The Barco Concession, as granted by the Colombian government in 1931, comprises slightly over 1,000,000 acres lying along the Venezuelan boundary in the upper Río Catatumbo drainage basin. The topography varies from gently rolling to low mountainous. The climate is wet equatorial. The area is nearly all virgin forest.

The sediments involved range in age from Lower Cretaceous to Pliocene-Pleistocene. The Lower Cretaceous rests on a basement of pre-Cretaceous metamorphic and plutonic rocks, is entirely marine, approximately 700 meters thick, and is divided into the Uribante and Cogollo formations. The Upper Cretaceous is dominantly marine but includes some brackish-water deposits in its upper part, is approximately 950 meters thick, and is divided into the La Luna, Colón, Mito Juan, and Catatumbo formations. The last named is here defined for the first time. No unconformity is known to exist between the Cretaceous and Tertiary systems; the change is transitional in this area, and the position of the Cretaceous-Tertiary contact is doubtful.

The Tertiary rocks, characteristically nonmarine but which include a few marine beds, are approximately 2500 meters thick within the Concession. This figure would be increased 75 per cent if the remainder of the Guayabo group as known just southeast of Cúcuta were included. The Tertiary is divided into the Barco, Los Cuervos, Mirador, Carbonera, León formations, the Guayabo group, and the Necesidad formation. Of these Barco, Los Cuervos, Carbonera, León, and Necesidad are new formational names here defined. No major unconformities are known in the Tertiary below the base of the Pliocene (?) Necesidad formation. A minor unconformity occurs at least locally at the base of the Mirador, and there may be another local unconformity at the top of this formation.

Within the Concession oil production has been obtained from the Uribante, Cogollo, La Luna, Mito Juan, Catatumbo, Barco, and Los Cuervos formations.

Numerous oil and gas seepages occur on truncated anticlines. Two dikes of solid hydrocarbon have been found on and near the Concession. Bituminous sandstones of the Catatumbo and Barco formations are common on truncated anticlines.

The Barco Concession lies near the southwestern edge of a northeasterly plunging lobe of the Maracaibo basin between the Sierra de Perijá on the west and the Sierra de Mérida on the southeast. This lobe is more or less bisected by a truncated anticlinorium extending from immediately north of the bifurcation of the Cordillera Oriental northward through the Petrolea anticline and its extension in the Tarra anticline of western Venezuela. The total length of this bisecting anticlinorium is at least 120 kilometers, of which one third is on the Concession. East of this anticlinorium lies the Zulia syncline, and to the west is the Sardinata structural depression. Along the western edge of the upper Maracaibo basin there is a zone of folding and faulting in the eastern foothills of the Sierra de Perijá. About midway between these foothill folds and the bisecting anticlinorium is a north-south trend of uplift called the folds of the Sardinata depression. Major folding took place during late and post-Miocene time, but it began on a minor scale much earlier.

The Petrolea field has six productive zones, all Cretaceous. Zone 3, the most extensive, has a proven area of approximately 1700 hectares (4200 acres). Well depths range from 26 to 513 meters. The field has 124 producing wells and is considered fully developed. The gravity of the oil averages 46º A.P.I. A 12-inch pipe line 421 kilometers (261 miles) long extends from the field to the Gulf of Morosquillo.

The Río de Oro field produces black oil of 32° to 40° A.P.I. gravity from sandstones of the Catatumbo and uppermost Mito Juan formations. The productive area is about 170 hectares (420 acres). It is partially developed with nine wells averaging 430 meters in depth.

The Tres Bocas field, on the Tibú anticline, has had only three tests—two small wells in the Barco formation and one good producer in the Tibú member of the Uribante formation. This field is just past the discovery stage and has not been defined by drilling.

The Socuavó field has had two tests, both commercially productive, one of which produces from the Barco formation, and the other from the Uribante formation. This field, also, is just past the discovery stage and has not been defined by drilling.

The Carbonera field has been developed by three small wells pumping black oil of 21° A.P.I. gravity.

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