The search for oil: wooden derricks give way to computers, satellites

The image of oil exploration portrayed in movies such as "Boomtown" is deeply etched into public consciousness. Yet the vision of Clark Gable and Spencer Tracy doing a madcap dance under the black rain of a gusher, and becoming instant millionaires, bears very little resemblance to the present-day petroleum industry.

The "oil game" remains an enterprise for high rollers -- with large risks, high stakes, and potentially large rewards. But in the years since the big Texas oil bonanza, the game has become increasingly sophisticated. Satellites, computers, and scientists garbed in coats and ties who spend most of their time in high-rise office buildings analyzing maps and instrument readouts have replaced the roughneck in cowboy boots as the dominant actors in the industry.

"I cringe when I watch those old movies: They are so unrealistic," confesses Steve Lange of Inexco, a small oil and gas company.

The days when a couple of tough-minded, independent souls could play a hunch, erect a wooden derrick, drill a few hundred or thousand feet and hit "pay" are long gone. Today drilling rigs typically cost $30,000 a day to operate. Even major oil companies like Amoco do not own their own rigs but lease them. These rigs are booked up as much as a year in advance.

Stronger steels and specialized drill bits for specific types of rock have increased reliability and enabled drillers to reach ever greater depths. It is not uncommon for modern oil and gas wells to penetrate more than 15,000 feet beneath the Earth's surface. Better materials also have allowed oil men to drill into areas where hydrocarbons are contained at tremendous pressures. Often these reserves have been known for some time, but attempts to tap them were considered too risky.

Increasingly, the process of deciding where to drill exploratory wells begins with satellite imagery. General Electric's space division has announced a new service for oil and gas exploration called Geopak. This consists of heavily computer-processed pictures of specified areas taken by Landsat, the US earth-resources satellite.

These images reveal subtle surface features that signal possible reservoirs of oil and natural gas to the experienced geophysicist, explains GE's Kent Stow. Satellite imagery is gradually replacing the more expensive use of aerial photography for minerals exploration, he says. Once a structure of interest is located, geologists use seismic profiling to peer through the thousands of feet of solid rock in search of underground traps that may be filled with valuable hydrocarbons.

Geophones, sound listening devices, are placed along a "line." An underground dynamite blast is detonated, and the sound reflections at each of the listening posts are carefully recorded. It currently costs between $6,000 to $8,000 per mile to run these seismic lines, says Leo Girard of Kenai Oil and Gas. And it frequently takes hundreds of miles of these lines to map out an entire field.

In the past, the geologists usually ran these lines along roads because it was easier and cheaper. Now, with the use of helicopters to carry men and equipment into remote areas, seismic lines are run in nearly inaccessible areas if satellite photos suggest they have high potential.

From these seismic measurements, maps of the acoustic properties of the underlying strata are compiled. On these maps, which consist of hundreds of wiggly lines, "we look for closures" explains Mr. Girard. A closure is an inverted bowl or trough of dense, impermeable rock such as basalt capping a layer of limestone or sand: a natural trap for oil and gas fluids.

The interpretation of these traces is anything but certain, however. Sound waves can behave in a number of strange ways when traveling underground. As a result spotting these underground traps remains as much an art as a science.

This is particularly true in areas of extremely complex geology such as the Overthrust Belt in the Rocky Mountains. This area, the current focus of oil and gas exploration in the continental United States, remained inaccessible to oil men until recently because of this complexity. It was not until sophisticated computer-processing techniques were applied to seismic mapping that it became possible to pinpoint the reservoirs in this area, petroleum geologists say.

"Even so, it took a lot of dreamin' to come up with structure [there]," says Mr. Girard.

Once a well is started, it is not enough simply to drill. Each foot of the way the rock must be analyzed, a process known as "well logging."

Drilling mud is forced down through the drill stem. Periodically, it is laced with a chemical tracer. The mud picks up chips of rock that the bit is grinding away and carries them to the surface. Microscopic analysis of these "fines" enables geologists to determine the type and pertinent characteristics of the various rock layers. If the well strikes "pay," this information helps determine the size and production potential of the reservoir. If it is "dry," the information helps in the siting of further wells.

Besides analyzing rock fines in the drilling mud, the scientists lower various types of instruments into the wells to increase their knowledge of underground structure. In this fashion they measure the electrical and acoustic properties of the rock. They also record the levels of penetrating, gamma radiation at various depths. Each bit of information helps them develop a basic understanding of the area and increases their chances of locating oil and gas, a process that remains a matter of playing the percentages despite all the sophisticated technology.