AMACURO, Member

TERTIARY (middle Miocene)

Territory of Delta Amacuro, Venezuela

Author of name: J. A. Simmons, 1949 (private report).

Original reference: Mencher et al., 1951, p. 59.

See LA PICA, Formation

LA PICA, Formation

TERTIARY (middle Miocene)

State of Monagas, Venezuela

Author of name: H. D. Hedberg and L. C. Sass, 1936 (private report).

Original reference: L. L. Logue and M. W. Haas, 1943, p. 178.

Original description: H. D. Hedberg, 1950, p. 1205-1206.

Logue and Haas (1943, p. 178) only mentioned the La Pica formation when discussing the occurrence of the foraminiferal genus Paranonion, stating that Paranonion venezuelanus is "common in the upper Miocene portion of the La Pica formation in wells drilled in the northern part of the State of Monagas, Venezuela".

Hedberg (1950, p. 1205-1206) stated that the type section of the La Pica formation is in the interval from 3100-5857 feet in the La Pica N° 1 well, District of Maturín, State of Monagas. He described the formation as consisting of gray brackish to marine shales and silty shales with important local development of fine-grained clayey sandstones. Hedberg pointed out that the maximum thickness may be considerably greater than that of the type section. He mentioned that the formation rests with strong angular unconformity on the Carapita shale in the Santa Bárbara area of northern Monagas. According to Hedberg the La Pica appears to be laterally gradational in part into the Las Piedras formation of the overlying Sacacual group. The La Pica is known principally from well sections in northern Monagas and Delta Amacuro. Hedberg claimed that the La Pica formation is probably in part equivalent in age to the Freites formation and "other members of the upper part of the Santa Inés group, but represents a much different facies". He mentioned that the formation is characterized by a meager but very distinct fauna of arenaceous, and dwarfed brown calcareous foraminifera. He considered the age of the La Pica formation to be probably middle and late Miocene.

Mencher et al. (1951, p. 51-52, 56) described the La Pica formation as consisting predominantly of a sequence of shales and sands. The shales are soft-to-medium hard, dark gray to grayish black, silty, micaceous, lignitic, and frequently with laminae of fine sand and silt. Minor beds of soft shale, clay shale, clay and claystone are sometimes encountered. The typical sands are soft to firm, fine-grained to very fine-grained, light gray to dark gray, laminated, silty, argillaceous, lignitic, and micaceous. Mencher et al. stated that the overall thickness of the La Pica formation in the Greater Jusepín Area varies from less than 200 feet in the north to more than 4,500 feet in the south. 25 productive sands are known from the lower part of the La Pica formation. They pointed out that the La Pica formation is unconformably overlapped by the Las Piedras formation, especially in the northern part of the Greater Jusepín Area, but this contact becomes conformable toward the south (basinward). In the Pedernales area (Territory of Delta Amacuro) the La Pica formation has been subdivided into the following three members, in ascending order: Amacuro member, Pedernales member, and Cotorra member. Of these the Amacuro and Cotorra members were mentioned for the first time in published literature by Mencher et al. (1951, P. 56).

It was stated that the Cotorra member, from the top of the La Pica to the top of the original producing section; the Pedernales member, the original producing section and the Amacuro member, from the base of the original producing section to the base of the formation, make up the La Pica formation of the Pedernales area. At present oil is produced from the Pedernales member and the upper part of the Amacuro member.

The name "Pedernales beds and formation" has been known already from previous work. Liddle (1928, correlation chart) showed the "Pedernales beds" in his column on eastern Venezuela and the Guayana highlands describing them as "soft shales and hard sandstones. Poorly exposed. Exact age uncertain. Oil-bearing". In this column these beds were placed in the upper Oligocene and lower Miocene.

Liddle (1946, p. 494-496) applied the name "Pedernales formation to Miocene beds exposed on Isla Pedernales and Isla Plata by the mainland of Venezuela at the northern end of the Orinoco delta. He described these sediments as dark gray to black petroliferous, micaceous, arenaceous clays and shales which are very bituminous; gray and brown, soft sandstones; and some extremely hard, white sandstones. Liddle claimed that there seem to be from 1,500 to 2,000 feet of section exposed. He pointed out that the Pedernales beds are of Miocene age even though the lower beds bear some resemblance to the "Monagas formation of Oligocene-Miocene age". All beds exposed in the Pedernales area dip normally to the north or to the south at more than 80°, are vertical or are overturned.

Information taken from private reports indicates that the age of the La Pica formation is middle Miocene. The beds which crop out at Isla Pedernales and at Isla Plata and which have been called "Pedernales formation" by Liddle (1946, p. 494-496) represent the middle portion, and original producing section, of the La Pica formation of the subsurface in Pedernales field. Therefore, present practice prefers to consider the "Pedernales formation" as the middle or Pedernales member of the La Pica formation in the Pedernales area. The upper La Pica formation of northern Monagas correlates with the combined Zeta, Eta, and Theta members of the Quiriquire formation.

Leo Weingeist