The Big Wells (San Miguel) and Big Wells (Lo East) fields, 75 mi (120 km) southwest of San Antonio, Texas, are productive from stratigraphic traps in sandstones of the San Miguel Formation (Upper Cretaceous). Ranging in producing depths from 5,200 to 5,800 ft (1,585-1,768 m), Big Wells (San Miguel) field covers approximately 30,500 surface acres (25,400 oil-producing and 5,100 gas-producing acres) in an area 14 mi (23.5 km) long and 3-4 mi (4.8-6.4 km) wide. Estimated original oil in place is 200 million bbl. Since discovery of the field in January 1969, 210 oil wells (on 80-acre spacing) and 9 gas-cap wells have been completed. Maximum daily production of 21,000 bbl was reached in March 1972. Daily production in mid-1973 was 14,000 bbl. In December 1973, daily production was 11,665 bbl. The cumulative production through 1973 was 17,476,275 bbl of oil plus 19,851 MMcf of gas. Ultimate recovery is estimated to be 43 million bbl of liquids plus 60 Bcf of gas.

The Big Wells (Lo East) field covers approximately 14,000 surface acres in an area 8 mi (12.9 km) long and 2 mi (3.2 km) wide. Producing depths range from 5,600 to 5,900 ft (1,707-1,798 m), 200 ft (61 m) lower in the section than the producing zone in Big Wells (San Miguel). Fifty-three wells had been completed in this reservoir by the end of 1973 and were producing 1,745 bbl/day. Through 1973, cumulative production was 2,327,485 bbl of oil plus 1,569 MMcf of gas. Estimated original oil in place is estimated as 35 million bbl. Ultimate recovery is expected to be 7 million bbl of liquids plus 4 Bcf of gas.