A new offshore play is emerging in the oil and gas exploration business – deepwater contourites. Elaine Maslin spoke to Professor Dorrik Stow about what and where they are and how they were found and how scientific deepwater drilling is helping to unlock their secrets.
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Photo from Joides Resolution Facebook page. |
For those of a certain age, the idea of deepwater scientific drilling might bring up memories of tales about adventures to the earth’s core or deepwater extra-terrestrial life conjured up by John Wyndham in his 1950s science fiction.
They would be right, up to a point. Today’s deepwater scientific drilling, stemming back to a tradition of drilling in deepwater to understand more about the earth’s crust and formation, is helping to discover life in the most unlikely places – including microbial life deep beneath the seafloor – as well as helping to understand planetary dynamics, through subduction and earthquake mechanisms, and environmental changes.
But, it is also helping to unlock new subsurface paradigms of particular interest to the deepwater exploration industry, including deepwater sedimentation, specifically contourites. Thanks to research carried out through scientific drilling, more is being learned about this new paradigm for deepwater exploration. It’s still a relatively new area scientifically, but, the oil and gas industry is starting to take note and fields are already being developed which fit the contourite description – offshore Norway and Brazil.
Scientific necessity
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Joides Resolution drillship out to sea. Photos from from Dorrik Stow. |
“Drilling is the only way to actually know what’s in the subsurface, says Professor Dorrik Stow, Director of the Institute of Petroleum Engineering at Heriot-Watt University in Edinburgh, Scotland. Stow has taken part in scientific drilling programs offshore Angola (1980), leading to the discovery of Cretaceous black shales in the subsurface, paving the way for oil discoveries offshore west Africa, and the Gulf of Mexico (1983), and the Bengal Fan (1987), and documenting the Himalayan uplift history and tectonic influence on deepwater sedimentation, which helped to understand deepwater turbidites – one of the world’s key reservoir systems.
More recently, he has been researching contourites. “Originally we thought the oceans were deep and quiet and nothing but slow pelagic run off. Early expeditions with research ships just recovered pelagic sediment. At the beginning of the 1950s, a new paradigm emerged, which showed sediment moved down stream to deepwater by turbidity currents.” Once formed and then buried, these form the deepwater reservoirs we see globally today. In the 1960s, “along slope” currents, which distribute sediment along slopes, were recognized as another mechanism. “In some cases, those currents are pretty strong and can move sand, etc. We call these contour currents and the deposits contourites.”
In 2011-12, International Ocean Discovery Program (IODP) expedition 339 set out to drill in the Gulf of Cadiz, in the east North Atlantic Ocean, using the Joides Resolution drillship. The 470m-long, DP Joides Resolution, operated by a subsidiary of Norway’s Siem Offshore, comes with non-riser drilling, coring, and wireline logging equipment as well as a fully equipped shipboard scientific laboratories (including sediments, physical properties, magnetics, geochemistry and micropaleontology). The vessel, currently working on the Bengal Fan, started life as the Sedco 471 in 1978, but was converted for scientific research in 1985 and is now part of the IODP. It is able to drill in up to 7000m water depth and has cored holes to 2000m below the seafloor.
During its work in the Gulf of Cadiz, seven sites were drilled, five in the Gulf of Cadiz and two in the West Iberian Margin, with 19 holes, 681 cores recovered, 8km of sediment drilled, and 6.3km cored.
According to the expedition report, the Gulf of Cadiz, due to its contourite sands, is the “world’s premier contourite laboratory,” providing a testing ground for the contourite paradigm.
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A core sample. |
The expedition recovered more than 4.5km of contourite cores, found more than the expected quantity of contourite sands, extensively distributed and “clean and well sorted,” representing “a completely new and important exploration target for potential oil and gas reservoirs,” the report says. While the basin was known to be dominated by along slope currents, coming from the Strait of Gilbralter around the Iberian Peninsula, they also found thick, extensive amounts – more than expected – of sandy contourites.
“We managed to drill through 225m of it, although it was quite a struggle because the sands kept collapsing on us and we got stuck,” Stow says, suggesting it could have been thicker at that point.
These are sediments the oil and gas exploration industry has already drilled though. Stow says Repsol had drilled a well in a similar area targeting a potential oil play below the contourite sequence. When they recovered data from the well, they found they had drilled through 800m of contourite, Stow says, over an area of about 4000sq km. “It could be the biggest sands we know of in modern oceans [as a play] and a significant rival to anything down slope and turbidite,” he says. Furthermore, some of the contourite muds that were drilled through during the IODP expedition had a more than 2% organic carbon content, suggesting they could also be source rocks. “We hadn’t realized that before,” Stow says. “Anything with more than 1% organic carbon content is a potential source rock.”
Which just shows the importance of scientific drilling. “Unless you drill through the rock and bring it back you don’t really know what is there and that applies to science and industry drilling,” Stow says.
“Scientific drilling is designed to ask the big scientific questions about the earth’s history and the workings of the planet as a whole. The spinoff is that all the results are applicable to industry, including the petroleum industry.”
The industry is getting more and more interested in contourites, Stow says. Offshore Brazil, Petrobras, has identified at least part of one of its deepwater fields, the Marlim field, as contourite sands. Statoil has also identified a field that is in a contourite sand sheet offshore Norway, Snorre.
Cairn Energy is also looking at some deepwater West Africa systems that could contain contourites.
“It is just getting known,” Stow says. “We are trying to set up a joint industry research project looking at contourites. About 12-13 companies came to the first meeting to explore funding.”
With the oil and gas industry becoming more accustomed to working in deepwater, as the easier to find resources dwindle, contourites could become a significant new target, Stow thinks, aided by scientific drilling.
“It is more and more difficult to make the big easy finds, so more and more you need to know your science to provide the most likely places hydrocarbons will be.”
Origin story
Scientific drilling dates back to 1968, when it was initiated to help prove the then new theory of plate tectonics. It has since moved on to look at the continental margins, looking at how sea levels affected sedimentation along continental margins and the build-up of continental margins – a key area for the offshore industry – as well as drilling into the ocean crust to discover of what it is made. More recently, the focus has shifted to climate change and environmental change and what ocean records can tell researchers about past environments.
Other research areas close to the petroleum industry’s heart include exploring black shale source rock, to find out where they form when and why.
Scientific drilling is internationally funded through groups of nations and their science research councils. Currently, Europe, Japan and the US are running the Ecord program, or European Committee for Ocean Research Drilling, supported by a number of other nations. It has two drillships operating out of Japan and the US, with different elements of the program run out of different centers, with Heriot-Watt focusing on the facilities and equipment.
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