Stratigraphic and mineralogic studies of Cretaceous-Tertiary (K-T) boundary sections demonstrate that the offshore oil-producing breccias and seals from oil fields in the Campeche marine platform are of K-T boundary age and that their mode of formation is probably related to the K-T impact event at Chicxulub. The oil-producing carbonate breccia and the overlying dolomitized ejecta layer (seal) found in several wells on the Campeche marine platform contain typical Chicxulub impact products, such as shocked quartz and plagioclase, and altered glass. These offshore units are correlated with thick (∼50–300 m) onshore breccia and impact ejecta layers found at the K-T boundary in the Guayal (Tabasco) and Bochil (Chiapas) sections. Regionally the characteristic sequence is composed of, from base to top, coarse-grained carbonate breccia covered by an ejecta bed and typical K-T boundary clay. The onshore and offshore breccia sequences are likely to have resulted from major slumping of the carbonate platform margin triggered by the Chicxulub impact. Successive arrival times in this area, ∼350–600 km from the crater, of seismic shaking, ballistic ejecta, and tsunami waves fit the observed stratigraphic sequence. The K-T breccia reservoir and seal ejecta layer of the Cantarell oil field, with a current daily production of 1.3 million barrels of oil, are probably the most important known oil-producing units related to an impact event.