The Eastern Venezuelan Basin1
The Eastern Venezuelan Tertiary basin is situated in the north-central and northeastern parts of Venezuela in northern South America. It is bordered on the south by the Guayana shield, on the west by the El Baúl swell, on the north by the Serranía del Interior, and on the east by the Atlantic Ocean. The basin, which trends eastward, is approximately 435 miles long and 140 miles wide, covering a land area of more than 59,000 square miles in Venezuela. The total volume of sediments, from the base of the upper Eocene to Recent, is about 74,000 cubic miles.
The Eastern Venezuelan sedimentary basin, as contrasted with the structural basin, is defined as a geosyncline which began its history in late Jurassic or early Cretaceous time as an orthogeosyncline which, following late Cretaceous orogeny, was transformed into a geanticlinal welt that forced the migration of the geosynclinal axis to the south and formed an exogeosyncline or foredeep which first appeared during the early Oligocene in the western part of the basin and which reached the eastern part during the Miocene. Late Miocene-Pliocene orogenic movements resulted in renewed uplift and thrusting of the welt toward the south as well as the development of normal faults along a structural hinge belt on the south flank of the basin. At the same time the Anaco uplift came into existence, and this high now separates the Maturín structural basin on the east from the Guárico structural basin on the west.
Sedimentation within the Tertiary basin was governed by the migrating foredeep and its position in relation to the southern shelf. Most of the oil fields on the south rim of the Eastern Venezuelan geosyncline are located on the shelf and upper slope of the foredeep, and as the regional dip of the foredeep is somewhat steeper than that of the shelf, the area along the bend is sometimes referred to as the hinge belt. The deposits within the foredeep itself are 30,000 to 40,000 feet thick. They are composed predominantly of sandstones and shales of the Merecure and Santa Inés groups and reflect an interplay of shallow-marine, brackish-water and intermittently continental conditions. These deposits become increasingly marine toward the east. The sediments on the relatively stable cratonic shelf to the south are one half to one third as thick as in the foredeep, and these shelf sediments, particularly of the Oligocene Chaguaramas, Periquito, and Oficina formations, indicate that the environment in which most of the oil was generated was paralic, with paludal conditions more dominant than marine. The section in the Greater Oficina and Anaco areas contains as many as 100 different oil-producing sheet and channel sands and up to 75 lignites; in the Chaguaramas formation of the Tucupido area about 150 lignites are present. Along the northeastern flank of the basin, in the Greater Jusepín area, the Miocene La Pica oil is trapped in up-dipping wedge belts of such character as to suggest modification of normal deposition by turbidity currents. Farther east, production in the Quiriquire field is predominantly from the continental deposits of the Quiriquire formation into which oil has migrated up dip or from below the angular unconformity at its base. Approximately 50 per cent of the oil found in Eastern Venezuela occurs in stratigraphic traps; the remainder is associated with normal faults, domes along thrust faults, faulted anticlines, and diapir-like folds.
Although much of the oil, at least in the Greater Oficina area, is believed to have been derived from sources close to present reservoirs, there is some evidence that elsewhere extensive migration may have taken place. It is suggested that the heavy crude in the "tar belt" along the south rim of the basin is either inspissated in place or migrated from an Oficina or Temblador source.
Cumulative production in Eastern Venezuela to the end of 1953 has been 1,800 million barrels of which about 2 per cent has come from the Cretaceous, 56 per cent from the Oligocene, 19 per cent from the Miocene, and 23 per cent from the Pliocene. Remaining reserves are estimated at 1,770 million barrels.