Promotion of Carbonate Precipitation by Microbial Iron Reduction

Diagenetic carbonate cements are important in petroleum systems due to due to their modification of depositional porosity and permeability [1]. Microbial sulfate reduction is known to promote precipitation of carbonate cements and this process has been well-studied in a number of experimental and in situ settings [2]. Alternatively, iron reduction produces far more alkalinity per unit carbon respired than sulfate reduction. Iron reduction has been implicated in carbonate cementation in a variety of settings [3], but little is known about the circumstances under which iron reduction may be effective in promoting cementation. Understanding the mechanisms of mineral precipitation during microbial iron reduction will contribute to facies-based models of carbonate diagenesis and therefore improve the predictability of early diagenetic modifications to reservoir porosity.

In order to investigate carbonate precipitation during microbial iron reduction, Shewanella oneidensis MR-1, a dissimilatory iron reducing bacterium, was cultured anaerobically (79%N2, 20%CO2 and 1%H2) in basal medium at pH 7.0-7.2 to trigger iron reduction. Lactate was used as the electron donor, and fresh ferrihydrite was supplied as the electron acceptor. Culture medium was amended with 10 mM CaCl2, MgCl2, or both to produce carbonate supersaturated solutions (~35 times calcite saturation). Escherichia coli (with fumarate added as an electron acceptor) was used to provide a comparison to live but non-iron- reducing cells. After 20 days incubation, precipitate was collected, washed and identified by Raman microspectroscopy and electron microprobe. Preliminary results indicate abundant calcium carbonate precipitation in all S. oneidensis cultures with Ca2+, but not in abiotic or E. coli controls. Iron reduction rate, pH and amount of precipitate were correlated. Calcium carbonate precipitates occurred as roughly spherical grains around 2 µm in diameter in the first 20 days of incubation and grew up to 1 mm in the next two months as ferrihydrite was progressively dissolved. Dolomite has not yet been observed despite saturation with this mineral. These results confirm that microbial iron reduction has the potential to promote significant carbonate precipitation. Ongoing experiments are identifying conditions under which different carbonate and oxide minerals are favored as diagenetic precipitates.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California