The Role of Water and Inorganic Minerals in the Formation of Oxygenated Organic Alteration Products during Petroleum Maturation
SEEWALD, JEFFREY S.
Laboratory experiments were conducted at 300 to 350 degrees C and 350 bars to constrain reactions that regulate the generation of oxygenated organic alteration products during the thermal maturation of petroleum in the presence of water. The experiments differed from conventional hydrous pyrolysis experiments by containing individual low molecular weight hydrocarbons and inorganic iron-bearing mineral assemblages that buffer the activities of dissolved hydrogen, oxygen, and hydrogen sulfide. This approach allowed careful mass balance on oxygen, hydrogen, and carbon and interpretation of results within a thermodynamic and kinetic framework.
Results of the experiments indicate that short-chain alkenes are highly reactive in the presence of water under subsurface conditions and readily react to produce their corresponding n-alkane, methane, ketones, alcohols, carboxylic acids, and carbon dioxide. Additional experiments have shown that alkenes are rapidly formed from their corresponding n-alkanes at relatively low abundances controlled by metastable thermodynamic equilibrium involving water and redox dependent inorganic mineral assemblages. These data suggest that although alkenes are not abundant in petroleum, they may represent important reaction intermediaries during oxidation reactions that produce oxygenated organic alteration products.
Sources of oxygen during the experiments included water and/or iron oxide minerals while hydrogen was derived from water and the initial hydrocarbons. The experiments demonstrate that in the presence of water, oxygen derived from, inorganic sources is readily incorporated into organic alteration products. Accordingly, predictive models that do not include inorganic sources of oxygen may underestimate the amounts of oxygenated organic alteration products such as carboxylic acids and carbon dioxide that can be generated within a sedimentary basin.