On a structural anomaly at Jackson, Mississippi, known since 1860, and surface structural closure proved in 1926, the first commercial gas well was completed in 1930. The proved and potential gas-bearing area should cover about 8,560 acres, and produce more than 128 billion cubic feet of gas. The gas reservoirs occur in the uppermost part of the Selma chalk (Cretaceous) and just below an unconformable contact with the Clayton limestone (Tertiary).
The initial well-head pressures were about 1,010 pounds per square inch; initial open-flow volumes ranged from 1 to 54 million cubic feet. The original water table, probably not horizontal, is believed to have been at about 2,200 feet subsea. Deliveries from 117 gas wells that are or were connected to pipe lines amount to 31,077 million cubic feet (from the date of discovery to September 26, 1934).
The Jackson gas field occurs on the top of a large uplift. Surface rocks, of Jackson age, show at least 325 feet of structural closure. More than 1,500 feet of structural closure are now known on the upper surface of the Selma chalk. On the highest parts of the uplift the Selma probably rests directly on igneous rocks of syenitic composition. No wells have been drilled through the Selma (normally about 450 feet thick) on top of the uplift. Flank wells encounter 320–450 feet of Selma and as much as 800 feet of Eutaw-Tuscaloosa. But wells drilled 160–300 feet below the crest of the uplift pass from Selma chalk into igneous rocks.