Roles of Dissolved Methane and Sulfide in Sedimentary Dolomite Formation

溶解甲烷和硫化物在白云石化作用中的作用研究MadisonEAR-0958000美国大部分油气藏与沉积白云岩有关，白云岩在地质记录中丰富，但在全新世和现代海洋沉积物中却很稀少。沉积白云岩的形成机制仍存在争议。尽管在硫酸盐还原菌和产甲烷地下水的培养介质中已有报道，但在室温下还没有无机或非生物合成白云石和原白云石的报道。抑制白云石和无序白云石成核结晶的主要因素是水中镁离子的强烈水化作用。PIS假设原白云石和无序白云石可以在含有溶解硫化物和/或具有低分子偶极矩的有机物的溶液中成核和生长。具有低分子偶极矩的溶解分子起到了催化剂的作用，削弱了水偶极子与镁离子之间的键，从而降低了水合镁离子脱水的动能垒。为了验证这一假设，PIS建议进行以下几组实验：(I)系统地研究温度、溶解有机物和硫化物的浓度对镁在白云石和HMC中掺入的影响。(2)白云石中钙镁有序化动力学研究。(3)利用密度泛函理论(DFT)对溶解甲烷对表面镁脱水的影响进行了计算机模拟，以了解甲烷中性物种在降低白云石结晶动能垒中的作用。所有的沉淀物都将使用X射线衍射、扫描电子显微镜和透射电子显微镜以及相关的化学和结构分析技术进行仔细的表征。由于溶解甲烷等低介电性有机物起到了降低脱水和白云石结晶动能垒的催化作用，沉积白云岩和砂岩与白云石胶结作用可能与烃源岩或受含油气流体影响的岩石直接相关。所提出的白云石化机制也可能为油气源岩和储集层的实际问题以及碳汇场所的长期绩效评价提供新的线索。这项拟议的研究将有助于选择合适的碳封存地点。

Investigation on the roles of dissolved methane and sulfide in dolomitization PI: Huifang Xu, University of Wisconsin ? MadisonEAR-0958000Most 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.