POROSITY AND PERMEABILITY DEVELOPMENT IN CARBONATE ROCKS AFTER LONG-TERM MICROBIALLY PRODUCED ACIDIFICATION

Salem Formation limestone and Smackover Formation dolomite were inoculated with bacterial cultures using vacuum pressure. Bacterial growth was promoted by alternating phosphate and nitrate solutions in simulated production water for either two days or for every other day for two weeks. Ethanol (concentration of 15 µM) was introduced and drained and refilled to 20 ml every other day for periods of 2, 4, 6, 9, 12, 18, or 24 weeks to induce the bacteria to produce acetic acid and promote dissolution. Two 4 x 6 cm cylinders of Salem Limestone were subjected to the same protocol for five months, and rate of fluid flow was measured at regular intervals.

Initially, Salem Limestone shows pressure solution resulting in microstylolitization, interpenetrating allochems, and spalling of ooid cortical layers. Primary interparticle porosity dominates. After nine weeks, dissolution begins to cross-cut allochems and enlarges primary interparticle porosity. After 24 weeks, vuggy dissolution affects ooids and large syntaxial calcite cement. Biofilms lining pores are as thick as 0.1 mm. After five months, vuggy porosity is visible with the naked eye. Dissolution affects allochems most profoundly, but large (1 mm) syntaxial overgrowths on echinoderm fragments are affected on margins and along fractures.

Initially, Smackover Formation dolomite is dominated by intercrystalline porosity with occasional moldic pores (>1 mm diameter). Paraffin impregnation did not penetrate all pores. After six weeks, no evidence of dissolution is clearly visible, but paraffin impregnation did penetrate all pores. After 12 weeks SEM photomicrographs reveal fractures enlarged by dissolution. After 24 weeks regularly spaced voids suggest crystallographically controlled dissolution of dolomite.