Tuesday, March 23, 2010

Salt

[Note: A version of this blog entry will appear in World Oil (April, 2010)]

Offshore, the new giants are rumbling. We see a stream of news about Angola's Kwanza and Congo Basin ultra deep water oil, Brazil's giant Tupi field, and deep water Gulf of Mexico headline discoveries like Tiber, Thunder Horse, and dozens more. Aside from being offshore, you might wonder what all these have in common. Two things, really.

First is a relentless push into deeper water, a natural expansion into less explored territory. Successful drilling in ten thousand feet of water has been reported, and five thousand is now almost routine. Like a medical scan, seismic is used to identify features of interest and reduce various kinds of exploration and production risk. From a seismic point of view, deep water presents no special problems. For years, academic geophysicists have been gathering and processing reflection data in some of the deepest water on earth. There are some peculiarities, like a sea floor reflection time of 14 seconds in the Marianas trench (11 km of water), but no intrinsic difficulties.

The second common denominator is salt: a massive headache, and an opportunity, for geophysicists. Salt is very simple and benign stuff when first deposited. It forms in low-slope coastal areas with tidal influx of sea water rich in minerals. Stranded waters evaporate to leave thin salt layers; the tide comes and goes, leading to more evaporation and more salt. In favorable circumstances the salt can build up to great thickness. But at this stage it is just a vast slab of salt. With geologic time, tectonic subsidence, and sedimentation, the salt is buried under an ever-thickening wedge of sandstone, shale, and limestone. As the sediments become more deeply buried, they lithify into rocks and, importantly, become more dense. Salt density changes little with burial, so at some point it is less dense than the overlying rock and it begins to move. Slowly over tens of millions of years, the buoyant salt grinds upward deforming, bending, fracturing, and faulting the overlying rock.

We see today a snapshot of this slow, powerful process. Gone are the days when we think of simple domes composed of smooth, ghost-like blobs of salt. We now understand that salt flows to form a vast and bizarre bestiary of shapes. But what is it that makes salt so seismically difficult?

The problem comes, not from density, but the speed at which seismic waves travel through salt. In the Gulf of Mexico, for example, as we pass down from the ocean surface we first have water with a seismic velocity of about 1500 m/s, then sediments at maybe 2000 m/s, below that a progression of shale and sandstone with wave speeds of 2500-3500 m/s (depending on various rock frame and pore fluid properties), and finally salt at

5000 m/s. This sets up a difficult situation. It is often useful to think about seismic waves as a family of rays, like pencils of laser light. When a ray travels through the sediment and hits the salt, it bends according to a simple rule called Snell's law. The law depends only on the velocity contrast and the angle of the ray relative to a line perpendicular to the salt face. Importantly, the ray bends gently in the pile of overlying sediment, but kinks dramatically at the top salt interface and again at the base when the ray passes back into sedimentary rocks. To make things worse, it turns out geologic salt bodies are rarely smooth. They are irregular, deformed interfaces kicking the rays off in crazy directions. A ray and its neighbor can end up miles apart after whacking into salt.

We care about rays because they must be accurately mapped for some kinds of seismic imaging to work. One of the lessons of the last decade or so is this: The salt is sometimes so complicated that no one can figure out the rays. But rays are a human invention, a useful and simplifying approximation when the earth is not too complicated. For extreme cases, like imaging through 2 km of salt in a soft-sediment basin, the ray idea breaks down or becomes enormously complicated. Consequently, there has been a subsalt push to move away from ray-based imaging (termed Kirchhoff migration) in favor of algorithms that use waves directly (wave equation migration). Unlike rays, wave fields are smooth, continuous, and easy to compute. The ultimate version of wave equation imaging is reverse time migration or RTM. Although RTM has been theoretically understood since about 1982, it is only recently that computer power has enabled people to do 3D prestack RTM on large surveys.

While we were building an understanding of salt tectonics, wave equation migration (and the computer science to make it work), there was a growing sense that something was missing. As researchers went to ever greater lengths to improve imaging algorithms, the improvements were becoming progressively smaller. Rays, waves, better physics, faster computers... it all started to look the same, like we were up against a wall; some kind of fundamental limit to image quality in complex subsalt areas. As it turns out, the next level of imaging came not from better algorithms or computers, but good old fashioned communication. Over the decades, two groups had grown up in offshore exploration, acquisition and imaging. The one a pragmatic field campaign of cables, airguns, and high seas. The other cloistered in research labs, deriving and programming equations on super computers. You can imagine how the company picnic split up.

There have always been voices calling out the message that a fundamental link exists between acquisition and imaging, and that significant advances can only come by tuning both. We know this new way of seismic shooting as wide or full azimuth, but it is hardly new in concept. Land 3D shooting has been full azimuth for decades. Now it is happening offshore.

It is the twin advance of wave equation imaging and wide azimuth acquisition that has allowed us to peer better into the deep, unlocking a subsalt treasure trove around the world.

2 comments:

Anonymous said...

sweet

Chak said...

Nice post.

One quick comment and one little querry.

Actually, the salt density decreases with depth. Ref. http://www.fugro-gravmag.com/resources/Technical%20Papers/cpxsa.pdf

With so much of ray-bending uncertainity, what is the reliability of subsalt Relative Amplitude preservation for offset-dependent elastic interpretations?
Cheers