DOLOMITIZATION OF THE SMACKOVER FORMATION AND ITS APPLICATION TO HYDROCARBON EXPLORATION IN MISSISSIPPI

The Smackover Formation (Upper Jurassic) was dolomitized to various degrees around the U. S. Gulf Coast. In most cases, dolomitization enhanced porosity and permeability of the unit, leading to the formation of excellent hydrocarbon reservoirs. Because dolomite is more resistant to compaction than limestone, Smackover dolomite reservoirs remain porous and permeable to depths greater than 6 km in this area.

In Mississippi, the Smackover Formation consists of limestone except on crests of high-rise salt structures where nearly the entire thickness of this unit is dolomitized. Dolomitized strata form excellent oil and gas reservoirs, whereas the adjacent limestones on the same structures lack porosity and permeability. This unique system is studied to evaluate mechanism, timing, fluid flow, and duration of dolomitization process in Mississippi and its application to hydrocarbon exploration in this area.

The study concludes that dolomitization postdated deposition of the Smackover Formation, but it was contemporaneous with the precipitation of the overlying Buckner Anhydrite. Dolomitization took place by the seepage of Mg-rich, Ca-deficient Buckner brine into Smackover grainstones through subaerial exposure surfaces that developed on crests of salt domes. This brine was derived by evaporation of Buckner seawater to anhydrite saturation in lagoonal environments between salt domes. Dolomitization process continued for the duration of Buckner Anhydrite deposition.

Dolomitization was possible only when the salt movement exposed Smackover grainstones close to the sea-level. A rapid rise of salt domes would have displaced grainstones beyond the reach of the brines. A slow salt movement would have left them in subtidal environments where deposition of anhydrite or lime mudstone would have shut down the brine seepage.

In high-rise salt structures of central Mississippi dolomitized Smackover can be found where the Buckner Anhydrite is thin or absent. But, this situation means the lack of Buckner seals resulting in the leakage of potential hydrocarbons from these reservoirs. Therefore, other reservoir – seal geometries need to be considered.