opens up exploration possibilities to hitherto unexplored ground above ancient subduction zones.

‘Fracking’ shale and US ‘peak gas’

Around 1970 the production of natural gas in the US reached its peak and has been slowly declining since then. The degree to which the US economy has grown to depend on natural gas and growing fears of becoming dependent on insecure supplies on the international LPG market has seen a stealthy growth in unconventional technologies to maintain indigenous supplies. The greatest growth has been in winning the useful fuel from ‘tight’ organic-rich shales that are usually regarded as source rocks for conventional petroleum rather than resources in their own right (Kerr, R.A.. 2010. Natural gas from shale bursts onto the scene. Science, 328, p. 1624-1626). The technology relies on drilling methods developed in the oil industry that allow several holes from a single platform to bend to pass at low angles through thin, gently dipping strata. That allows far larger volumes to be tapped than through a single, vertical well. Oil shales are not yet targeted for liquid petroleum because of the cost, but as Richard Kerr, a news writer for Science, reveals they are supplying an increasing proportion of US gas demand: from 1% to 20% since 2000. Being less of a source of carbon dioxide than coal or oil that might seem to be a ‘good thing’ all round, but there are worrying and little known problems with the technology.

To get the gas out demands that the permeability of shale is artificially increased by jacking open joints and fractures using very high-pressure fluids that carry sand to wedge them open when production begins open: this is ‘fracking’ in driller-speak. Not only gas starts to move, but also water locked into the shale for millions of years and often highly toxic. Drillers hope that all the fluids will follow the holes, but that is by no means guaranteed and some may make their way into aquifers and up to the surface. The fluids used in fracking are deliberately full of chemicals that help open up cracks and even biocides that keep them from being clogged by bacterial films: around 15 million litres used per well. Although aimed to be recycled these noxious fluids can escape, sometimes in massive blowouts. Uncontrolled gas and formation water escapes can cause explosions and kill of forested areas by disrupting tree-root biota.

Groundwater depletion measured from orbit

The NASA and German Aerospace Centre Gravity Recovery and Climate Experiment (GRACE) launched in 2002 aims to measure variations over time in the gravity field by gauging tiny changes in distance between two satellites using radar. The only significant changes in the short term are due to movements of water in one form or another. The best-known result from GRACE is its assessment of shrinking ice caps, and it can also detect shifting ocean currents and the drainage of lakes. The GRACE science team has noted a major change in gravity since launch over a nearly 3 million km2 area of NW India centred on Delhi. The only conceivable mechanism is gradual loss of groundwater though irrigation of the Gangetic plains (Rodell, M. et al. 2009. Satellite-based estimates of groundwater depletion in India. Nature, v. 460, p.999-1002). The authors estimate a decline of around 109 km3 of groundwater since 2002 – more than twice the storage capacity of India’s largest reservoir. In places local farmers are reportedly having to sink deeper and deeper wells as the water table sinks by over 6 m each year. The area is one of Asia’s largest producer of food grains and occupied by 600 million people. Most likely there has been a surge in withdrawal for irrigation during poor monsoons in the early 21st century, for pumping rates seem to be 70% greater than they were in the 1990s.

 

Gas hydrates soon to come on stream?

 The looming prospects of petroleum production outside of Arabia passing its peak and flexing of Russian economic power that stems from its control of the largest  untapped natural gas reserves are spurring evaluation of methane production from gas hydrates in onshore frozen peat mires and marine sediments. Gas hydrates are more equitably distributed than are much older petroleum reservoirs: even Japan, which is currently entirely dependent on foreign supplies, has what appear to be huge offshore reserves of gas hydrates. Estimates of the world‘s potential resources are enormous, at around 2 x 1016 m3 (annual US natural gas consumption is ~ 6 x 1011 m3) but in a variety of sands and muds at different concentrations (Boswell, R. 2009. Is gas hydrate energy within reach? Science, v. 325, p. 957-958). Experiments in northern Canada (see Onshore gas hydrate reserves close to recovery in March 2004 issue of EPN) indicates that drilling to induce lower pressure in gas hydrate bearing sediments induces dissociation of the hydrate crystals to release methane while retaining also present within their structure. Injection of CO2 into deposits should displace methane while CO2 enters the crystalline structure: killing two birds, including carbon sequestration, with one stone. The main technical stumbling block is that gas hydrates occur in unconsolidated sediments that may be destabilised during production, resulting in uncontrollable release of the powerful greenhouse gases as well as collapse of surface structures. From an environmental standpoint, all gas hydrates do is sustain reliance on carbon-based fossil fuels and continue emissions of greenhouse gases, though burning methane is a good deal ‘cleaner’ than coal or oil.