SUBSALT GULF STRIKE PROVES CRUCIAL IDEA

Cheering an oil or gas discovery is hazardous business. Initially robust flow rates and pressures can quickly subside. Water can encroach. Appraisal wells can yield unwelcome information. Economic hurdles can simply be too high. But all wildcat wells test ideas. And some ideas hold the potential to change longstanding beliefs. Any wildcat that upholds such an idea deserves notice, whatever its commercial fate.

Oct. 11, 1993

3 min read

Results from wildcats of this pivotal type are coming to light in the Gulf of Mexico. Phillips Petroleum Co. and partners Anadarko Petroleum Corp. and Amoco Production Co. this month reported test rates as high as 7,256 b/d of oil and 9.944 MMcfd of natural gas through a 32/64 in. choke with 7,063 psi flowing tubing pressure from a well in 370 ft of water off Louisiana. More important than these impressive numbers is the idea that the wildcat tested.

THE SALT PROBLEM

The idea is simply that exploratory targets beneath formations of salt can be identified and depicted in seismic records with a fair degree of accuracy. Because sound travels through salt several times faster than it does through sedimentary rock, seismic energy turns to hash when it encounters salt formations. Not so long ago, it was common to assume that there were no sediments beneath salt or that, if there were, seismic reflections from them would be hopelessly scrambled by salt-sediment velocity contrasts.

Recent advances in geophysical technology have changed all that. It turns out that underground salt comes in a greater variety of shapes than was once believed. Many of those shapes involve extensive horizontal reach. And beneath those horizontal sheets of salt lie sediments. A deepwater well drilled by Exxon Corp. and Conoco Inc. in 1990 on Mississippi Canyon Block 211 showed that hydrocarbon pay can exist beneath salt. The Phillips well shows not only that hydrocarbon-bearing sediments exist beneath salt but that carefully processed seismic data can create images of subsalt structures.

It's not easy. To make accurate velocity corrections, geophysicists have to know thicknesses of horizontal salt bodies. That means they must start with an accurate image of the top of salt, which, especially on the Gulf of Mexico shelf, can be greatly deformed. Then they must develop an accurate image of the base of salt, which presents another set of challenges. Once they know salt thickness they must calculate the velocity of sound traveling through it and account for how the velocity differences between salt and adjacent sediments influence seismic energy. The process is confounded by the tendencies of salt bodies to be bounded by faults and to have steep flanks.

But the payoff may be huge. Horizontal salt occurs under as much as 60% of the Gulf of Mexico shelf. Until now, the subsurface beneath all that salt was thought to hold no potential. The Exxon/Conoco and Phillips/Anadarko/Amoco wells thus place a huge volume of sedimentary rock in play.

NEW WAY OF THINKING

One welcome consequence is that the Gulf of Mexico, which already was making a comeback this year thanks to rising natural gas prices, once again looks like a world-class exploration theater. But subsurface salt is common elsewhere, which means that the gulf discoveries have added to exploratory potential around the world.

Even if the gulf strikes never prove commercial, they have ratified a new way of thinking about the underground. They have given explorationists reason to look for hydrocarbons in places they once ignored. What's most important, they have reminded the world of the awesome power of ideas and eternal promise of technology.