The South Mississippi Salt Basin is one of the three interior basins of the Gulf Coast region of the United States characterized by structures formed by movement of late Jurassic Louann Salt. An analysis of pathways of migration within the basin has revealed that it is possible to explain why hydrocarbons have accumulated in some structures, yet are absent from others that would appear to be favorable. Of the more than 840 fields within the basin, 74, including the largest known accumulations, have hydrocarbons stacked in reservoirs in more than one formation. These stacked reservoirs result from vertical migration brought about by faulting. More than 750 fields have hydrocarbons confined to one formation in traps associated with four distinct trends of production that decrease in age systematically from the margin of the basin into the interior. The hydrocarbons in these trends have accumulated by intrastratal migration, without the agency of faulting, from a nearby source in the same unit as the reservoir. On the northwest side of the basin, migration between units brought into contact along unconformities has resulted in ten fields. Vertical migration brought about by faulting around shallow salt diapirs has allowed hydrocarbons to escape, so that only 5 of 56 such structures have ever produced oil or gas. These conclusions, based only on geological deductions, have been confirmed by geochemical analysis of the hydrocarbons. The products from each of the sources are chemically distinct, and oil from the deepest source, the Smackover formation, can be recognized in reservoirs many thousands of feet higher in the Cretaceous section.

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