Abstract Reviews the sequence of reconnaissance and detailed gravimetric, magnetometric, and seismic refraction and reflection surveys leading to definition of the north-south-trending faulted anticlinal structure in the Alturitas area west of Lake Maracaibo, where oil was subsequently encountered in middle and upper Cretaceous, Paleocene, and basal Eocene formations. Coal beds present in the Paleocene gave rise to a strong reflection initially interpreted as the Cretaceous Socuy limestone horizon some 4,000 feet lower in the section, so that drilling depths were considerably greater than originally anticipated.