The East Texas Oil Fiel, located in central Gregg, western Rusk, southern Upshur, southeastern Smith, and northeastern Cherokee counties in the east central part of the state, is the largest and most prolific oil reservoir in the contiguous United States. Since its discovery on October 5, 1930, (by Columbus Marion (Dad) Joiner) some 30,340 wells have been drilled within its 140,000 acres (570 km²) to yield nearly 5.2 billion barrels of oil from a stratigraphic trap in the Eagle Ford-Woodbine group of the Cretaceous. The source of its primary recovery was a strong water drive. Because the field is so geographically large the first wells, located several miles apart, were originally regarded as discovery wells in separate fields. After the spaces between the first wells were drilled, it was revealed that all sectors drew oil from the same Woodbine sands. The giant field was named for its geographic region.