More than 5 billion barrels of oil have been produced from the Anahuac and Frio formations in the Texas and Louisiana Gulf Coast. The Frio, with many excellent blanket sands, accounts for practically all of this total; the Anahuac, which is primarily shale, produces only a small percentage from limited sand development. Oil activity on salt-dome structures in the Frio trend dates from the founding of the present oil industry with the discovery of oil at Spindletop and Jennings in 1901. Since that time, the oil industry has discovered approximately 455 fields in Texas, and 160 fields in Louisiana. A great variety of geologic structures, together with favorable sedimentation conditions, resulted in forming many traps for oil and gas in the Anahuac and Frio formations. The p oducing belt for these formations ranges from 40 to 60 miles wide, and extends approximately 675 miles from Mexico to the state of Mississippi.