Dolerite, the Karoo’s Fracking Game-Changer

By Julienne du Toit

Photographs by Chris Marais

About 182 million years ago, all hell broke loose in Southern Africa.

From the latitude of Beaufort West in the central Karoo to southern Malawi, from East London to the Okavango and beyond, the earth began to crack open and molten basalt poured out.

It oozed upwards from a hundred kilometres or more below the surface through all the Karoo’s layered sediments. It filled cracks, crevices, hollows and faults, creating level sills, saucer-shaped rings and vertical dykes.

As the molten magma came into contact with wet sediments or groundwater, hydrothermal explosions punched upwards, creating thousands of holes in the Earth’s crust – through which hydrocarbon gases poured. Today many of these ancient ‘chimneys’ are visible as breccia outcrops.

Nearly two million cubic kilometres of molten rock flowed out of the Earth.

This catastrophic episode of the Karoo’s geological life coincided with the tearing apart of the Gondwana supercontinent into its jigsaw parts of Africa, Antarctica, Australia and South America.

Dolerite Sills and Dykes

As geologist Nick Norman explains in his 2013 book Geology Off the Beaten Track: “… tensional cracks heralding the break-up of Gondwana permitted the ascent of pressurised basaltic magma from great depths. Up through the thick cake of Karoo sediments it came – long after the Ecca shale through which it cuts had been deposited – to flow over the surface as sheet after sheet of lava; where it didn’t make it to the surface, it squeezed between layers of sediments to form ‘sills’.”

The geological maps of South Africa that are pinned to walls in most geologists’ offices show the Karoo basin thickly laced with red. That marks the dolerite – the magmatic rock that cooled and hardened underground.

The characteristic flat-topped mountains and hills of the Karoo are the most visible remnant of the dolerite’s tough sills, protecting underlying sandstone from erosion.

Less visible are the underground cracks left in the adjoining sediment as the superheated dolerite rose. It is these fractures that are so useful to Karoo people today. Groundwater collects in the crevices around dolerite pipes, sills and dykes. Anyone looking to drill for water in the Karoo will do so in the vicinity of dolerite.

Of the lava that managed to reach the surface, the most recognisable remainder lies in the jagged basalt peaks of the Drakensberg mountain range.

The Karoo Basin’s Complexity

As Dr Luc Chevallier of South Africa’s Council for Geosciences can confirm, the dolerite created a kind of geology in the Karoo Basin that is unique in the world.

“Drilling one borehole can reveal such complexity,” he says.

South Africa’s leading authority on Karoo dolerite is Julian ‘Goonie’ Marsh, Professor Emeritus of Geology at Rhodes University in the Eastern Cape. He says it’s still uncertain how long this ancient fiery apocalypse took to play out.

“In geological terms, it was brief, but intense. We don’t have an exact time frame. It could have happened over a few decades or up to a few hundred thousand years. But the volumes were incredible, amounting to at least 1.5 million cubic kilometres of molten rock.”

The Karoo Igneous Event

Marsh says such massive magma outpourings have occurred very few times in the Earth’s history. The first recorded instance was 250 million years ago, coinciding with the Permian extinction, which killed off more than 90% of all life on the planet. Then came the Karoo igneous event 182.5 million years ago.

In South America and part of Namibia (Parana and Etendeka, which were once joined) there are traces of such a dolerite event dating back 132 million years. There was also a massive magma outpouring 60 million years ago in India’s Deccan area.

Most of them triggered global evolutionary crises characterised by climate change, acidic oceans and severely stressed ecosystems.

But no one knows what causes this kind of igneous intrusion to happen, or whether it will ever happen again.

What did this surprising event do to the methane trapped in the shale (Ecca) layers?

Some scientists think the gas that the frackers seek is long gone. Or at least gone in commercially viable quantities.

The late Dr Billy de Klerk of Rhodes University was a palaeo-geologist with a PhD in magmatic processes. He pointed out that the doleritic magma was at about 1200 deg C when it intruded.

Dolerite, Methane and the Karoo Basin

“It’s entirely likely that the enormous heat of this dolerite, hotboxed for millennia by the insulated layered sediments, would have driven off most of the volatile hydrocarbon gases like methane.”

Dr Ingrid Aarnes, formerly of the University of Oslo’s Geosciences department concurs, to some extent. Her doctorate was on the Karoo and included calculations on the thermal effect of dolerite on the adjoining shales.

“The results of those studies indicate that a vast maturation of the surrounding shales could be expected.” (Aarnes explains that maturation of shale is the process in which the organic matter in the shales transforms into oil and gas upon heating. This process usually occurs at 120 deg C over a long period.)

“So in theory there should be a lot of gas generated,” she says. “In fact it seems that the quantities could add up to be enough to trigger global warming. The fate of all these gases remains unresolved.

“However, due to several thousand mapped pipe/chimney structures which begin in the matured zone close to the dolerite and go all the way up to the surface in the Karoo, we (my colleagues, such as Henrik Svensen, and I) infer that most of these gases were released to the atmosphere within a quite short period of time after the intrusions.

“In the bigger picture such a release could potentially explain the sudden disturbance in the carbon cycle and subsequent global warming also observed at this time in Earth’s history.”

The Karoo’s Singular Geology

She goes on: “Assuming that the pipe-structures were acting as transport channels for the methane from underground where they are formed to the atmosphere, they could have continued to drain the shales for a long time after the initial outburst. Was it all drained? That is very difficult to answer without actually drilling down to have a ‘look’.”

Having a look, however, has become an issue fraught with difficulties. All three current applicants (Bundu, Falcon and Shell) face many legal hurdles before their drill rigs penetrate the Karoo.

The other hurdle will be the Karoo’s peculiar geology. Chevallier confirms that apart from dolerite dykes and sills, and breccia pipes, there are also kimberlite pipes under the Karoo, many of which have not erupted onto the surface.

“So there are different types of intrusion.”

The Karoo’s underground geology, says Chevallier, might change every 20 or 30 km or so, which will make financial predictions of gas reserves very difficult.

“Obviously you can use the same technique and approach while drilling, but the results may differ with different wells. Here you might have three sills and one dyke. A little further you will have something quite different. Here there might be gas in financially viable quantities, but a little further, there might not.”

Many geologists will tell you there is definitely gas in the Karoo. So will farmers.

When water boreholes are dug on Karoo farms, it’s not completely unusual for gas to come out. Some farmers have tapped the gas to their homes for years before it peters out.

Karoo Shale Gas or Water?

Dr Aarnes explains: “The heating from the sills is potentially both ‘good’ and ‘bad’ for the shale gas. The intrusions would have heated the basin so that large volumes of shales most likely reached gas-type maturation.

“This will increase the potential amount of gas available in the shales. On the other hand, gas is very mobile, and if the reactions are happening fast, then the overpressure will most likely lead to fracturing and escape of the gases.

“Evidence for the latter can be linked to the thousands of pipe-structures mapped in the region by my colleagues. Also, due to the long time in question, I would expect a slow seepage as erosion uncovers the shales.

“I guess that close to the intrusions, where the maturation was most intense, I would not expect too much of the gases to linger on today. However, in shales that haven’t been subject to intense fracturing through pressure build-up, but still felt the heat wave, the amounts could potentially be increased.”

Dr Aarnes feels strongly that fracking the Karoo would be a mistake:

“And let’s say the gas is actually there, what about maybe preserving that for future generations? People can live without gas, but they cannot possibly live without water.”

Could the heat from magma below and lava above have matured organic-rich formations into gas above the Ecca formation?

There was a methane gas explosion when three drilling teams were excavating the 83km Orange-Fish River Tunnel under the Suurberg Mountains between Gariep Dam and Steynsburg in the early 1970s. The resulting fire burnt for months and by all accounts, melted surrounding rocks.

Faulting and Karoo Dolerite

To this day, the tunnel exhales methane. Petrol vehicles are not allowed into the tunnel during maintenance periods for this very reason.

Hydrogeologist Zoe George is working on the potential impacts of unconventional gas activities on groundwater. She confirmed that dolerite could complicate deep drilling.

“My personal belief is that the undersides of the sills are likely to be highly fractured and contain pockets of gas under high pressure which carries the danger of blowouts, as happened when Soekor drilled a deep well on the farm Cranemere near Pearston.

“I expect pockets of gas all the way up the rock pile where there is shale, not just in the Whitehill Formation. Blowouts are uncontrolled or uncontrollable eruptions of drilling mud and formation water (both poisonous) out of the borehole onto the land surface, which means groundwater and surface water could be contaminated. It would be almost impossible to rehabilitate the groundwater.”

In short, the dolerite-intruded shale fields are undeniably problematic. Upstream general manager of Shell South Africa Jan Willem Eggink has already confirmed he wants to stay well away from dolerite because it is so hard, is linked to faulting and because it is recognised as a preferential pathway for water.

Dolerite’s Threats to Shale Gas

Dolerite’s threats to successful shale gas extraction are neatly expressed in four facts:

• Falcon Oil & Gas were ahead of Shell to get a shale gas exploration concession in the Karoo. They could take their pick of spots. They chose about 30 000 square kilometres in the only part of the Karoo without dolerite.
• In June 2013, the US Energy Information Administration downgraded South Africa’s potential shale gas reserves from 485 trillion cubic feet (which put us fifth in the world) to 390 tcf (eighth in the world), in part because of “geologic complexity caused by igneous intrusions”.
• In August 2013, Petroleum Agency of South Africa principal geologist John Decker told Mining Weekly he felt the potential of shale gas might be “thwarted by the existence of dolerite rock in the Karoo.” He mentioned several academic studies which proposed that “the intrusion of dolerite 183 million years ago might have caused an explosive degassing of shale in some parts of the Karoo”. In addition, Decker thought that rather than 485 tcf or 390 tcf, South Africa’s recoverable shale gas reserves might be closer to 30 tcf. (It’s worth remembering that speculation about shale gas quantities in South Africa leans heavily on records from the 20-odd deep wells drilled by Soekor in the mid to late 1960s in an area larger than Germany.)
• There was a now-famous incident in 1967 where a Soekor drilling team lost drilling fluid at about 2400 metres at Skietfontein Farm outside Aberdeen.The same composition of chemicals (containing the deflocculant chrome lignosulphate) popped up near Klipplaat via a farmer’s borehole six weeks later, having taken this staggeringly short time to rise and travel more than 30km. In other words, water moves quickly along the deep sediments via cracks around dolerite intrusions. This makes drilling and fracking operations a risky proposition for the dozens of Karoo towns depending on groundwater.

Dr Aarnes concludes: “The fact that the drilling fluid loss so quickly contaminated other wells far away is a quite large warning sign of the potential of messing up the whole water system, and I would say that could really be the second ‘inferno’ of the Karoo.”

Falcon, Bundu and Tectonic Pressure

Does this mean we should all rush out and buy Falcon Oil and Gas shares because they got the most promising part? Alas for them, not necessarily. Their concession area between the dolerite line – which stretches through Somerset East and Aberdeen to Beaufort West and beyond – and the Cape Fold Mountains has its own challenges.

Dr Billy de Klerk said “This area was influenced by tectonic pressure exerted by South America and Antarctica. It would have heated up too, possibly driving the volatiles off.”

Also, Dr Chevallier points out, the shale layers are very deep in Falcon and Bundu’s concession. Their wells would have to go down 4 to 5km or so, which would be a very expensive exercise.

Dolerite’s other role has been its importance to human evolution and South Africa’s culture.

This takes at least two forms. Firstly, all across the Karoo you can find Stone Age artefacts and tools. They were made out of hornfels, a hard, fine-grained surface shale that was heated (metamorphosed) by dolerite.

Hornfels (also known as lidianite) is very hard and like glass or obsidian, it breaks with a ‘conchoidal’ fracture. With it, Stone Age people created tools for cutting, scraping and stabbing.

Dr Goonie Marsh says: “I have found some places along dolerite dykes intruding shales, where the ground is littered with hundreds of worked hornfels fragments. One can imagine such sites to literally have been stone tool ‘factories’. Without dolerite to make hornfels, one can wonder whether this Stone Age culture could have thrived as it did across the Karoo Basin.”

Dolerite and South African Culture

Secondly, dolerite has qualities linked to early human creativity.

Many large dolerite boulders near the top of flat-topped hills give off a bell-like sound when struck. Ancient strike-marks show that Bushmen and Khoi people used them for drumming.

Archaeology professor John Parkington of the University of Cape Town writes in his co-authored book Karoo Rock Engravings:

“Karoo rock gongs are almost without exception located among clusters of engravings and are recognisable as sets of hammered or pecked patches usually on dolerite boulders, nearly always on the lips of rocky ridges.”

Dolerite boulders were also used as ‘canvases’. As ironstone (dolerite) ages, the exposed area darkens to reddish brown and then pitch black, as if the rock were becoming sunburnt. This is called a rock varnish, patina or oxidation.

Some of the planet’s earliest human artistic expression is etched on dolerite, not just here but at sites all over the Karoo.

Rock Etchings on Dolerite

In just one rock etching site near Nelspoort (north of Beaufort West), there are literally hundreds of etchings and engravings of animals and geometric symbols, many thought to date back thousands of years, made by South Africa’s Bushmen (San) and Khoi.

These ‘First People’ engraved eland, black rhinoceros, giraffe, hartebeest, zebra, bird-human figures, spirit beings, jackal, blesbok and elephant.  Dark patina has reclaimed many of them again. The oldest ones can only be seen at certain angles, in certain lights.

More recent ones, a few hundred years old, show paler geometrics, sunbursts, apron shapes, flower-like patterns. They are probably made by the Khoekhoen herders, also called ‘Khoi’ people.

A century or two ago, European travellers, settlers and soldiers spent time here too, carving initials and images on the rocks. There are also engravings of trains, cars and women with skirts.

Dolerite the Fracking Game Changer

In short, dolerite has shaped the Karoo and its people like no other rock formation.

• Its minerals fed the Karoo’s clay soils so that this is one of the more fertile arid regions in the world.

• Dolerite’s sills and dykes created the Karoo’s distinctive flat-topped hills and dramatic landscapes.

• Dolerite created some of the rock that Stone Age people used for tools.

• Dark dolerite boulders helped provide the canvas for human artistic endeavour and evolution.

• The cracks it created in the earth became water reservoirs that made possible farming and towns in a semi-desert.

• Dolerite has long been invaluable for building and road-making.

Now it is becoming a major character in an intriguing oil and gas drama.

This surprising rock formation has always been game-changer. But perhaps not in the way that Government or the oil and gas companies would have wished.

Postscript: In late February 2014, the Petroleum Agency South Africa (PASA) announced to Parliament that its “best estimate” of shale gas recoverable reserves in the Karoo Basin was 40tcf.  PASA’s website notes that dolerite intrusions, which occur in much of Karoo Basin, are “a major exploration risk factor”.