Investigation of Microbially Mediated Clay Mineral Transformation

Deb P. Jaisi
Department of Geology, Miami University
Oxford, OH
[email protected]

The smectite-illite (S-I) reaction has been used as an indicator for many geological activities such as hydrocarbon maturation and pore water chemistry because development of 60-80% illite in the mixed layered illite/smectite is correlated with the beginning of oil generation from kerogen. With progressive transformation of smectite (>85%) to illite organic matter over-matures and gas begins to produce. A recent study has shown importance of microbes in promoting the S-I reaction by dissolving smectite and precipitation of illite. The smectite dissolution was made possible by microbial reduction of Fe(III) in the smectite structure. However, this microbial role in the S-I reaction has not been quantified in the previous S-I reaction kinetic models.

Building on previous results I propose to systematically study the S-I reaction by using Shewanella putrefaciens CN32 emphasizing the modeling aspect of the research. The study will be divided into laboratory and modeling components. For the laboratory study, experimental parameters (time-course rate and extent of microbial reduction of Fe(III) in the smectite structure and subsequent reaction of smectite to illite) as a function of electron donor and acceptor (smectite) concentrations will be collected. The decrease in the reaction rates with time due to sorption of reduced smectite into cell surface will be identified. The mineralogical investigation of microbial enhanced transformations of S-I reaction will be performed using X-ray diffraction, and environmental cell transmission electron microscopy (EC-TEM).

The smectite to illite reaction kinetics largely depends on reaction mechanism. Recent studies have shown that the reaction is via dissolution-precipitation, and that the reaction kinetics consists of two steps: dissolution of smectite and precipitation of illite. It is important to determine the rate-limiting step. Fe(II) sorption capacity on the reactant smectite samples is important and will be plotted against Fe(II)_aq and smectite concentrations. The experimental data will be fitted to identify saturation kinetics of Fe(II) as a function of smectite concentration. Rate of formation of illite layers in S-I reaction will be normalized to Fe(II) sorption capacity of the reactant smectite and plotted as a function of electron acceptor and donor concentrations. Experimental illite formation rates will be numerically fitted to generate a function, similar to that for microbial reduction of goethite. Simulations of bacterially mediated smectite to illite conversion will be performed. The applicability of kinetic parameters will be tested against results from smectite to illite conversion curves for natural systems, such as Nankai Trough.

AAPG Search and Discovery Article #90033©2004 AAPG Foundation Grants-in-Aid