The structure of the Santa Ana oil field in central Anzoategui comprises a subsurface thrust fault (Anaco thrust), with approximate strike N. 65° E. and northwest dip of about 45°, and three elongated oil-bearing domes oriented along the fault in its northwestern segment. The field is part of the Anaco structural trend which, with the same characteristics, continues northeastward for more than 40 kilometers to determine the San Joaquin, Guario, and Santa Rosa oil fields. Repeated sections measured in Santa Ana wells, ascribed to the Anaco thrust, vary from 80 to almost 1,500 feet, but stratigraphic throws are still larger in San Joaquin. In Santa Rosa, where no wells have yet penetrated the thrust, displacements may be of the order of 3,000 feet or more. The thr st fault is best known in the Santa Ana area, where ample subsurface control is available; it appears to die out upward at the Oficina-Freites contact.

A detailed thickness analysis of the sedimentary column (upper Periquito and Oficina formations) in the Santa Ana area, supplemented by similar but less detailed information from adjoining areas, has revealed a continuous record of deformation, in concomitance with sedimentation, and has resulted in a structural history which confirms an earlier outline postulated on the basis of seismic evidence. According to this history, normal faulting (the present-day thrust) and doming were already present in Periquito time, that is, from the earliest available record. While folding continued uninterruptedly possibly to the Recent, normal fault growth stopped toward the close of Naranja time. A reversal of fault movement started before the end of Oficina time to produce the Anaco thrust, which h s the same attitude as the original normal fault. Several lines of evidence, such as the thickness trend of the Miocene Freites formation, indicate the uplift continued during post-Oficina sedimentation.

The writers are not aware that a similar history has been proposed for other thrust faults, but it is quite possible that the Santa Ana-type fault is not unique. The data and conclusions of this work may be of interest to those concerned with the mechanics of deformation, which reason has prompted the writers to present their findings.