Let’s get deep

Ultra-deepwater exploration on the Atlantic margin could be the opportunity the offshore industry is looking for, according to geoscience firm Spectrum. Elaine Maslin reports.

Maersk Venturer drillship.Image from Maersk Drilling.

In today’s low oil price, cash strapped environment, drilling in ultra-deep water might not seem an obvious gambit. However, Spectrum Geo thinks there are some good, and big, reasons for doing so – and there isn’t a better time than now.

The Norway-based firm, having filtered through 2D seismic acquired over the last two years, has identified a new ultra-deepwater Atlantic margin play, which focuses on the basin floor (or abyssal plain). This is an area beyond the various Atlantic margin shelves, which are nearer to shore, or slopes, where recent deepwater discoveries offshore Senegal and Mauritania and others have focused industry attention. The basin floor stretches out from the base of the slope to some 150km out into the Atlantic, offering potentially large, widely spread resources.

With rig rates at a significant discount, drilling technology now available for such feats and, crucially, the prospect of massive finds, making exploitation economically attractive, Spectrum thinks there’s a huge opportunity.

So, why hasn’t the industry looked at this before? It’s a resource that hasn’t been found because no one has been looking, says Neil Hodgson, Spectrum’s executive vice president of geoscience. “It is an oddity of exploration in that we have been shooting seismic in deepwater for a long time. But when in 3-4km of water, people haven’t been too interested in looking for prospects because they didn’t think they could drill it.”

The Atlantic margins. Images from Spectrum Geo.

That has now changed and, despite the low price, deepwater drilling records are set to be broken this year. India’s ONGC holds the current record at 3174m deepwater offshore west India. This is set to be broken by France’s Total when it drills the Raya exploration well offshore Uruguay in 3400m water depth. Later this year, Anadarko is expected to drill an even deeper well offshore Colombia, Hodgson says, tapping resources that could open up another gas basin like Rovuma offshore Mozambique, in terms of scale.

Interestingly, Hodgson notes that, except for a very short period of time, most deepwater drilling records have been set when oil has been at less than US$50/bbl. “You don’t need a higher oil price in deepwater because it works against you – increasing exploration costs,” he says. In fact, thanks to $30-40/bbl oil prices, deepwater rig rates have plummeted from $650,000/d some 19 months ago, to around $250,000/d. “That means a $150 million well 19 months ago would now cost you $60 million. All of a sudden you realize this is the only time to drill ultra-deepwater.”

Deepwater production technology has also been on an upward curve, Hodgson says, especially for gas. He cites the speed at which Noble Energy was able to develop the Tamar deepwater gas field offshore Israel. It was drilled in 2009 in 1500m water depth. “Noble noticed if they could do a subsea tieback to shallow water infrastructure and get it in stream within three years, which is what they did. And that’s unprecedented.

Play model for south Atlantic floor fans.

“This will be the way to develop these gas resources in the future, providing it is a nice dry gas and it can be tied back fairly easily.” It is likely to be the same type of solution for [Eni’s] Zohr [offshore Egypt], which could be a subsea tieback to Temsah [production facilities]. For gas, this is more economical and then you are only limited by your access to markets. Total also recently brought onstream the Laggan Tormore gas condensate fields, on the UK Continental Shelf, as a 140km subsea tieback to shore.

Hunting elephants

Spectrum started first by looking at its seismic data differently, converting it into depth images rather than time images (which use the time it takes the sound in seismic shoots to travel down through the rock and back again). “All of a sudden, you can see trapping potential down to 4km of water depth,” Hodgson says. “We haven’t seen them because we have not looked there before and we have always looked in time, not depth. It was a big surprise to me.”

It’s not just as simple as seeing the trapping mechanism. Source rock has to exist, and it does, throughout almost all of the Atlantic basins, Hodgson says. National Oceanic and Atmospheric Administration mapping suggests its existence and wells drilled deep enough, to date, offshore Namibia, by HRT, and Brazil, in the Sergipe basin, by Petrobras, and others have proved it.

Seismic reconstruction across the conjugate margin.

It comes from when the super-continent Gondwana broke up, at the end of the Jurassic and in the early-cretaceous periods, creating a 300km wide basin. This was then filled in with organic rich mud. As the two continents, Africa and South America spread further apart, these layers remained attached to their continents, creating 150km belts along the coastline with underlying source rock. These muds have now been buried deep enough, at 3.5-4km, to create the temperatures to generate oil and gas – 100-150°C.

Next, there need to be reservoirs in which this oil or gas could collect and these were found; sands, which fan out on the basin floor, deposited some 100 million years ago. The difference between these and the sands that have been drilled on the Atlantic margin slopes – the sloping areas which drop from the shelf down to the basin floor – are that the slopes are dominated by channels of sand, like gutters, transporting the sand down the slope, which means higher risk in terms of drilling, for finding seals and to target the right areas. The Sergipe wells offshore Brazil have targeted these channels, Hodgson says.

On the basin floor, the sands fan out, however, creating – yet to be drilled – huge potential deposits. These are covered by multiple kilometers of mud stone, which hold the oil in place. Then, traps, to create pools of oil in the sands, have been created by the pressure of the shelf by the weight of layers and layers of mud stone, which cause “plate-scale” subsidence, creating huge traps. “They are actually phenomenal,” Hodgson says. “Some of the biggest structures would stretch from London to Manchester.”

So, when might we see this play tested? Hodgson is hopeful that Mauritania might be a good proving ground. Kosmos Energy has drilled on the slope here in 2400m water depth. The basin floor offshore Mauritania is in a modest 3000m water depth, making it reachable, Hodgson says. This is due to the Cape Verde islands – intrusions near Mauritania, which lifted up the basin floor. Mauritania currently has some open acreage, which – as a result of the above – Hodgson thinks could be the most prospective in the world at the moment.

Meanwhile, Total’s work offshore Uruguay will help increase confidence in deepwater exploration. “It is trying for a slightly different target [to Spectrum’s concept] and it has got a very high chance of success, which will help increase confidence in ever deeper water exploration,” Hodgson says.