GAMMA, Sand

See OFICINA, Formation

OFICINA, Formation

TERTIARY (Oligocene to lower States of Anzoátegui and Monagas, Venezuela Miocene)

Author of name: H. D. Hedberg, 1933 (private report).

Original reference: H. D. Hedberg, L. C. Sass and H. J. Funkhouser, 1947, p. 2107-2108.

Original description: ibid.

The Oficina formation was named by Hedberg (1933, private report) from its type section in well Oficina n° 1, District of Freites, Anzoátegui, and first published by Hedberg et al. (1947, p. 2107-2108) as a sequence of gray to brownish-gray shales, light gray, fine to coarse-grained sandstones and siltstones, with thin lignites, lignitic shales, green to light-gray claystones, sideritic glauconitic sandstones, and thin cone-in-cone limestones; individual well sections may have up to 50 thin lignite beds; sandstones are thicker, coarser and more plentiful near the base of the unit. Sands and sand groups throughout the producing section of the Oficina formation were given letters from A to U in order of descending stratigraphic position; subdivisions of these groups were designated by sub-numbers of the terms upper, middle and lower. In the upper part of the Oficina formation above the AB sands, the sand bodies were at first designated by the greek letters Alpha, Beta, Gamma, Delta and Theta. With the exception of the Theta sand, which marks the top of the formation, the remaining terms were replaced, when these upper Oficina sands were discovered to be productive in the East Guara field, by a numbered series extending downward from near the top of the formation (Oficina-1 sand) to the sand just above the AB sands (Oficina-13 sand).

The Oficina formation grades transitionally into the overlying Freites formations, the contact being placed where the brownish-gray, more carbonaceous Oficina sediments give way to the typical greenish gray Freites shales. It lies unconformably on the Cretaceous Temblador formation: this contact is difficult to recognize, since adjacent sections of both units are characterized by thick, lithologically similar sand bodies. The thickness of the Oficina formation increases from south to north and ranges from 2000 to 4000 feet in this area. It contains many foraminifera and mollusks which indicate an Oligocene-Miocene age; the formation correlates with prts of the Santa Inés group of northern Anzoátegui.

Funkhouser et al. (1948, p. 1866-1870) described the Oficina formation in the Greater Anaco area, central Anzoátegui, as 7,500 to 10,000 feet of sediments, somewhat arbitrarily subdivided into the following members (from top to bottom): (1) Blanco member, 1230-1635 feet of gray carbonaceous and 1ignitic shales, interlaminated shale and sandstones, with green claystones, greenish-gray sandstones, thin greenish-gray limestoneis and lignites; (2) Azul member, 1450-1750 feet of interlaminated dark gray, silty and fissile shales and light-gray, fine-grained, micaceous and shaly sandstones, with some thin limestones, claystones, lignitic shales and lignite streaks; (3) Moreno member, 700-1700 feet, thickening northeastward, mostly at the top of the member, which suggests a mild unconformity; dark-gray fissile shale with some thin calcareous sandstones, cone-in-cone limestones, lignites and green claystones; (4) Naranja member, 1230-1800 feet of gray fissile shale with some thin sandstones, limestones, lignites and claystones; (5) Verde member, 850-1150 feet of dark gray shales and thin, fine to medium-grained sandstones, with some thin limestones and lignitic shales;. (6) Amarillo member, 650-904 feet of dark-gray shale with only few sands; (7) Colorado member, 1100-1574 feet composed mostly of shale, but containing several light-gray, fine-grained and some coarse-grained sandstones. The main differences between the Oficina formation of the Greater Anaco Area, and that of the Greater Oficina Area are a greater predominance of shales and less coarse, gritty and glauconitic fossiliferous sandstones in the Greater Anaco than in the Greater Oficina Area. Moderate changes take place in the fossil faunas, detrital minerals and character of the contained petroleum. The contact of the Oficina formation with the Freites formation in the Greater Anaco Area seems to be slightly unconformable. When overlain (unconformably) by the Sacacual group, the top of the Oficina formations is determined by its typical gray shales, interbedded shales and sandstones, greater lithification, brackish-water indigenous foraminifera and appearance of persistent electric-log markers. The contact with the conformably underlying Merecure group (Periquito formation) is placed at the top of the latter's nearly continuous sandstone section. Recent information definitely indicates that the upper part of the Periquito formation in the Greater Anaco Area correlates with the basal part of the Oficina formation (U sands) in the Greater Oficina Area.

As to the age of the Oficina formation, Funkhouser et al. (1948, p. 1870) suggest a middle Oligocene to middle Miocene age. However, more recent paleontological work indicates a predominantly Oligocene age for the formation, whose uppermost part extends into the lower Miocene.

Athough Hedberg (1950, p. 1204-1205) includes the Oficina formation into the Santa Inés group, as a time-equivalent of the outcropping Suata and older Santa Inés group units farther west, all geologists are not in agreement with this designation.

Mencher (1950, p. 69) indicate a thickness of 600 feet, increasing westward, for the Oficina formation in the Temblador area. The unit becomes steadily shaker towards the east of the Temblador field; its contact with the underlying Temblador formation is difficult to determine and is based primarily on changes in the heavy mineral suites.

Cecily Petzall