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Big Wells field is a rapidly developing oil field in northeastern Dimmit and southeastern Zavala Counties, Texas, approximately 75 mi southwest of San Antonio. Big Wells is a large and significant stratigraphic trap. Production is from a San Miguel sandstone of the upper Taylor Formation (Upper Cretaceous), a section noted for "tight" sandstone conditions and small--5-10 well--oil fields in anticlinal structures generally associated with small volcanic extrusives. This trend has been considered high risk for "economical" fields and therefore ignored by most operators, both large and small. Big Wells has instigated the expected flurry of activity when a "dead" trend suddenly springs back to life.
Ranging from 5,300 to 5,700 ft in depth, with a minimum of 200 ft or possibly 400 ft of oil column, the field was found by Sun Oil in January 1969. Development drilling began in the north part of the field where low-permeability sandstone was encountered casting doubt on the economics involved. Southward of the early drilling, much better sandstone reservoirs were found, resulting in full allowable flowing wells (142 BOPD in March 1971).
The sandstone reservoir is very fine grained, with an average porosity of 18-20% and permeability ranging from less than 1 md to 100 md, generally less than 5 md. The San Miguel sandstone in the Big Wells field area is a linear sandstone body interpreted to have been deposited as an offshore bar.
Today the field is 12 mi long and 3 mi wide. Nine rigs were operating in the field in March 1971. With 160 wells completed by the end of May 1971, the limits of the field are fairly well defined. Drilled on 80-acre spacing, approximately 200-250 wells are anticipated. Production in March 1971 was between 15,000-16,000 b/d of oil.
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