J. J. Mandujano V.* and J. D. Keppie M.**

* Instituto Mexicano del Petróleo, Subdirección de Exploración y Producción. Avenida Lázaro Cárdenas 152, Colonia, San Bartolo Atepehuacán, Delegación Gustavo A. Madero, C. P. 07730, México D. F. Edificio 20 primer piso cubículo 131, México. J. J. Mandujano V.

** Instituto de Geología, UNAM, Ciudad Universitaria, México D. F., C. P. 04510, México.

The NW-SE trending Cantarell structure in the Gulf of Campeche is the largest hydrocarbon-producing complex in Mexico. The oil occurs predominantly in latest Cretaceous-earliest Tertiary breccias with subsidiary reserves in Upper Jurassic (Oxfordian and Kimmeridgian) and Lower Cretaceous oolitic and partially dolomitized limestones, dolomites and shaly limestones. Cantarell has been interpreted both as a fold-and-thrust zone and a dextral transpressional structure. Analysis of structure contours at 100 m intervals, on the top of the Tertiary breccia and the Kimmeridgian (Upper Jurassic) dolomite, indicates that the structure is an upright cylindrical fold with gently plunging conical terminations and a conical portion in the middle part of the structure. The axes of the middle, NW and SE cones are subvertical. This geometry indicates that the two fold terminations and the middle cone are aprons rather than points, with the NW and middle cone axes intersecting the cylindrical fold axis at the point where the geometry switches from conical to cylindrical. The apical angle (i.e. the angle between the fold and cone axes) varies as follows: (i) in the NW cone, it is ~70° in the breccia and ~76° in the Kimmeridgian dolomite; (ii) in the middle cone, it is ~77° in the breccia and ~73° in the Kimmeridgian dolomite; and (iii) in the SE cone, it is ~64° in the breccia and ~57° in the Kimmeridgian dolomite. This indicates that whereas the fold opens with depth in the NW cone, it tightens with depth in the middle and SE cones. Assuming a parallel fold geometry, these apical angles indicate an increase in volume in the NW cone (i.e. larger hydrocarbon reservoirs), compared to the middle and SE cones. Theoretical considerations indicate that the curvature increases dramatically towards the point of the cone. In the case of the Cantarell structure, the apices of the cones are located at the conical-cylindrical fold junctions, where the highest curvature may have produced a higher degree of fracturing. The coincidence of maximum curvature and the intersection of the conical and cylindrical fold axes in the fold culminations, with porous and permeable reservoir rocks have made these locations favourable for hydrocarbon accumulation.

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