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The Santa Ana, San Joaquin, Guario, and Santa Rosa oil fields, which may be grouped together as the Anaco fields, are in the central part of the state of Anzoategui in eastern Venezuela. Since the completion of the discovery well of the Santa Ana field in 1937, five additional domes have been drilled and found productive. As of January 1, 1947, 71 wells have been drilled, resulting in a production of more than 27,000,000 barrels and a daily potential of 22,650 barrels.
The six drilled domes of the Anaco area have a northeast-southwest trend and extend over a distance of 30 miles. The four southwestern domes are indicated on the surface by outcrops of middle Miocene beds surrounded by upper Miocene-Pliocene beds, but older beds of the two northeastern domes are completely and unconformably covered by the younger beds. The closest known seepage is
Fig. 1. INDEX MAP
more than 35 miles distant. The discovery well was located on surface and aerial geology and the remaining domes were drilled on such information plus reflection seismograph excepting for the two northeast domes which were based on reflection seismograph alone. Subsequent development has been guided mainly by reflection-seismograph and subsurface geological work.
The section encountered by wells in the Anaco fields is as follows.
Sacacual group (upper Miocene-Pliocene). 0-1,600 feet thick
Freites formation (middle Miocene). 0-2,100 feet thick
Possible unconformity on structural highs
Oficina formation (Oligocene-Miocene). 7,500-10,000 feet thick
Merecure formation (upper Eocene-lower Oligocene). More than 1,900 feet thick (base not yet reached)
All of the Anaco domes are asymmetrical with steep (as much as 80°) southeast flanks and relatively gentle (as much as 25°) northwest flanks; the several domes are separated by saddles or by sharp synclines which are probably faulted. All the domes are believed to be formed by drag over a northwest-dipping zone of thrust faulting. Most of the domes are relatively simple with only a few normal faults but the Santa Rosa dome is noteworthy because of its many normal and reverse (strike-slip) faults. Accumulation is mainly controlled by the structural closure of the domes with sand lenticularity of some importance in localizing the accumulations. Some oil has been produced from the first beds below the thrust fault.
Production is from sands in the Oficina and Merecure formations. Twenty-eight Oficina sands have produced oil and at least 15 additional Oficina sands have so far tested gas at high positions. In the oil-productive part of the Oficina formation only 8 per cent of the section consists of sandstone beds. In the Merecure formation, which consists of 50 per cent sandstone and 50 per cent shale and claystone, it is possible that only one reservoir is present in any one segment; in other words, the many sands may have complete connection with each other. Where tested, the Merecure formation has been gas- or oil-productive without exception. Oil-productive depths range from 3,830 to 10,705 feet, and gas accumulations have been encountered just below the surface. The total productive surface rea amounts to nearly 29,000 acres.
Reservoir pressures in the Oficina formation are locally 500-1,000 pounds per square inch in excess of hydrostatic pressure, whereas Merecure pressures are essentially in accord with hydrostatic pressure. The temperature gradient averages about 44 feet per 1°F., and the maximum recorded temperature is 276°F. All production to date has been either 33-47° API wax oil or 44-57° API condensate, the latter accounting for only 2 per cent of the total. The wax oils have a wax content of about 15 per cent by weight and a pour point of about 85°F. Apparently all reservoirs have gas caps and some reservoirs contain gas only. One oil zone has a thickness of 1,400 feet. Formation waters range from only 1,500 to 5,000 parts per million of chloride ion through the Oficina f rmation but show a marked increase in salinity to 15,000-18,000 parts per million of chloride ion in the Merecure formation.
Dual-zone completions are common practice and for the past several years only one sand has been perforated in each zone. All recent completions have been through gun perforations. Well spacing is on the equilateral triangle system and varies from 25 to 126 acres or more; boundary and structural adjustments have locally caused close spacing but only a small part of the productive area has been drilled. Pressure-maintenance projects are being discussed. The outlet of the fields is through the 155-kilometer 16-inch pipe line extending from the Greater Oficina area through the Anaco area to Puerto La Cruz on the Caribbean coast.
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