Investigation on the roles of dissolved methane and sulfide in dolomitization PI: Huifang Xu, University of Wisconsin ? Madison EAR-0958000
Most hydrocarbon reservoirs in the US are related to sedimentary dolomites which are abundant in the geologic record, yet scarce in Holocene and modern marine sediments. The formation mechanism for sedimentary dolomite is still under debate. Although ankeritic dolomites have been reported in cultured media of sulfate-reducing bacteria and methanogenic groundwater, dolomite and protodolomite have not been synthesized inorganically or abiotically at room temperature. The main factor inhibiting dolomite and disordered dolomite nucleation and crystallization is the strong hydration of aqueous Mg2+ ions. PIs hypothesize that protodolomite and disordered dolomite can nucleate and grow in solutions containing dissolved sulfide and /or organics with low molecular dipole moments. The dissolved molecules with low molecular dipole moments behave as catalysts that weaken the bonds between the water dipoles and Mg2+ therefore lowering the kinetic energy barrier of dehydration of the hydrated Mg2+. In order to test the hypothesis, PIs propose to carry out the following sets of experiments: (i) a systematic investigation on the effects of temperature and concentrations of dissolved organics and sulfide on Mg incorporation into dolomite and HMC. (ii) Study on Ca-Mg ordering kinetics in dolomite. (iii) Computer modeling of the effect of dissolved methane on surface Mg2+ dehydration using Density Function Theory (DFT) in order to understand the role of neutral species of methane in lowering the kinetic energy barrier of dolomite crystallization. All of the precipitates will be carefully characterized using X-ray diffraction, SEM, and transmission electron microscope (TEM) with associated chemical and structural analysis techniques. Because dissolved methane and other low dielectric organics serve as catalysts for lowering the kinetic energy barrier of dehydration and dolomite crystallization, sedimentary dolomites and sandstones with dolomite cementation may be directly related to hydrocarbon source rocks or rocks influenced by hydrocarbon-bearing fluids. The proposed dolomitization mechanism may also shed new light into practical problems of hydrocarbon source rocks and reservoirs and long-term performance evaluation of sites for carbon sequestration. The proposed study will help choose appropriate sites for carbon sequestration.
