GAMMA, Member

TERTIARY (upper Miocene?-Pliocene)

State of Monagas, Venezuela

Author of name: F. W. Johnson and R. J. Loeffler, 1943 (private report).

Original reference: H. D. Borger, 1952, p. 2302, 2306-2307.

See: QUIRIQUIRE, Formation.

QUIRIQUIRE, Formation

TERTIARY (middle Miocene to Pliocene)

State of Monagas, Venezuela

Author of name: H. D. Hedberg, 1936 (private report).

Original reference: J. H. Regan, 1938, p. 192-196.

Original description: ibid.

According to Regan (1938, p. 192-196), the formation derives its name from the Quiriquire field in northern Monagas. Regan claimed that the formation includes all the Pliocene rocks in the vicinity that had been called "Llanos" by Liddle (1928, p. 15-16). The formation is described as composed of irregularly interbedded lenses of boulders, cobbles, gravels, sands, silts, sandy clays, and various kinds of clay-conglomerates that have practically no stratification and are poorly indurated. These sediments were deposited in a mixed continental and semi-marine environment. Regan stated that the thickness of the Quiriquire formation varies from zero to 5,000 feet. He subdivided the formation into four main "zones", three of which were based entirely upon their economic importance and one upon its persistent geologic characteristics. From top to bottom these "zones" are as follows: (1) Upper Heavy Oil Zone; (2) Laminated Marker; (3) Light Oil Zone; (4) Lower Heavy Oil Zone. The "Laminated Marker" is characterized by laminae of shale and fine sand averaging 3 millimeters in thickness.

Hedberg (1950, p. 1206) placed the Quiriquire formation in the Sacacual group. He stated that farther south in Monagas, both wells and outcrops show the formation to consist dominantly of claystones and siltstones. These are commonly light gray, greenish gray or yellowish brown, and locally mottled red. There are also some poorly consolidated sandstones and conglomerates. Farther west in the State of Anzoátegui, Hedberg pointed out, the formation consists of tan-brown claystones and siltstones and tan or red and light gray mottled claystones. According to Hedberg, the Quiriquire formation forms the dominant part of the Sacacual group adjacent to the northern mountain front in northern Anzoátegui and grades laterally and downward into the Las Piedras formation of the Sacacual group.

Mencher et al. (1951, p. 30, 54) claimed that the "Sacacual beds" of the Quiriquire area are known as the Quiriquire formation. In their correlation chart they showed the Quiriquire and Las Piedras formations making up the Sacacual group in northern Monagas in a way that one of these formations thickens or thins at the expense of the other. Thus, the Quiriquire formation is shown to be equivalent or younger than the Las Piedras formation. Mencher et al. indicated an upper Miocene and Pliocene age for the formation. They mentioned the Zeta, Eta, and Theta members to be the basal members of the formation.

Borger (1952, p. 2302, 2304-2307) selected the well Q-228, in the east-central part of Quiriquire field, as the type section for all the members into which the formation is subdivided above the basal Theta member. The Theta section of the well Q-135 was chosen to complete his type log of the Quiriquire formation (p. 2302). Borger pointed out that there is an extreme variation and lenticularity of individual beds of the formation and the group of sands are correlative only over limited areas. There are instances of a 40-foot sand present in one twin well which is absent in the twin well 23 feet distant. Borger claimed that nearly without exception, recognizable zones afford a basis for member correlation. Correlation is on equivalent time rather than sand-to-sand basis. Utilizing this method of correlation, Johnson and Loeffler (1943, private report) divided the Quiriquire formation into eight members, and the Greek alphabet was used to name the members, beginning with Alpha at the top, Beta, Gamma, Delta, Epsilon, Zeta, Eta, and ending with Theta at the base. The Alpha, Beta, Gamma and Delta members are lithologically similar and consist of boulders, cobbles, gravels, sands and unconsolidated to firm beds of gray to mottled red and gray clays. The Epsilon member, varying from 230 to 290 feet in thickness, is predominantly a sandy section containing thin clay beds. The top of the Zeta member was placed on a prominent, positive, self-potential kick on the Schlumberger log which is Freie's "laminated marker" (Regan, 1938, p. 194195). The thickness of the Zeta member varies from 380 to 475 feet. The Eta member pinches out against the unconformity in the northwest part of the field, Borger stated, and has a thickness of 625 feet in the southern part. The Theta member is the basal member of the Quiriquire formation and varies from 0 feet, where it pinches out against the unconformity along the northern edge of the field, to more than 700 feet in thickness along the southern limits of the field. This basal member contains considerable reworked glauconitic material and green clays in some parts of the field. This part of the section represents part of a number of glauconitic beds of different ages referred to in older reports as "Green Series". Borger described the Quiriquire sediments as a whole, as sandy clays interbedded with unsorted clastics ranging in size from silty sands to conglomerates and boulder beds. Thin beds of lignite, lignitic clay, tufa, travertine, and asphalt are present from the base of the formation to the surface. Borger stated that the areal extent of the formation is restricted to and parallel with the immediate mountain front area of northern Monagas. Where distinguished from the underlying beds, the formation is in unconformable contact with the older beds. Borger pointed out that it is very difficult to differentiate the formation limits at relatively small distances from the type area. The over-all thickness of the Quiriquire formation is approximately 1,600 feet in the northern limits of the field pinching out northward against the Cretaceous outcrops. To the south, the formation reaches a thickness of 5,440 feet. Farther south in the exploratory wells, it has been impossible to allocate the Quiriquire formation in the section due to facies changes. Borger mentioned that macrofossils have been recovered from cores although these occurrences are neither abundant nor widespread. Among these he listed Polinices sp., Unio sp., Modiolus sp., and Corbicula (Cyanocyclus) sp. Borger considered the formation to be Plio-Pleistocene in age.

Private information indicates that the age of the Quiriquire formation is middle Miocene to Pliocene. The combined Alpha to Epsilon members are upper Miocene and Pliocene and the combined Zeta, Eta, and Theta members are middle Miocene in age. The combined Alpha to Epsilon members correlate with the Las Piedras formation of Monagas and with the Sacacual group of Anzoátegui. The combined Zeta, Eta, and Theta members correlate with the upper La Pica formation of Monagas.

Borger (1952, p. 2305) pointed out that the Quiriquire formation is restricted to the northern mountain front of northern Monagas and cannot be distinguished to the south of Quiriquire field due to facies change. Then the beds farther south in Monagas which were placed by Hedberg (1950, p. 1206) in the Quiriquire formation, could very well represent the Las Piedras formation considering the lithologic description given by Hedberg. Those beds in Anzoátegui placed in the Quiriquire formation by Hedberg (1950, p. 1206) may represent the Caicaíto formation (Mencher et al., 1951, correlation chart) of the Sacacual group.

A. N. Dusenbury, Jr.