BARCO Formation

TERTIARY (lower Eocene)

Department of Santander del Norte, Colombia

Author of name: F. B. Notestein et al., 1944.

Original reference: F. B. Notestein et al., 1944, p. 1190.

Original description: ibid.

The Barco formation is the middle part of the Orocué‚ group and is named from the East Barco Ridge, a prominent escarpment formed by resistent sandstones of this formation along the east flank of the Petrolea anticline in the Barco concession, Colombia. As type section Notestein (1944, p. 1190) designated a section on the East Barco Ridge, 3 kilometers south of the Barco Venezuela triangulation station; though incomplete, this is the best available section. The formation consists of a series of interbedded sandstones, shales, and claystones. It is topographically important, forming prominent strike ridges. The sandstones occur in beds ranging in thickness from 0.3 to 20 meters, are in large part gray argillaceous very fine to medium-grained, well-sorted cross bedded and cross-laminated, and locally contain abundant micaceous carbonaceous partings and numerous shale laminae. Some siltstones and rarely coarse-grained sandstones have been noted. A distinct type of sandstone, which is everywhere found in the Barco formation and forms a fair proportion of the sandstone content, is the so-called "sparkling sandstone". Sandstones of this type are more common in the middle and lower parts of the formation and are relatively clean fine to medium-grained sandstones in which secondary growth of the sand grains has formed myriads of crystal faces. These faces sparkle in the sun, hence the name.

Interbedded with the sandstones are shales and claystones, commonly gray or dark-gray, in part silty, micaceous, and carbonaceous, and locally rich in minute siderite spherules. Brown clay-ironstone in the form of thin lenticular masses and small nodules is common. Some coal, in one or more thin beds, is generally present in the upper part of the formation. The shales and claystones normally form one-third to half the total thickness of the formation.

Locally they may form about three-fourths of the thickness, and in three wells which the Barco drilled on the Tibú anticline, shales and claystones form four-fifths to eight-ninths of the formation. However, these extreme cases are not at all typical.

The formation ranges from 150 to 278 meters in thickness in 12 typical outcrop sections on and close to the Concession, the average being 194 meters. In wells the formation ranges from 76 to 198 meters in thickness with an average of 130 meters. Wells on the Sardinata, Tibú, and Socuavó anticlines show thin sections, bringing down the average thickness. It is probably significant that these wells are on the crestal portions of the anticlines, whereas the majority of surface sections have been measured on flanks of folds. Well evidence alone has suggested that a down-flank increase of sandstone exists on at least some folds. Regionally the Barco thins to the northeast.

Production is obtained from the Barco sandstones on the east flank of the Petrolea anticline (Carbonera fault zone) and in the Tibú-Socuavó area. The formation carries gas on the Sardinata anticline. In the Río de Oro field, the sandstones are exposed but carry some heavy inspissated oil.

The base of the Barco formation is drawn where the light-colored sandstones of the formation give way to the dark sandy shales of the Catatumbo formation. The contact is apparently conformable and is generally distinct. The top of the formation is drawn at the first fairly prominent sandstone below the coal series of the basal Los Cuervos formation, and the contact is apparently conformable.

Sutton (1946, p. 1665) recommends that the terminology in use in the Barco Concession be extended into Colón and southern Perija and to call the Tabla sands the Barco formation. According to Schaub (1948, p. 220) the subdivision used in the Barco Concession is well defined and checks closely with his observations in the Colón district and the State of Táchira. The age of the Barco formation is considered to be lower Eocene by Sutton (1946, p. 1665).

The combined Barco and Los Cuervos formations are considered by Sutton to be correlative with the Angostura, Marcelina and Trujillo formations.

W. A. Mohler