Influence of Primary Ooid Mineralogy on Porosity Evolution in Limestone and Dolomite Reservoirs: An Example from the Eastern Gulf of Mexico Basin

Abstract Ooids in the Jurassic Smackover Formation of the eastern Gulf of Mexico Basin 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 Womack Hill Field 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, which resulted in porosity reduction from 25% to less than 5%. Aragonite cortices in bimineralogic ooids were preferentially leached or dolomitized, which resulted in a minor contribution to porosity (<1%).