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08/04/94

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Method for age-dating oil is developed

STANFORD -- The Jurassic (180 million to 140 million years ago) was a very good age for oil formation. So too was the Cretaceous (140 million to 65 million years ago). But, until now, oil industry experts have lacked a direct way to date their crude.

That may change, however, with the development of a biological marker system developed by a joint academic-industry research team headed by J. Michael Moldowan, a research professor at Stanford University. The new dating method is described in the August 5 issue of the journal Science. Other members of the team are Jeremy Dahl from the Chevron Petroleum Technology Company, Bradley J. Huizinga from ARCO International Oil & Gas Company, Stanford science and engineering technician Frederick J. Fago, Leo J. Hickey from Yale University, Torren M. Peakman from the University of Bristol, England, and David W. Taylor from Indiana University-Southeast.

Their dating system measures the relative amount of an organic compound, oleanane, in the oil. Oleanane is highly associated with the angiosperms, flowering plants that have evolved and spread since the early Cretaceous. Many flowering plants produce derivatives of oleanane, called oleanoids, that are toxic to predators.

Because angiosperms become more plentiful as time progresses, younger organic deposits that are converted into oil are likely to contain more angiosperms. Because the geological processes involved convert the oleanoids in the angiosperms into oleanane, the more oleanane found in a given oil sample, the more recent it is likely to be.

The method is not a very precise yardstick. Oils of any age can lack oleanane if flowering plants were not part of the material from which it formed. But lack of oleanane is a significant clue that the oil may have formed in the Jurassic or older times, before angiosperms evolved. If the compound is present in relatively small amounts, the crude is almost certainly Cretaceous or younger. If it contains large amounts of the organic substance, on the other hand, its pedigree most likely dates from the post-Cretaceous or Tertiary Age (65 million to 5 million years ago).

But "even this crude level of dating can be very useful in petroleum exploration," said Moldowan, who recently moved to Stanford from Chevron.

Dating the oil in a given reservoir provides important clues to its source formation, something that often is not obvious because oil can migrate large distances underground. Therefore, dating can help petroleum geologists determine the specific "plays" - geological environments - on which they should concentrate their efforts, Moldowan said.

To calibrate this scale, the scientists first measured the amount of oleanane in 103 samples of petroleum source rock that could be dated by conventional means.

The researchers knew that they could not use the absolute concentrations of oleanane because the amounts of this compound can vary by a factor of a thousand or more, depending on how much heat a given sample has undergone. They have attempted to correct for this problem by using the ratio of the oleanane concentration relative to a very similar chemical, called hopane, that is ubiquitous in crude oil and whose precursors are produced by bacteria. Because hopane should degrade at about the same rate as oleanane, the ratio of the two should be relatively independent of the petroleum's thermal history, the researchers said.

When they obtained these ratios and compared them with the age of their samples and the number of families of angiosperms in the fossil record, they found that, although there were some differences, variations in the level of the biological marker are broadly consistent with the fossil record. These differences support Taylor and Hickey's hypothesis that the ancestral angiosperm were herbaceous.

The researchers did get a surprise, however, when they analyzed a 307-million-year-old concretion associated with Pennsylvanian Age coal seams in Illinois. Despite the fact that this "coal ball" dates to well before the period when flowering plants first came on the scene, they found measurable levels of oleanane.

"It must be that specific piece of coal," Moldowan said. "To the best of my knowledge, oleanane has never been found in a sample this old before. That could mean that we are seeing oleanane being produced by the ancestors of the angiosperms. This lineage has recently been referred to as 'stem-angiophytes.' "

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