Geochemical distribution of helium, hydrogen, carbon dioxide, and methane in Sakhalin Island mud volcanoes, hot springs, and cold seeps

Abstract

The Asian-Pacific - continental margins have been the object of intensive geochemical research in recent years, as areas within this zone have been found to contain large hydrocarbon deposits in a variety of forms including: oil, natural gas, gas condensate, and gas hydrates. One such area is Sakhalin Island, located in Russia's Far Eastern Federal District. Genetically, the territory of Sakhalin Island is a part of the shelf of the Hokkaido-Sakhalin fold area, therefore, the nature of the sub-vertical gas migration is controlled by deep faults, along which the concentrations of natural gases change, and is also influenced by high seismotectonic activity of the Okhotsk region. This paper presents data regarding the distribution of helium, hydrogen, carbon dioxide and methane from mud volcanoes, thermal springs, mineral waters and other seep types on Sakhalin island and the surrounding marine margins, along with an interpretation of the geological and tectonic factors that have likely led to the present distribution of these gases. In general, the results of geothermometry equations based on the concentration and stable isotopic composition of carbon dioxide indicate much higher temperatures than equations involving the saturation concentrations of hydrogen gas. We interpret this to indicate that deeply-sourced carbon dioxide does not reequilibrate at near-surface conditions, whereas the equilibration time for hydrogen gas is relatively rapid. Areas on the island which are observed to have active degassing, coupled with high apparent reservoir temperatures also provide indicators of geodynamics along the regional tectonic structures. Mud volcanoes to the south of the island are predominantly situated at the juncture between NW-SE trending faults which extend the entire length of Sakhalin and continue as far south as Hokkaido Island, and more recently developed NE-SW trending faults. Despite the relatively low water temperatures of these mud volcanoes, the high reservoir temperatures indicated by gas chemistry suggest a source of fluids and gas at several km from the surface.