Sedimentary rocks downdip from present lower Miocene production in Louisiana and Texas, and in the Texas offshore area on strike with this producing trend, offer the best potential for major future petroleum reserves in lower Miocene-Oligocene beds of the western Gulf basin. These prospective beds are present in an area about half the size of the present lower Miocene trend; the area is divided into two roughly equal parts--a probable producing area where an environment favoring hydrocarbons is known to have existed, and a possible producing area where such an environment reasonably can be inferred.

Future lower Miocene-Oligocene discoveries should be found under conditions similar to those controlling present production--where reservoirs are typically sandstone and traps are usually associated with salt domes, fault closures, anticlines, residual highs, and, in some places, stratigraphic changes. Depositional environment is critical, the ideal habitat for petroleum being a thick section of deltaic or shallow-neritic sandstone interbedded with marine shale. Depositional environment limits the extent of production downdip, because deepwater shale succeeds sandstone.

Miocene-Oligocene strata of Mississippi, Alabama, and Florida are thin, nonpetroliferous carbonate rocks. They give little promise for future discoveries, although thicker, deeper reef-carbonate rocks may be present east of the Mississippi delta.

Additional reserves will be found from the Rio Grande to southeastern Louisiana within present producing trends, where the most productive areas in successively younger intervals commonly are found progressively downdip and northeast along strike. Production from the middle Oligocene Frio and the lower Miocene beds is especially prolific.