Aqueous geochemistry of low molecular weight hydrocarbons at elevated temperatures and pressures: Constraints from mineral buffered laboratory experiments

Organic matter, water, and minerals coexist at elevated temperatures and pr essures in sedimentary basins and participate in a wide range of geochemica l processes that includes the generation of oil and natural gas. A series o f laboratory experiments were conducted at 300 to 350 degreesC and 350 bars to examine chemical interactions involving low molecular weight aqueous hy drocarbons with water and Fe-bearing minerals under hydrothermal conditions . Mineral buffers composed of hematite-magnetite-pyrite, hematite-magnetite , and pyrite-pyrrhotite-magnetite were added to each experiment to fix the redox state of the fluid and the activity of reduced sulfur species. During each experiment the chemical system was externally modified by addition of ethene, ethane, propene, l-butene, or n-heptane, and variations in the abu ndance of aqueous organic species were monitored as a function of time and temperature. Results of the experiments indicate that decomposition of aqueous n-alkanes proceeds through a series of oxidation and hydration reactions that sequen tially produce alkenes, alcohols, ketones, and organic acids as reaction in termediaries. Organic acids subsequently undergo decarboxylation and/or oxi dation reactions to form carbon dioxide and shorter chain saturated hydroca rbons. This alteration assemblage is compositionally distinct from that pro duced by thermal cracking under anhydrous conditions, indicating that the p resence of water and minerals provide alternative reaction pathways for the decomposition of hydrocarbons. The rate of hydrocarbon oxidation decreases substantially under reducing conditions and in the absence of catalyticall y active aqueous sulfur species. These results represent compelling evidenc e that the stability of aqueous hydrocarbons at elevated temperatures in na tural environments is not a simple function of time and temperature alone. Under the appropriate geochemical conditions, stepwise oxidation represents a mechanism for the decomposition of low molecular weight hydrocarbons and the production of methane-rich ("dry") natural gas. Evaluation of aqueous reaction products generated during the experiments wi thin a thermodynamic framework indicates that alkane-alkene, alkene-ketone. and alkene-alcohol reactions attained metastable thermodynamic equilibrium states. This equilibrium included water and iron-bearing minerals, demonst rating the direct involvement of inorganic species as reactants during orga nic transformations. The high reactivity of water and iron-bearing minerals suggests that they represent abundant sources of hydrogen and oxygen avail able for the formation of hydrocarbons and oxygenated alteration products. Thus, variations in elemental kerogen composition may not accurately reflec t the timing and extent of hydrocarbon, carbon dioxide, and organic acid ge neration in sedimentary basins. This study demonstrates that the stabilities of aqueous hydrocarbons are st rongly influenced by inorganic sediment composition at elevated temperature s. Incorporation of such interactions into geochemical models will greatly improve prediction of the occurrence of hydrocarbons in natural environment s over geologic time. Copyright (C) 2001 Elsevier Science Ltd.