Ben Hewett is a Shell geophysicist and leader of the seismic team that located the Cardamom deposit now flowing oil and gas through an updated Auger platform:
“It’s unlikely we would have found the Cardamom field without wide azimuth seismic, and if we did, drilling would likely have been much more expensive.”
Guided by wide azimuth (WAZ) seismic imaging and using fiber optic drilling technology, Shell drilled the record-setting Cardamom discovery well from Auger in 2010 – snaking the drill pipe down under a salt dome overhang to nearly 8.5 kilometres (5 miles) below the sea bed and 5 kilometres (3 miles) away from the platform’s drilling rig.
Hewett explained the salt that often hides the oil is also why it’s there in the first place. In much of the Gulf of Mexico, oil producing sediments were 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 the enormous heat and pressure from miles of subsequent rock deposits, the salt softens and tends to push up through the rock in columns, somewhat like the colored blobs in a Lava Lamp, forming mushroom-like canopies. Some rise more than six kilometres (nearly 4 miles) from the deep base salt layer.
As they push upwards these columns and overhangs often create “traps”, accumulating oil and gas released from rocks fractured by the rising salt. Many historic oil discoveries, such as the famous gusher at Spindletop, occurred near salt formations.
But, while the salt traps and holds the oil and gas, it also hides it. The speed of sound in salt varies, jumbling the return from seismic echoes and clouding the view beneath. That’s where WAZ can help.
If you think of seismic sound transmissions as a kind of “light” illuminating the rocks beneath the sea bed, then wide azimuth is like a floodlight on the salt formation, allowing a larger, clearer picture of what’s down there, like stepping back from a table to see what’s under it.
The data Hewett and his team of geoscientists analyzed to create their subsurface picture came from electronically linked ships towing seismic sensors across the area in a swath several miles wide. That separation produced the necessary “standoff” distance to allow them to see underneath the salt dome cap to the oil trapped below.
With the successful discovery well and decision to develop Cardamom, Shell called in a newer seismic technology to guide the drilling program. Ocean bottom seismic (OBS) places seismic sensors, called “nodes”, directly on the sea floor. Just as pressing your ear to a wall lets you better hear sounds in the next room, placing sensors on the sea floor delivers a clearer picture of the underlying rocks. Hewett says another round of OBS is planned to monitor Cardamom and better manage production.