Impact enthusiasts searched the globe for the


massive crater that would have resulted from such an impact. Attention focused on the Gulf of Mexico after reports of iridium-rich K-T material overlying poorly sorted carbonate-rich sediments in Texas in the midst of mudstones typical of deep- water sediments.36


An impact-generated tsunami


had been put forward as the cause of the anoma- lously coarse-grained sediment layer. Through geophysical surveys conducted in the 1980s and 1990s, researchers rediscovered the circular gravity low that PEMEX had identified decades earlier (below).37


The crater is buried beneath up to 1 km [0.6 mi] of younger sediments.


Geophysical data show the outer ring of the


crater to be approximately 180 km [110 mi] across. Modeling studies indicate that the tran- sient crater may have been up to 100 km [60 mi] across, displacing material to a depth of 34 km [21 mi] and excavating target rock to a depth of 14 km [9 mi].38 Intense shaking, whether from the impact or


from tsunamis it generated, caused widespread collapse of the continental slopes of North America, South America, West Africa and Europe.39


The carbonate platforms of the Yucatán


Peninsula slumped into deeper water and were covered by a layer of ejecta. With time, these car- bonate rocks became important oil reservoirs.


The fields of the Villahermosa area and the


prolific Bay of Campeche, including the Cantarell complex—Mexico’s largest oil field—produce from these carbonate debris-flow breccias.40


With


35 billion bbl [5.6 billion m3] initial oil in place, the Cantarell complex produces 60% to 70% of its oil from the Chicxulub impactites. The reservoirs comprise approximately 300 m


[1,000 ft] of highly productive dolomitized lime- stone breccia underlying a 30-m [100-ft] less pro- ductive zone of reworked breccia and ejecta (next page, top).41


In the lower layer secondary


vuggy porosity is common and average porosity ranges from 8% to 12%. Permeability is 3 to 5 D. Overlying the layers is an impermeable seal


22°


about 30 m thick. This bed, which has also been dolomitized, is made up of fine-grained impact ejecta, including shocked quartz and feldspar, and clay minerals interpreted as alteration prod- ucts of impact glass (next page, bottom). These


36. Bourgeois J, Hansen TA, Wiberg PL and Kauffman EG: “A Tsunami Deposit at the Cretaceous-Tertiary Boundary in Texas,” Science 241, no. 4865 (July 29, 1988): 567–570.


21°


37. Penfield GT and Camargo Z A: “Definition of a Major Igneous Zone in the Central Yucatan Platform with Aeromagnetics and Gravity,” Expanded Abstracts, 51st SEG Annual International Meeting and Exposition, Los Angeles (October 11–15, 1981): 448–449.


Hildebrand AF, Penfield GT, Kring DA, Pilkington M, Camargo Z A, Jacobsen SB and Boyton WV: “Chicxulub Crater: A Possible Cretaceous/Tertiary Boundary Impact Crater on the Yucatán Peninsula, Mexico,” Geology 19, no. 9 (September 1991): 867–871.


U S A 20°


38. Kring DA: “Dimensions of the Chicxulub Impact Crater and Impact Melt Sheet,” Journal of Geophysical Research 100, no. E8 (August 25, 1995): 16979–16986.


90°


Chicxulub structure


M E X I C O Bay


of Campeche Cantarell field


Villahermosa Bochil


0 0


km 300 mi 300


C E N T R A L A M E R I C A


> The Chicxulub impact crater. A series of concentric features in the gravity signature (top right) reveals the location of the crater. The coastline is shown as a white line. This image was constructed from gravity measurements taken by PEMEX beginning in 1948 and augmented by recent work of researchers from the Geological Survey of Canada, Athabasca University, the Universidad Nacional Autónoma de México and the Universidad Autónoma de Yucatán. White dots represent the locations of sinkholes (solution-collapse features common in the limestone rocks) called cenotes. A ring of cenotes tracks the outermost gravity gradient feature. The cenotes are developed in near-surface Tertiary limestones overlying the crater. Somehow the crater is able to influence the properties of the younger rocks that cover it. (Image courtesy of Alan Hildebrand.)


89°


39. Day S and Maslin M: “Linking Large Impacts, Gas Hydrates, and Carbon Isotope Excursions Through Widespread Sediment Liquefaction and Continental Slope Failure: The Example of the K-T Boundary Event,” in Kenkmann T, Horz F and Deutsch A (eds): Large Meteorite Impacts III. Boulder, Colorado: Geological Society of America: GSA Special Paper 384 (2005): 239–258.


40. Grajales-Nishimura JM, Cedillo-Pardo E, Rosales-Domínguez C, Morán-Zenteno DJ, Alvarez W, Claeys P, Ruíz-Morales J, García-Hernández J, Padilla-Avila P and Sánchez-Ríos A: “Chicxulub Impact: The Origin of Reservoir and Seal Facies in the Southeastern Mexico Oil Fields,” Geology 28, no. 4 (April 2000): 307–310.


Magoon LB, Hudson TL and Cook HE: “Pimienta- Tamabra(!)—A Giant Supercharged Petroleum System in the Southern Gulf of Mexico, Onshore and Offshore Mexico,” in Bartolini C, Buffler RT and Cantú-Chapa A (eds): The Western Gulf of Mexico Basin: Tectonics, Sedimentary Basins, and Petroleum Systems. Tulsa: The American Association of Petroleum Geologists, AAPG Memoir 75 (2001): 83–125.


41. Grajales-Nishimura et al, reference 40.


Murillo-Muñetón G, Grajales-Nishimura JM, Cedillo-Pardo E, García-Hernández J and Hernández-García S: “Stratigraphic Architecture and Sedimentology of the Main Oil-Producing Stratigraphic Interval at the Cantarell Oil Field: The K/T Boundary Sedimentary Succession,” paper SPE 74431, presented at the SPE International Petroleum Conference and Exhibition in Mexico, Villahermosa, February 10–12, 2002.


For more on dolomitization: Al-Awadi M, Clark WJ, Moore WR, Herron M, Zhang T, Zhao W, Hurley N, Kho D, Montaron B and Sadooni F: “Dolomite: Perspectives on a Perplexing Mineral,” Oilfield Review 21, no. 3 (Autumn 2009): 32–45.


26


Oilfield Review


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