Oil Fields of Greater Oficina Area Central Anzoategui, Venezuela

The oil fields of the Greater Oficina area are situated in the south-central part of the state of Anzoategui in the midst of the vast interior plains of Eastern Venezuela. Since the completion of the discovery well in 1937, 16 fields with a total of 512 wells have been drilled, resulting in a production of more than 127 million barrels of oil to January 1, 1946. The region is covered by Quaternary and uppermost Tertiary deposits, and surface indications of structure are meager. No seepages are known within a distance of more than 100 kilometers. The discovery well was located on the basis of torsion-balance and refraction-seismograph work, and subsequent development has been guided principally by geophysical, structure-drill, and subsurface geological work. The section encountered by wells in the Greater Oficina area is as follows. End_Page 2089------------------------------ Mesa formation (Pleistocene). 0-500 feet thick Sacacual group (upper Miocene-Pliocene). 400-3,000 feet thick Freites formation (middle Miocene). 1,100-2,000 feet thick Oficina formation (Oligocene-Miocene). 2,000-4,000 feet thick Unconformity Temblador formation (Cretaceous). 500-1,500 feet thick Unconformity Basement igneous and metamorphic rocks The Greater Oficina area is a segment of the southern limb of the Eastern Venezuelan structural basin. Dips are very gentle (less than 5°) throughout the area and there is little folding. Accumulation is controlled largely by normal faults constituting barriers to migration of oil southward and westward up the regional dip of the basin. Stratigraphic pinch-outs are locally important adjuncts to accumulation. Production is from sands in the Oficina formation. There are 65 distinct, productive sands in 46 different accumulation segments, constituting 340 separate, productive reservoirs. Producing depths range from 4,000 to 7,000 feet. Total surface productive area to date amounts to more than 25,000 acres. Net productive sand thicknesses in individual wells may amount to as much as 550 feet. Reservoir pressures show a regular straight-line increase with depth within the Oficina formation in accordance with the calculated hydrostatic gradient but are slightly out of equilibrium with hydrostatic pressures in near-surface sands. The temperature gradient in the area is about 51 feet per degree Fahrenheit. Oil gravities range from 8° to 57°; the oil being produced averages about 34° API. Gravities in general are heavier with depth. The Oficina oils are largely of intermediate base. Some reservoirs contain oil with a wax content up to 15 per cent by weight. Almost all reservoirs have gas caps but few produce gas only. Formation waters vary in salinity from about 4,500 parts per million in the shallow producing zones to 22,000 parts per million in the deep zones. It is believed that the oil produced in the Greater Oficina fields originated within the Oficina formation and in close proximity stratigraphically and geographically to the reservoirs in which it is now found. There is definite evidence, however, of a certain amount of lateral migration within the reservoir sands up the regional dip of the Eastern Venezuelan basin. Dual- and triple-zone completions are common practice but only a single sand is opened in any one zone. Well spacing is on the equilateral-triangle system and varies from 14 to 42 acres or more, according on local factors. The relatively flat open terrane allows skidding of rigs and derricks from one location to the next. Pressure-maintenance and secondary-recovery projects are in prospect. The outlet of the fields is through a 155-kilometer pipeline to the Caribbean coast at Puerto la Cruz.