MICROBIALLY MEDIATED POROSITY ENHANCEMENT IN CARBONATE ROCKS

Microbially mediated porosity enhancement is the process of applying nutrient solutions followed by ethanol to induce bacteria to generate acids, thereby enhancing porosity and permeability in carbonate rocks. The goal of this study is to document the role of microbes in dissolution of three rock types: 1) Completely dolomitized Jurassic Smackover Formation from Container Corp. 34-9, Monroe County, Alabama; 2) Cretaceous rudist packstone from the Sligo Formation, Mobil McElroy-1, Webb County, Texas; and 3) quarried Mississippian packstone from the Salem Formation, Indiana.

A vacuum pump and filtration apparatus were used to introduce cultured bacteria into rock samples 0.5 to 1.5 cm thick. Bacterial growth was promoted by alternating nitrate and phosphate nutrient solutions for either two days or every other day for two weeks. Ethanol was introduced and maintained every other day for either two weeks or four weeks to promote dissolution.

SEM, confocal laser microscopy and petrographic thin sections revealed that bacterial biofilm lines pores in all three rock types. The most abundant biofilms were seen in samples treated for two weeks with nutrients. No clear evidence of dissolution was observed in Smackover Formation samples. Sligo Formation samples treated with nutrients for two days and ethanol for two weeks were impregnated with biological paraffin prior to making thin sections. The paraffin preserved rod-shaped bacteria, which line pores. SEM images of Salem Formation control samples, which were not inoculated, revealed smooth surfaces on large crystals (>40 µm) and euhedral small crystals (1-2 µm). The samples with the greatest evidence of dissolution were Salem Formation samples treated with nutrients for two days and ethanol for four weeks. These samples contained rounded small crystals and regularly distributed, irregularly shaped pores in large crystals.