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La Sara field, on the Island of Tierra del Fuego at the southern tip of Argentina, was discovered in 1959, developed and exploited by pressure maintenance with crestal gas and semiperipheral river-water injection. In three years 4,731,000 bbl of oil have been realized from the undersaturated reservoir by injection of 13,743,000 bbl of water and 4,600,000 Mcf of casinghead gas. Geology, development, reservoir analyses, injection systems, injection-well preparation and operations are discussed.
The La Sara field is located 31 miles north of Rio Grande and 5 miles south of San Sebastian Bay on the eastern side of the Island of Tierra del Fuego at the southern tip of Argentina, South America [Fig. 1]. The two oil horizons in the field are the Upper and Lower Springhill sands with tops occurring at 6,250 and 6,430 ft respectively in well LS-36. Tennessee Argentina, S. A., discovered the first commercial production in La Sara with the drilling and completion of Well LS-1 in both horizons in Dec., 1959. This report deals with La Sara field development, pressure maintenance programs and operations. No other successful secondary recovery project utilizing injection of water existed in Argentina at the time these programs were initiated. Upper Springhill pressure maintenance consisted of crestal gas injection in an undersaturated reservoir with a solution gas-depletion type of drive. This initial gas injection is being supplanted by downdip water injection. Lower Springhill pressure maintenance consists of downdip semi-peripheral, fresh water injection in an undersaturated reservoir of non-marine type sediment. A small amount of crestal gas injection was done to augment initial low rate of water injection. Evidence indicates partial water drive or aquifer expansion, the effect of which decreases with continued production. Calculations and production history indicate that both reservoirs have been in communication since early in the field life.
The La Sara field is located in the Magellan Basin of South America. Oil entrapment is controlled by the La Sara fault which is a normal fault of 490 ft maximum displacement dipping northeast up to the basin. The producing structure consists of two domes which lie against the fault on the upthrown side. The principal dome is to the south and is elongated in nature with limited [fault-controlled] relief to the northwest. The secondary oval-shaped dome is toward the northwest end of the field and has relief in three directions [Fig. 2]. The average dip of the sands is 350 ft/mile.
