WHAT was bad for the dinosaurs 65 million years ago has turned out to be very
good for Mexico. The asteroid that hit Yucatan may have wiped out the giant
reptiles, but it also helped form the country鈥檚 richest oilfield鈥攁nd other
reserves may still await discovery.
The Cantarell oilfield is Mexico鈥檚 largest by far, producing 1.2 million
barrels a day. The 鈥渂lack gold鈥� originated in 150-million-year-old sediments
rich in organic material, says Manuel Grajales-Nishimura of the Mexican
Institute for Petroleum in Mexico City.
When the asteroid hit shallow water about 350 kilometres away from Cantarell,
it threw up debris and triggered tsunamis that swept limestone over the
sediments, trapping hydrocarbons under a layer that is now about 300 metres
thick. Finer debris then settled on top, creating a thinner, impermeable clay
seal. A final layer, which capped the oilfield under a dome, formed later and
was unrelated to the impact.
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During the next 50 million years, chemical reactions transformed the
limestone into magnesium-rich dolomite, a more porous material better suited to
trapping oil. Underground heat began to break down the buried organic compounds,
and hydrocarbons started to fill the reservoir layer about 15 million years
ago.
The impact did not create the oil, Grajales-Nishimura says, but it did create
the right geology to trap it for drilling. The layers formed by the impact are
responsible for about one third of the country鈥檚 oil production and the
discovery suggests that other thick layers of impact debris around the Caribbean
may also hold oil.
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Source:
Geology (vol 28, p 307)