ABSTRACT

The Upper Jurassic Smackover Formation is a major oil reservoir in the Eastern Gulf Coastal Plain. In marginal areas of the Manila and Conecuh Sub-basins, Smackover facies that accumulated in shallow marine carbonate settings over pre-Jurassic basement paleohighs are important exploration targets. Microbial reefal buildups consisting of thrombolitic boundstone and ooid-oncoid-peloidal grainstone/packstone representing shoal/shoreface deposits are the reservoir facies. These sediments onlap the flanks of paleotopographic highs and pinchout in the most proximal areas, as observed in the Vocation field in southwest Alabama. In lower elevated structures that were completely submerged during the transgression of the Smackover sea, these facies are distributed on the crests of the paleohighs as observed in the Appleton field in southwest Alabama.

In addition to paleotopography, there are other key factors that control the quality, thickness and distribution of these reservoirs. Diverse diagenetic processes have substantially modified the primary petrophysical properties of the reservoirs. Tectonic and isostatic subsidence are critical in the thickness of the reservoirs. In Vocation field, paleoenvironmental conditions limited the accumulation of the microbial buildups to the northern and eastern sides of the structure.

The exploration strategy in the basement ridge play should be founded on the interpretation of 3-D seismic data which is used to accurately define the location and geometry of the paleostructure as well as the distribution of the Smackover section and its potential reservoirs. Integration of the seismic interpretation with the available well data, and the application of depositional and diagenetic concepts can substantially improve the predictability of reservoir location and characterization.

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