ABSTRACT

The Jurassic Smackover Formation is an important and prolific hydrocarbon producing geologic unit in the onshore eastern Gulf of Mexico area, including onshore Alabama. To date, however, no Smackover strata containing commercial accumulations of hydrocarbons have been reported in Alabama or panhandle Florida state coastal waters or in the adjacent parts of the Federal OCS Mobile, Pensacola, Viosca Knoll, and Destin Dome areas. To assess the hydrocarbon potential of the Smackover in the offshore eastern Gulf of Mexico, it is necessary to understand the regional geologic framework and petroleum geology of the Smackover Formation and sub- and superjacent strata in both onshore and offshore areas.

The Smackover Formation is a regionally extensive carbonate unit that subcrops around the Gulf Coast basin. Depositional patterns of the Smackover Formation, which relate directly to the distribution of reservoir strata, were controlled to a large degree by pre-Smackover geology and paleotopography. The Smackover Formation in the Alabama onshore and offshore area and in the extreme western part of the panhandle of Florida accumulated in a modified carbonate ramp setting with an inner ramp complex in the Mississippi interior salt basin and the Conecuh and Manila embayments and an outer ramp south of the Wiggins arch complex. South of the Pensacola-Decatur ridge complex offshore of the Florida panhandle, the Smackover Formation probably accumulated in a ramp setting that was contiguous with the outer ramp in offshore Alabama.

Numerous petroleum fields are developed in Smackover strata on the inner ramp, where the informal middle member of the Smackover is the inferred source rock and the overlying Buckner Anhydrite Member of the Haynesville Formation provides a regional seal. Reservoirs, which are principally developed in the informal upper member of the Smackover, include dolostone and lesser limestone. On the outer ramp south of the Wiggins arch complex, reservoir-quality rocks are limited to the coastal dolostone lithofacies which is not found in the more down-dip areas. To date, only one field has been established south of the Wiggins arch. Reservoir-grade porosity has been found in dolomitized Smackover strata in the offshore Florida panhandle area; these strata may be genetically related to the coastal dolostone of extreme southern and offshore Alabama. Lithofacies relationships of Smackover strata in the Pensacola, Destin Dome, and Apalachicola OCS areas are unknown (because few rock samples are available) and must be inferred from geophysical log signatures. Carbonate strata that are probably rich in organic material occur in the lower and middle outer-ramp lithofacies and although no significant hydrocarbon accumulations have been found in the Smackover in this area, these strata are interpreted to be the source rocks for the large amounts of natural gas found in the Norphlet Formation offshore.

Considering the widespread occurrence of apparently organic rich, prolific hydrocarbon source strata in the Smackover Formation, the most important factors controlling the accumulation of hydrocarbons in the unit in the study area are the distribution of porous and permeable strata and the development of hydrocarbon traps. Within the study area, four structural settings have been identified. These are based on the potential for trap formation and on the kinds of traps expected within each setting. These settings, which are designated Settings A, B, C, and D, are related to the presence and thickness of Louann Salt. Further, four geographic regions, which reflect overall similarities in structural and/or paleogeographic settings, have been delineated. These are designated Regions 1, 2, 3, and 4. Within Regions 2 and 3, subregions (2a, 2b, 2c, 3a, 3b) have been defined to identify internal variations in rock types. These settings and regions are used to characterize the potential for the discovery of significant hydrocarbon accumulations in the Smackover Formation in the study area.