COULD it be that many of the world's oil fields are refilling themselves at nearly the same rate they are being drained by an energy-hungry world?
A geochemist at the Woods Hole Oceanographic Institution in Massachusetts says she believes that hitherto undetected gas and oil reservoirs lying at very great depths within the earth's crust could stave off the inevitable oil depletion much longer than many experts have estimated.
The scientist, Dr. Jean K. Whelan, whose research is part of a $2 million Department of Energy exploration program in the Gulf of Mexico south of New Orleans, has found evidence of differences in the composition of oil over periods of time as it flows from greater to shallower depths. By gauging degradative chemical changes in the oil resulting from action by oil-eating bacteria, she infers that oil is moving in quite rapid spurts from great depths to reservoirs closer to the surface.
Skeptics of Dr. Whelan's hypothesis acknowledge that oil is almost certainly flowing into certain reservoirs from somewhere, but say her explanation remains to be proved, as does the exact extent of the phenomenon.
A site in the gulf of particular interest to the Pennzoil Exploration and Production Company and several independent scientific teams, including Dr. Whelan's group, is Eugene Island Block 330, which is not an island but a patch of sea floor 700 feet beneath the water's surface. Discovered in 1972, an oil reservoir some 6,000 feet beneath Eugene Island 330 is one of the world's most productive oil sources; it has yielded more than one billion barrels, or 42 billion gallons, and is still going strong.
But Eugene Island 330 is remarkable for another reason: Its estimated reserves have declined much less than experts had predicted on the basis of its production rate.
Dr. Whelan's somewhat controversial hypothesis is a possible explanation.
Although the reservoir from which Pennzoil is pumping oil was formed at the time of the Pleistocene epoch less than two million years ago, oil now being recovered from the reservoir has a chemical signature characteristic of the Jurassic period, which ended more than 150 million years ago, Dr. Whelan said. The implication, she believes, is that highly pressurized oil from lower levels of "stacked" reservoirs is frequently breaking through geological barriers and "burping" upward, eventually reaching the reservoir from which oil is being pumped. The source of the pressurized gas Dr. Whelan believes to be powering the process is a bed of Jurassic period "source rocks," more than 30,000 feet deep, which are rich in very hot hydrocarbons.
Oil created during the Jurassic period from the decayed bodies of plants and animals that lived during the age of dinosaurs would initially have been a liquid, but in some cases, geological processes are believed to have gradually dragged oil reservoirs downward to great depths, where pressures are enormous and temperatures are greater than 700 degrees Fahrenheit. Under these conditions, oil is "cooked," decomposing into hydrocarbon gases consisting mainly of methane.
The methane, however, cannot exist in its normal gaseous form at such pressures and temperatures, but is transformed into a "supercritical fluid" -- neither a gas nor a liquid but something in between. Large amounts of oil can be dissolved in supercritical methane, and the oil-and-methane mixture is probably capable of flowing upward from deep reservoirs through faults, cracks and geological "plumbing" to higher reservoirs. At the higher levels where temperatures and pressures are lower, methane can no longer remain in its supercritical state, and the mixture of gas and oil separates into its respective components.
"It could be," Dr. Whelan said, "that at some sites, particularly where there is a lot of faulting in the rock, a reservoir from which oil is being pumped might be a steady-state system -- one that is replenished by deeper reserves as fast as oil is pumped out." Extensive fault systems like those along the California coast might offer such a geological environment, she said.
The discovery that oil seepage is continuous and extensive from many ocean vents lying above fault zones has convinced many scientists that oil is making its way up through the faults from much deeper deposits. Faulting associated with such seepage is common along the Pacific Coast, in parts of the North Sea, and the Gulf of Mexico, among other areas, Dr. Whelan said.
The amounts and types of hydrocarbons lying at great depths are highly uncertain, because oil companies have had little incentive to sink exploratory wells to depths of 20,000 feet or greater. The cost of drilling increases exponentially with depth, and there are still ample supplies of oil in relatively shallow reservoirs.
Dr. K. K. Bissada, a geochemist at Texaco Inc. in Houston, is doubtful that a true steady state exists in reservoirs that seem to be refilling themselves.
"I think we pump oil out much faster than oil can come in," he said. "But from a long-term perspective, I believe that hydrocarbons are coming in from great depths and are filling the newer reservoirs at shallower depths.