INFLUENCE OF PRIMARY OOID MINERALOGY ON POROSITY EVOLUTION, UPPER JURASSIC SMACKOVER FORMATION, EASTERN GULF COAST PLAIN, USA

Ooids in the Smackover Formation at Womack Hill field, southwestern Alabama, have tangential, radial, and mixed tangential and radial fabrics. Based on well established petrographic criteria, tangential ooids were originally aragonite, radial ooids were originally magnesian calcite, and mixed tangential and radial ooids were originally bimineralic (aragonite and magnesian calcite).

Ooid grainstones of the Smackover Formation at this location occur at the tops of three upward-shoaling parsequences. Magnesian calcite ooids are dominant in the two lower parasequences, where the depositional environment was likely normal marine. Aragonite ooids are dominant in the uppermost parasequence, which was deposited near the termination of Smackover deposition when the basin was becoming more restricted. The Buckner Anhydrite overlies the Smackover Formation, indicating increased basin restriction.

Primary ooid mineralogy had a direct influence on porosity evolution. Aragonite ooids were commonly leached during diagenesis, resulting in high oomoldic porosity. Interparticle pore space is commonly occluded by cement, resulting in low permeability unless the rock is dolomitized or fractured. Magnesian calcite ooids were neomorphosed to calcite, and present porosity in these rocks is interparticle and solution-enlarged interparticle. These rocks have high porosities and permeabilities except where extensively physically and chemically compacted, resulting in porosity reduction from 25% to <5%. Aragonite cortices in bimineralogic ooids were preferentially leached or dolomitized, which resulted in a minor contribution to porosity (<1%).