It all started in 2004. Shell geologists were using state-of-the-art seismic imaging for below the ocean’s surface combined with the best available computer analysis. But they weren’t seeing the underground detail needed to confidently explore for new oil under a salt dome in the already productive Mars basin.
While their seismic images were hazy, it was crystal clear that the “Deimos” prospect’s geology was a lot more complex than what the scientists were seeing.
Sub-surface salt was the problem, scattering the seismic sound waves in multiple directions, making it nearly impossible to read the geologic story. So, the geologists went back to the lab, developed more powerful computer analysis tools and advanced technology for gathering subsea seismic data.
“Try looking at the floor directly through the top of a table. You can’t see it. That is about the equivalent of an older version of seismic. Now, move away from the table so you can see under it. That is the advanced form of seismic we have today. It provides us with a different perspective and allows us to enhance our portfolio with both new and existing projects.”
Ocean bottom seismic
An important new technology, called “ocean-bottom seismic”, allows geologists to see formations below salt more clearly than ever. Much like you might press your ear to a wall to better hear something in the next room, underwater robots array hundreds of seismic receivers in a large grid, directly on the seafloor.
Sound waves, generated with compressed air, pass through the ocean into the earth where they reflect off underground rocks and are recorded by receivers on the seabed. The recordings are processed with computers to create a detailed picture of what’s underground.
The result is brighter, clearer images that can show Shell engineers exactly where to drill. In 2009, they returned to “Deimos” armed with this insight and made a major discovery. Additional ocean-bottom seismic surveys uncovered more reservoirs nearby, adding up to Mars B – another promising Shell project in the Gulf of Mexico.
Ocean-bottom seismic is delivering remarkable improvement in subsurface picture clarity – particularly in areas of complex geology. This improved resolution has already revealed oil and gas reserves that, otherwise, might have been overlooked.
The new Mars B, West Boreas, and South Deimos fields, producing through the Olympus platform, are testament to the exploration success possible with new technologies.
The Gulf of Mexico’s twisted underground landscape
In much of the Gulf of Mexico, oil producing sediments have been deposited on top of a thick, ancient bed of rock salt – the remains of shallow seas repeatedly evaporated as the Gulf slowly opened hundreds of millions of years ago.
Under heat and pressure from overlaying rock layers, the salt softens and tends to push up through the rock in columns, like the colored blobs in a Lava Lamp, forming mushroom-like canopies. Some formations rise as much as six kilometers from the deep base salt layer.
These salt canopy overhangs often act as structural “traps”, accumulating oil and gas released from rocks fractured by the rising salt. But, while the salt traps the oil and gas, it also hides it. The speed of sound in salt is variable and can be twice that of the surrounding sediments.
This deflects seismic sound waves in many different directions, clouding the view of any rock layers beneath. Because of this, understanding what lies below a salt structure using older narrow seismic surveys from the ocean surface, is something like looking at the world through a slit covered by frosted glass.