ABSTRACT

Big Wells fields is a rapidly developing oil field in northeastern Dimmit and southeastern Zavala Counties, Texas, 75 miles southwest of San Antonio. This field is a large stratigraphic trap with production from a San Miguel sand of the Upper Taylor (Upper Cretaceous). The section is noted for tight sand conditions and small (5-10 well) oil fields in anticlinal structures usually associated with small volcanic extrusives. This trend has been considered high risk for "economical" fields, and, therefore ignored by the great majority of operators. The Big Wells discovery has instigated the expected flurry of activity when a "dead" trend suddenly springs back to life.

Ranging in depth from 5300 to 5800 feet, with 200 to possibly 400 feet of oil column, the field was found by Sun Oil in January 1969. Development drilling began in the north part of the field where tight sand conditions were encountered, casting doubt on the ultimate economics for the field. Southward of the early drilling, much better sand reservoirs were encountered in March 1971, yielding full allowable (142 BOPD) flowing wells.

The San Miguel Sand is very fine grained, with an average porosity of 19% and permeability ranging from less than 1 Md. up to 100 Md., but commonly less than 5 Md. The sand in the Big Wells field area is a linear sand body interpreted to have been deposited as an offshore bar.

Today the field is 12 miles long and 3 miles wide. At one time nine rigs were operating in the field and by the end of May 1971, 160 wells were completed, and the limits of the field were fairly well defined. Drilled on 80 acre spacing, the field will ultimately contain 200-250 wells. Daily production in March 1971 was between 15,000-16,000 barrels of oil.