geologists were hired to project the sedimentary rock layers outcropping on the surface of the ground back into the subsurface to map traps that could hold oil. Later, the seismic method was developed to detect these traps in the subsurface.


1.2.2 The Cantarell Story


Oil fields have even sometimes been found by accident. Out in his little fishing boat in the Campeche Sound one day in 1961, roughly 100 km off the small Mexican town of Ciudad del Carmen at the western edge of the Yucatán peninsula, the fisherman Rudesindo Cantarell Jiménez saw something shiny floating on the sea surface. It was oil stains. Initially, he believed the slicks came from leaks spilling from ships passing through the area, but the oil continued reappearing. What he had discovered were natural seeps rising to the surface through 40m of sea water from the oil-filled formations beneath. In 1968, when he visited the city of Coatzacoalcos, Veracruz, to sell huachinango (red snapper), he told the state-owned oil company Petróleos Mexicanos, or Pemex, about his findings. Tey did not react immediately, but three years later, in 1971, samples were taken and analysed, leading to the discovery of the supergiant oilfield Cantarell, named after the finder. Pemex began producing oil at Cantarell in 1979, and at its peak, in 2004, it produced 2.1 MMbopd. But the story does not end here. Following


the Cantarell discovery, Pemex started an oil exploration programme, including geophysical surveys in south-eastern Mexico, which yielded surprising results. In 1978 a consultant geophysicist with Pemex, Glen Penfield, set out to acquire an airborne magnetic survey of the Gulf of Mexico north of the Yucatán peninsula. Te magnetometer measures the magnetic field of the rocks in order to map out their composition beneath the surface and thus determine the likelihood of finding oil. In the data, however, he observed a huge underwater arc with extraordinary symmetry in a ring 70 km across. Tis observation was inconsistent with what he knew about the region’s geology. Feeling curious, he looked into old gravity data acquired in the 1960s where he found another arc on the peninsula itself. Comparing the magnetic and gravity maps, he saw that the separate arcs neatly formed a circle, 180 km wide, centred near the village of Chicxulub (pronounced cheek-shoe-lube). Together with Pemex geologist Antonio Camargo-Zanoguera they concluded that the huge ring, half on land, half under the Gulf of Mexico, must be an impact crater. Tey felt certain that the ring shape had been created by a cataclysmic event in geological history, such as a meteorite hitting the earth. Te two Pemex geoscientists were allowed to present their


findings at the 1981 conference of the Society of Exploration Geophysicists. Teir talk attracted scant attention, and the ‘exciting news’ of an impact crater that could be the clue to


Figure 1.13a: Gravity map revealing the Chicxulub crater, which formed 65 million years ago in one of the largest collisions in the inner solar system to have occurred in the last four billion years. The crater is believed to be a multi-ring basin with an outer ring about 300 km in diameter (the black circle outlines the ~180-kilometre diameter crater). The colours represent variations in the magnitude of the gravity field at sea level. Positive gravity anomalies (blue is maximum) indicate dense rocks created by melting of the crust, forming a thick pool of impact melt that eventually solidified into a dense mass of igneous rock. Negative gravity anomalies (lower than normal gravity; red) indicate that the near- surface crustal rocks created by impact-generated shattering of the rock have relatively low densities. The Chicxulub basin appears as a circular region in which gravity values are usually lower than the regional values. Observe the ring-like variations in the gravity field. The irregular black line marks the shoreline of the Yucatán peninsula, and the straight lines mark province borders. The original petroleum exploration borehole locations (C1, S1, and Y6) are shown where a drill core was recovered. That core was sufficient to prove the crater had an impact origin. A scientific borehole Yax-1 was drilled in 2001-2002 and produced continuous rock core sequence through the impact melt-bearing rocks in the crater. A borehole was drilled at sea (Chicx-03A) in 2016.


the great mass extinction of the dinosaurs at the end of the Cretaceous received little notice. Although they had a lot of geophysical data to present, they had no rock cores or other physical evidence of a meteor impact, so they reluctantly went back to their Pemex work. However, a Houston Chronicle reporter who was present at


the convention put a story on the front page of the paper on New Year’s Eve, 1981 and he kept telling the story to whoever might listen. Finally, one day in 1990, at a science meeting in Houston, a graduate geology student, Alan Hildebrand, did listen. He had published on arth impact theories and was searching for candidate craters, so he contacted Penfield, who was able to find drill samples from old Pemex wells in the area. Hildebrand analysed the samples, which clearly showed shock-metamorphic materials, providing the first proof that Chicxulub crater in the Yucatán peninsula was the impact site for a 10–15 km diameter meteorite which hit Earth 65 million years ago, at the end of the Mesozoic Era. We now know that when the huge Chicxulub meteoroid


collided with the earth, a massive crater 300 km wide and many kilometres deep was formed. Te explosion was equal to that caused by 100 million megatons of TNT, producing shock waves with a pressure of 660 gigapascals – a pressure greater


9


David A. Kring


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