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From Biblical record, Noah was the first successful oilman. Throughout history, people have used the petroleum that seeped from the ground for boat building, warfare, as an ingredient in Greek fire, a primitive form of napalm used by the Byzantine navy, and as a laxative. And there are still plenty of natural seepages to be found today.

The petroleum geology research group at the Royal School of Mines, at Imperial College in London, has collated nearly 200 places in Britain where petroleum seeps out through soils and outcrops of rock. Although the seeps may originate from modern pollution, this is unlikely. Many were documented before the development of the internal combustion engine. In addition, natural petroleum has a different composition to refined oil; geochemical analysis can tell them apart. Nor must the oily iridescence produced by iron salts floating on stagnant pools in peat bogs be confused with the iridescence produced by petroleum. The former breaks into jagged rafts when you prod it, while the genuine article flows in smooth curves, like cream in Gaelic coffee.

Most petroleum comes originally from sedimentary rocks – ‘source rocks’ in the language of the oil business – containing plenty of organic matter usually formed in lakes or shallow seas. If these rocks were heated up to the right temperature when they were buried, the organic molecules reacted to make oil and gas. Oil reservoirs form when the hydrocarbons move away from the source rocks and accumulate in a permeable area of rock, often sandstone. In places, the petroleum oozes from the ground; elsewhere, it fills fissures and pores within the rock as what is called an impregnation.

Here we take a tour around Britain’s natural seepages of hydrocarbons. From our survey, we found that most of the natural petroleum is in the Orkneys and northeast Scotland, in the Midland Valley of Scotland, around the southern end of the Pennines, along the Welsh borderlands, in the Mendips, Devon, Cornwall, Dorset and Sussex. One oil seep in the Midland Valley of Scotland prompted D’Arcy Exploration Company, the forerunner to BP, to drill a well on the crest of the adjacent Cousland fold, near Edinburgh.

The seepages in the Orkneys and the mainland of northeast Scotland are principally within sedimentary strata – the Old Red Sandstone – but they also fill veins and fissures in metamorphic rocks, such as the schists at Castle Leod near Inverness. In the 19th century the local blacksmith used to quarry these veins of pitch to fire his forge. The seepages and impregnations of northeast Scotland come principally from layers of shale rich in organic matter, within the Old Red Sandstone itself. These shales were mud in lakes some 400 million years ago; the oil of at least one North Sea field probably came from the same source.

Elsewhere in Scotland there are reports of petroleum in strange places, including impermeable igneous and metamorphic rocks that the conventional petroleum prospector would regard as barren. The hydrocarbons are far away from conventional source beds from which they could have migrated. For example, there are lumps of ozocerite, a plastic, waxy paraffin oil, on the shores of Loch Fyne; bitumen impregnates granites at Aberdeen and elsewhere. A few kilometres south of Aberdeen, there was a seepage of petroleum from fractured schist sufficiently abundant and pure for a local farmer to run his tractor on it in the 1970s. Suspecting that the farmer had found his fuel from some recent spill or a leaking tank somewhere, Customs and Excise officers tried to trace it. But even their best efforts could not find a suitable, taxable source.

Many coal mines throughout the English Midlands also bear petroleum. Seepages at the surface are familiar around the southern end of the Pennines where the permeable strata called the Coal Measures (because they are the main coal-bearing rocks of Britain) spread across impermeable older rocks. There is a line of seepages along the eastern edge of the Derbyshire Peak district, where the Coal Measures lie on Carboniferous Limestone. The most noticeable example of this is at Castleton, where something akin to axle grease oozes from fractures in the limestone crags. This elaterite, as it is called, is also a feature of the Blue John mine, and other former lead and zinc mines nearby. Hydrocarbons are often found with low-temperature mineral deposits such as these.

But oil and pitch are not the only natural forms of petroleum. In 1722 William Camden, the antiquarian, noted a pond near Wigan from which sulphurous gas bubbled in sufficient quantities ‘to boyl eggs, meat, etc, tho the water itself be cold’. In 1843, oil was discovered in a peat bog in nearby Formby. Several oil wells were drilled in the vicinity during the 1930s and 1940s. At first, it was thought that this oil had formed from modern algae, but chemical analysis disproved the theory.

In the Welsh borderlands, there are impregnations of petroleum in rocks of many different ages, from Rose Hill in the north to Bristol in the south. Again, most of them are where older, impermeable rocks lie at a different inclination to the Coal Measures above – a junction called an unconformity. Petroleum generated deep within the basins of the Coal Measures has migrated along and upward through permeable beds until it reached impermeable older rocks. Here it stopped, and seeped out along the unconformity. There are now some curious impregnations in places many kilometres west of the present extent of the Coal Measures. But the Carboniferous rocks once stretched much farther west. These seeps may have come from the unconformity, before the Coal Measures eroded away. But elsewhere in the borders, heat from minor intrusions of igneous rock has generated small amounts of petroleum from organic shales in the same older rocks.

One of the most famous seepages in the Welsh borders is at a village with the appropriate name of Pitchford. This, or one of the adjacent seeps, may have been the source for the bitumen used in the construction of the Roman fort at nearby Wroxeter. The Pitchford petroleum seeped at such a rate that it used to be bottled and marketed as ‘Betterton’s British Oil’ (no doubt a deeply moving experience, as it proved to be a powerful laxative).

Further south, at a limestone quarry on the banks of the River Avon in Bristol, the oil that leaked from the joints in the rock also had a medicinal use – as an embrocation to ease the pain in the quarrymen’s joints. There are also petroleum seeps in the Carboniferous Limestone of the Mendip Hills. After an earthquake in 1892, several barrels of oil were recovered from a water well at Ashwick Court.

But the southwest of England has the highest concentration of natural petroleum in the whole of Britain. Oil seepages have been recorded from Barnstaple and Torquay in Devon, where oil contaminated a water well. The highest concentration of petroleum is in and around the granites of Cornwall. The seeps are in tin mines, associated with veins of ore. The best documented instance is at the South Crofty tin mine, where there is so much oil that it drips down the necks of the miners.

There has been much speculation about the source of petroleum in granites in general, and the Cornish ones in particular. Oil produced commercially comes from rocks such as sandstone or limestone, not from igneous rocks. This link between oil and an igneous rock delights those scientists who favour theories that petroleum comes not from ancient underwater life, but rather from the Earth’s mantle, or even outer space. But there is a more mundane and conventional explanation: granites solidified beneath a cover of sedimentary rocks. Those in Devon and Cornwall formed beneath a sequence of muds and turbidite sands rich in organic debris that were deposited in deep water during the Devonian and Carboniferous periods between 300 and 400 million years ago. These strata were subsequently folded and metamorphosed into a series of slatey rocks, now called the Culm Series. As the molten rock of the granites moved up in the crust and eventually solidified, they heated the rock above. Water flowed in convection cells through communicating fracture systems deep below ground.

Petroleum normally forms at temperatures between 60 and 120 °C. This is well below the temperatures to which the rocks forming the present surface of Cornwall have been cooked. Researchers think that petroleum formed in the cooler rocks of the Culm Series, which have since eroded away above the granites. The convenience flow system carried it downwards to settle in pores and fractures within and adjacent to the granites. Erosion has removed the source beds. All that can now be seen at the surface are the deeper rocks that were heated to a temperature too high to have generated petroleum.

Farther east, there are a number of petroleum seepages along the Dorset coast, from Osmington Mills in the west, to Anvil Point in the east. Most of these are onshore, in coastal cliffs, but scuba divers off Anvil Point reported finding a gas seepage underwater. Most of the seepages arise where layers of permeable sands dip to the north and intersect major faults. Petroleum migrated up the faults, then across into the permeable sand and so up to the surface. The sand saturated with oil at Osmington Mills tells a petroleum geologist that there was once a major accumulation of oil in a fold in the rock in what is now Weymouth Bay. Sadly, erosion has removed most of the reservoir and its overlying cap rock; only the floor of the oil-soaked rock now crops out on the seabed.

One of the best known of the Dorset sites is at Mupe Bay, a few kilometres east of Lulworth Cove. This is a particularly interesting place because there is not only a modern seep, but also a fossil oil seep. The cliffs at Mupe Bay contain rocks formed about 140 million years ago, in an early Cretaceous river channel. The channel is now filled in, and floored by pebbles and boulders of sand cemented by heavy black oil with little smell. This is known as dead oil because all the volatile components have evaporated. The sand between the pebbles and above the floor of the channel is, in contrast, impregnated with a sweet-smelling brown oil – live oil. These strata, deposited horizontally, now dip steeply north towards a major fault system. Geochemical analyses show that the oil was generated from organic-rich shales formed in the Early Jurassic, 200 million years ago. Oil generation clearly started some 110 million years ago and polluted an alluvial flood plain adjacent to a fault in Early Cretaceous times. Meandering rivers eroded the oil-impregnated sands and made pebbles of oil-cemented sand grains. These were transported along the river bed, came to rest on the channel floor, and were finally buried beneath sands that subsequently filled the channel. The present saturation of live oil in the Cretaceous alluvial sands shows that petroleum is still migrating up the fault and seeping to the surface along the permeable channel sands.

The Dorset seepages stimulated a quest for commercial sources of petroleum that began with the drilling of a well at Poxwell in the 1930s, and led subsequently to the discovery of the Kimmeridge Bay and Wytch Farm fields. Two major source beds for petroleum crop out along the Dorset coast: the lower Liassic clays of the Lower Jurassic, which is about 200 million years old, and the Upper Jurassic Kimmerdige Clay, which is 153 million years old.

Both of these formations have demonstrated their potential as sources of petroleum in the past by occasionally igniting of their own accord, to the horror of the natives and the delight of tourists. This phenomenon has happened at the Liassic cliffs of Lyme Regis, the Kimmeridge Clay cliffs at Kimmeridge Bay itself, and at a place near Ringstead Bay, known appropriately as Burning Cliff. The burning cliffs probably owe their flames to the weathering of pyrites (iron sulphide) that is spread in tiny grains throughout the shales. The change from pyrites to oxides of iron and sulphur gives out enough heat to trigger the spontaneous combustion of the organic matter mixed with it.

In search of the source of rocks

More conventional methods of geochemical analysis show that the Liassic beds and the Kimmeridge Clay can also generate petroleum. But detailed studies show that only the Liassic shales have been buried and heated enough to generate the petroleum that is now pumped commercially from Dorset. The Kimmeridge Clay has not had the chance to generate petroleum here. But beneath the North Sea it is deeply buried and is the source of much of the oil and gas that is now produced from the northern part of the North Sea.

This tour of seepages of petroleum in Britian concludes in leafy Sussex. There have been about a dozen oil and gas seepages in an area bounded by Heathfield to the northwest and Hastings to the southeast. The first recorded discovery was in 1836 at Hawkhurst near Heathfield. During the drilling of a water well an Irish navvy heard a bubbling sound coming from the bottom of the shaft. His curiousity aroused, he injudiciously struck a match and peered over the side to investigate. History records that he lost his facial hair, not to mention all further interest in the proceedings. Subsequently several other wells were sunk for gas in the area. This led to the development of a company in 1902 that collected the gas and piped it to Heathfield, Polegate and Eastbourne.

Our survey of British petroleum seepages can never be complete, because new instances are constantly reported. Oil and gas seepages happen in many parts of the world -often in a far more dramatic way than in the British Isles, but in similar geological settings. You can expect them near faults, and where permeable strata lie above impermeable rocks. Sometimes seepages indicate that commercial accumulations of petroleum lie under the ground nearby, but this is by no means always the case. Many petroleum impregnations are the residues of larger accumulations that have long since vanished through erosion.

It is interesting to note that this pollution by petroleum is entirely natural, and has been going on for millions of years before the days of Noah. But this particular form of pollution has beneficial effects in the long term. Given time, petroleum will break down naturally, releasing its nutrients back into the biosphere whence they came.

Richard Selley is professor of applied sedimentology and head of the department of geology at the Royal School of Mines, Imperial College, London University.