Formation of geochemical anomalies in hydrocarbon migration in the permafrost zone of Western Siberia

Abstract

Climate warming can be caused by global changes due to emissions of the greenhouse gases, which are mainly carbon dioxide and methane. Although vertical migration of hydrocarbons (seepages) to the sur- face from oil and gas fields has been known for many years, this important environmental factor has not yet received due attention in the study of the Arctic and Subarctic regions. The major hydrocarbon-induced chemical and mineralogical changes within the permafrost stratum were investigated in the south of the Taz Peninsula in Western Siberia. The samples of frozen core from the deep (35 m) hole, which had been drilled from the top of hydrolaccolite, were examined to analyze the cryogenic texture of the frozen rock mass, to estimate the gas content in rock and ice, and to determine the authigenous mineral association using SEM and EDX spectroscopy analysis. It is shown that the migration of hydrocarbon gases through the permafrost stratum is caused by shear deformations with the formation of cryogenic crack-type textures on the sliding surfaces, which are characterized by the presence of gas-saturated ice crystallites and high jointing of quartz. It has been established that the migration of hydrocarbons, primarily methane, frozen in sedimentary strata causes significant changes of the pH/Eh parameters: local anaerobic conditions may be changed by micro- aerophilic ones through the formation of oxygen during crystallization of the water in the slide area; mainly neutral and weakly acidic conditions can locally be changed by the alkaline ones due to the cryogenic con- centration of chlorides during freezing. It was found that the impulse character of hydrocarbon migration in permeation zones of frozen strata causes mosaic distribution of sulfate and iron reduction processes, which control the neogenesis (including as a result of microbiological processes) of various forms of iron com- pounds: sulfides - carbonates - oxides.