For those who despair that old dogmas can't be taught new tricks, there's hope - Marathon Oil explorationists have defied conventional wisdom, drilled into a sand injectite in the North Sea and discovered oil.

This is a departure on many levels. In the risk-averse climate that surrounds the oil and gas industry these days, anyone taking a chance is newsworthy in and of itself. But what Marathon did was even more surprising given that sand injectites have been poorly understood and often avoided with the drillbit at all costs. If anyone drilled into one in the past it was a mistake, not a drilling plan.

Sand injectites are formed when depositional sand beds become pressured during burial. The fluids in the sand try to escape, with the result that spikes and sheets of sand squeeze up through the overburden. The lighter the fluids within the sand, the more easily they move and the more buoyant they become.
Geoscientists and drilling engineers have typically trodden carefully around these features because of the belief that they probably contain quantities of gas under pressure, making them drilling hazards. But a research consortium led by the University of Aberdeen has been studying injectites with the idea that they may, in fact, contain commercial quantities of hydrocarbons and might also provide links to lower layers of hydrocarbon-rich depositional sand layers.

Dr. Andrew Hurst from the university has studied injectites for years. "The interest started when we recognized in a lot of the turbidite reservoirs that we were looking at that not all of the rocks that we were seeing were depositional," Hurst said. "We were looking at core images that didn't look like the sand had been deposited; something else had happened to them. We recognized that these features were in fact produced by some sort of remobilization of the sand."

In turn, researchers noticed that the features correlated with odd geometries which cross-cut sedimentary layering on all scales, from a few inches to hundreds of feet.

Though little was known about the injectites themselves, they were easier to study in the North Sea because of so much other useful data. "There are several deepwater turbidite fields that have these features on them," Hurst said. "People have been producing from these fields for years and have been discovering as they've gone along that the fields weren't what they once thought they were. They're not big depositional piles of sand; they're depositional piles of sand with arms and legs on them, almost. And discovering those arms and legs has changed the way they've developed the fields."

Seven companies were involved in the original consortium, including companies that owned acreage with injectite characteristics. Marathon was not involved in the original consortium but approached the group when the research was complete.

"They had purchased some acreage in Norway that had conventional exploration targets but also this very strange feature, called the Hamsun feature," Hurst said. "We had done some work on the feature in our consortium and had some very nice images of it in our 3-D visualization center. We also showed them pictures of some outcrops in California that look like this.

They then took one of our masters students, a geophysicist doing her degree with us, and she did a really nice piece of work for them looking at this feature."

Marathon themselves conducted detailed seismic interpretation and built a 3-D geocellular model describing the project. Their geoscience team became convinced that the best explanation for the seismic anomalies was that they were produced by highly porous oil-filled sands.

They were right. They drilled a well in February that discovered an oil column of more than 300 ft (100 m) at a relatively shallow depth. Extensive cores were taken, which in turn have found their way back to the University of Aberdeen for further study.

"The symbiosis between us has continued," Hurst said. "They're hopefully going to support more generic research, and they've already supported some research on their own data."

Educated guesses

Prior to the Marathon well, only two boreholes through sand injectites (both in the Hamsun feature) existed, though wellpaths had accidentally tagged the sides of them on occasion. The original consortium relied on a variety of data to form its conclusions about the features.

"They were actually first described in 1827," Hurst said. "And we've made little progress since then."
Better seismic has resulted in better imaging, at least on the macro scale. But there are still numerous questions about why they form, the orientations they form in, the way they're structured internally and their microscale features.

Vast development of sand injectites is not, by itself, likely to have a significant impact on global reserves. But their connection to underlying depositional sands is intriguing. "Very substantial proportions of fields that were thought to be depositional can actually have their oil in injected sands," Hurst said. "And that usually means that they're in the shallower parts of the section, which is a bit cheaper to get to."
In addition to the North Sea, sand injectites are fairly common in the large fields offshore West Africa as well as other parts of the world. "These things are like huge sheets, and they're protruding through fractures in the Earth's surface," Hurst said. "But the fractures are very small. They're all inter-connected, so it's an amazingly connected system of extremely permeable sand in a background of mud.

"In areas like offshore West Africa, there are ancient canyons filled with sand. They also have very thick layers of sand and mud that do not contain hydrocarbons, and those act as pressure seals between the hydrocarbon-bearing sands. If there are injectites going through these layers, it gives the operators greater permeability on a field scale than they had originally envisaged."

In other words, they're like well bores with multiple perforations, only much, much bigger. An understanding of the injectites' role in hydrocarbon movement through the field can help operators avoid such things as water injection wells since the features improve the water drive of the whole petroleum system. A water injection well drilled in the wrong spot will send water into the injectite rather than pushing it throughout the reservoir.

A changing mindset

The fact that Marathon discovered a good-quality oil-bearing sand rather than an overpressured gas accumulation is changing the way the industry views sand injectites. Its success has paved the way for a second consortium and more interested parties. Companies who turned down the invitation to participate in Phase 1 are showing a more positive response to Phase 2. Hurst credits the exploration team at Marathon for taking a risk that was well worth taking.

"The industry is so conservative, and it doesn't like change," he said. "I think Marathon deserves a huge pat on the back for listening to academics, for doing some good work and for sweet-talking their management. Imagine what would happen if nobody ever took any chances!"

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