Evaluation of indigenous anaerobic microorganisms from Mexican carbonate reservoirs with potential MEOR application

The success of biotechnological processes for oil recovery depends on adequate understanding of the system components: microorganisms, oil and porous media. Nine oil samples were collected from a carbonate oil reservoir in Cordoba Platform, Veracruz, Mexico. Geochemical characterisation demonstrated heavy oils with: API gravity of 10.7 to 14.5 degrees, 3 to 5% sulphur, 36.58 to 54.06% aromatic hydrocarbons and -23.98 to -24.24 (parts per thousand) delta C-13 isotopic values in total oil. Additionally, the pristane/phytane ratio (Pr/Ph) <1 indicated an anoxic depositional environment. Results showed that native microbiota could have contributed to the biogeochemical transformation of the oil in the reservoir. Anaerobic, thermophilic, halotolerant and fermentative enrichment cultures were obtained from the nine oil samples (A1-A9). Metabolites such as CO2, CH4, ethanol, acetone, acetate and biosurfactants were detected. Mixed culture from the A7 sample showed the highest activity among all the mixed cultures screened, growing under 50 to 80 degrees C, 5 to 35 g NaCl and 0.8 to 14.2 MPa pressure. Its kinetic parameters were mu(max) = 0.321 h(-1) and Ks = 0.54 g L-1. These results were used to determine the conditions for oil recovery assays. A7 culture enhanced recovery of up to 12% of heavy oil (11.16 degrees API) in oil impregnated carbonate granular porous media. Analysis of the V3 region of the 16S rRNA gene of the A7 culture showed that the predominant taxon was Thermoanaerobacter. Phylogenetic approximations demonstrated that similitude percentage of 99.9% corresponded to Thermoanaerobacter ethanolicus, 99.6% to Thermoanaerobacter pseudethanolicus and 98.9% to Thermoanaerobacter brockii and Thermoanaerobacter finii. (C) 2011 Elsevier B.V. All rights reserved.