POTRERITOS, Formation

TERTIARY (upper Eocene)

State of Zulia, Venezuela

Author of name: H. D. Hedberg, 1927 (private report).

Original reference: F. A. Sutton, 1946, p. 1675-1677.

Original description: ibid.

Sutton (1946, p. 1675-1677) used the name Potreritos formation to designate an essentially shale and sandstone formation which crops out east of Lake Maracaibo near Potreritos ranch in the northwestern part of the District of Bolívar, Zulia. The formation comprises one of the major subdiviaions of the upper Eocene. In the Bolívar Coastal field, the formation exhibits a near-shore brackish-water to shallow-marine type of desposition and is composed mainly of dark gray to black, carbonaceous, laminated, sandy shales, interbedded with fine white sandstones. Fine-grained to coarse and porous lenses and major sandstone beds are present in places. The most importante of these is the porous "Main Sandstone" which has an average thickness of 200 feet and which yields most of the oil found in the Eocene oil the Bolívar Coastal field. Sutton cited a maximum thickness of an estimated 1,371 meters for the formation in the Bolívar Coastal field. He stated that the Potreritos formation thins to the north and west. The Potreritos is believed to be transitional with the underlying Misoa formation in the Bolívar Coastal field, according to Sutton. Westward, the Misoa formation wedges out, and in the Districts of Maracaibo and Mara, in the La Paz-Netick-Tetones area, the Potreritos is unconformable with the underlying Paleocene Guasare formation. Where post-Eocene faulting and erosion have not interfered, the Potreritos is always conformable and gradational with the overlying Las Flores formation. The known outcrop occurrences of the Potreritos formation are limited to the area immediately adjacent to the Bolívar Coastal field in the northwestern Bolívar and southern Miranda districts of Zulia. It is present in the subsurface of the Bolívar Coastal field and has been encountered in most of the wells drilled west and north of the city of Maracaibo. With its equivalents, it is known in most of the outcrop areas surrounding the Maracaibo basin and is believed to umderlie most of the basin. The Potreritos, with the sole exception of the Misoa formation, is the least fossiliferous formation of the Eocene of the Bolívar Coastal field. Sutton (p. 1676, 1677) gave more details on the faunal recoverles. He stated that the Potreritos grades by lateral transition southeastward mto the lower Paují formation. He claimed that the Potreritos is the direct equivalent, based on heavy mineral content, of the upper part of the Paso Diablo formation of Hedberg and Sass (1937, p. 91). Then Sutton pointed out that on the eastern side of the Manuelote syneline, west of the Mara field in the District of Mara, the lower or Barqueta member of the Mostrencos formation of Hedberg and Sass (1937, p. 92) is lithologically, mineralogically and faunally, identical with the Potreritos formation.

Schaub (1948, p. 219) pointed out that he believed that the Potreritos formation of Sutton could be tentatively correlated with the upper part of Sutton's Misoa formation.

Leo Weingeist