Fuel, loose and fossil-free

NEARLY two millennia ago, Pliny the Elder, a Roman chronicler of natural history, spotted flames seeping from bare rock at a site that is probably Yanartas, in modern Turkey. It was an extraordinary find, though not for reasons that Pliny could have guessed. The gas that fuelled the flames, and fuels them still today, comes from a source that geologists would once have decried as implausible, if not altogether impossible. But that source is turning out to be a significant one.

The overwhelming majority of the methane extracted commercially and 90% of what is in the air is biotic—that is to say, it comes from the decomposition of the stuff of life. However, natural gas can also form abiotically, as a result of chemical reactions in a kind of volcanic rock called peridotite. Add to such rock the heat and pressures of depth, throw in a splash of water, and eventually out comes methane.

But it is not just explaining away fires on Earth that is of interest; serpentinization, as this process is called, was invoked to account for plumes of methane spotted by orbiters around Mars that might otherwise have been indicative of life (such methane plumes are now in doubt). Well-understood as the phenomenon is, however, such "seeps" of abiotic methane were until recently believed to be an exceptionally rare thing: between Pliny's observations in 77AD and 2012, only three more were discovered.

That view is now changing. In 2013 and so far in 2014, a further five abiotic gas seeps have been reported—in Portugal, Greece, Italy, Japan and another in Turkey. The man behind many of these finds is Giuseppe Etiope of the Italian National Institute of Geophysics and Volcanology, in Rome. Together with colleagues in Canada and Spain, Dr Etiope has discovered two more abiotic seeps that the team will report in December, at the annual meeting of the American Geophysical Union.

With the addition of these two sites, in Spain and the UAE, the number of known abiotic seeps has more than doubled since the beginning of 2013. Six more, from Europe to North and Central America to the South Pacific, are currently being analysed for suspected abiotic origins. It is not that the world is springing an abiotic methane leak, however. The recent spate of discoveries can be ascribed in part to advances in technology—a new generation of sensors in wide use can measure the flow of methane on diverse sites—and to growing scientific interest in the details of serpentinization.

What started as geochemistry curiosity could yet have economic implications as the number of abiotic seeps—which have never been commercially exploited—rises further. Based on geological similarities with existing sites, Dr Etiope strongly suspects that there may be more in Cyprus and Bosnia. A similar hunch prompted the search for the seeps that were discovered in Spain and the UAE.

Dr Etiope has received research funding from Petrobras, Brazil's state-owned oil giant, to formulate an estimate for the worldwide abiotic methane reserve. He reckons that it will take at least a year to arrive at even a rough guess. But Pliny may finally be of some help here. Given the well-dated observation of the fires at Yanartas and a good estimate of how much methane is required to keep them going, Dr Etiope believes the seep has expelled some 400m cubic metres since then—comparable to the output of some conventional gas fields. That is likely to be an underestimate; presumably, it had been going for a while before Pliny showed up.

As a newly appreciated potential source of gas, it will take some time to understand just how many abiotic seeps there are, and how much methane each might produce. The general view of hydrocarbons such as oil and gas is one of dwindling reserves. But there is still some exploring to be done.