The Middle Mesozoic Drift and Cooling Phase begins with the main phase of sea floor spreading, slowly but steadily opening the Gulf of Mexico basin. Initially hypersaline conditions resulted in basin-wide deposition of an original thickness of 4 km of evaporites (halite and updip anhydrite), called the Louann Salt, which likely formed with episodic seawater influx from the Atlantic Ocean. Strontium seawater analysis suggests 170 Ma as a proxy age for the Louann Salt. The arid eolian Norphlet Formation is subsequently deposited, followed by marine carbonates, evolving from ramp microbalites (Smackover) to platform margin reef systems of the Haynesville and Cotton Valley. Rafting apart of the Smackover and Norphlet in the northeast Gulf of Mexico began in this phase, possibly associated with oceanic crustal cooling which created a dip slope to the south and west. This set up a major new petroleum province which is host to several new giant oil discoveries. Periods of reduced bottom circulation resulted in at least two phases of source rock development, in the Oxfordian and Tithonian stages, that are linked to petroleum generation for both conventional and unconventional plays.

The Gulf of Mexico petroleum habitat is broad and diverse, with virtually every depositional unit or supersequence producing hydrocarbons onshore or offshore in the USA, Mexico, or Cuba. Oil and gas fields and undiscovered resources follow a concentric trend, with Mesozoic hydrocarbons resources surrounding the prolific Cenozoic basin center. The most recent and expected future discoveries are in the deepwater subsalt domain of the USA and Mexico, though a potential pre-salt frontier remains to be tested. Characterization of emerging (deepwater Tuscaloosa and Norphlet), existing (deepwater Wilcox), and mature (Plio-Pleistocene minibasin) conventional exploration plays yields new insights but also important exploration lessons, such as the Perdido fold belt BAHA wells, which ultimately set-up deepwater Wilcox exploration in the Gulf of Mexico, with large discoveries as recently as 2017. Unconventional onshore plays are well-established (Eagle Ford), emerging (Agua Nueva), or technically challenged (Tuscaloosa Marine Shale). The seismic technology evolution that underpins current success in the subsalt of the US sector will undoubtedly impact new exploration in the Campeche salt province of Mexico.