The first production of hydrocarbons from post-Paleozoic rocks in Mississippi resulted from the discovery of the Jackson gas field in 1930. Production was from the "Jackson gas rock," a local reef facies, in the upper part of the Selma Group of Late Cretaceous age. The field now is depleted after having produced more than 117 billion cu ft of gas, and is used for gas storage. Since the discovery of the Jackson gas field, nonassociated-gas production has been established from Tertiary Wilcox, Cockfield, and Sparta; Upper and Lower Cretaceous; and Upper Jurassic strata. By far the most important production has been from the Upper Cretaceous sandstones of the Eutaw and Tuscaloosa. Dissolved or associated gas is produced from many oil wells in Mississippi. However, 70-80 per ent of all the gas produced in Mississippi from post-Paleozoic reservoirs in the past 10 years was nonassociated. Many of the nonassociated-gas fields also produce condensate in varied quantities.

In 1964, Mississippi ranked eighth among the states in estimated proved reserves, and ninth in net production of natural gas. Over the past 10 years, Mississippi has shown a net decrease of 88,233 Mcf in estimated proved recoverable reserves.

Most of the known natural gas accumulations in Mississippi are controlled structurally, but this control is modified by changes in character of the producing reservoirs and some accumulations are in combination structural-stratigraphic traps. Most of the known local structural features are the result of salt movement, although shallow salt diapirs have not afforded significant production of either oil or gas in Mississippi. All of the major gas-producing fields in Mississippi are at least 10 years old, and there have been no large significant gas discoveries since Maxie field was found in 1952. The future supply of natural gas in Mississippi, therefore, will depend on success in location of gas-bearing stratigraphic traps and additional structures in the Upper Cretaceous and Tertiary, and on deep exploration of Lower Cretaceous and Jurassic strata. The lowermost Cretaceous and Jurassic have been explored only partially because of depth and high costs of exploration and development. Production has been established from these strata in several wells. Most of the Upper Jurassic gas, particularly from the Smackover, is reported to contain large quantities of hydrogen sulfide and inert gases.