Geological characteristics оf subpermafrost gas hydrate reservoir оn the Taimyr shelf of the Kara Sea (Eastern Arctic, Russia)

Abstract

The conditions for the formation of gas hydrates associated to subsea permafrost in the Kara Sea are predicted based on numerical modeling. The forecast of the distribution of the relic submarine permafrost and related methane hydrate stability zone is given on the basis of solving the equation of thermal conductivity. According to modeling data, an extensive thermobaric relict submarine permafrost zone is predicted within the Kara Sea shelf. The greatest thickness (up to 600 m) of the permafrost is confined to the Taimyr shelf. Based on the results of the analysis of our model, drilling seismic data, the southwestern shelf of the Kara Sea is characterized by insular or sporadic permafrost. In the northeastern part, the nature of permafrost is also discontinuous, despite the greater thickness of the frozen strata. For the first time, accumulations of cryogenic gas hydrates on the Taimyr shelf have been characterized. The new drilling data obtained, seismic data reinterpretaion and numerical modeling have shown that the gas hydrate reservoir is confined to unconformably occurring Silurian‒Devonian and underlying Triassic-Jurassic strata. The thickness of the gas hydrate reservoir varies from 800 to 1100 m. Based on the interpretation of CDP data and their comparison with model calculations, frozen deposits and sub-permafrost traps of stratigraphic, anticline and anticline-stratigraphic types were identified for the first time. These pioneering studies allowed to characterize the thickness and morphology of the gas hydrate reservoir, giving a preliminary seismostratigraphic reference, and to identify the potentially gas-hydrate bearing structures. Due to favorable thermobaric and permafrost-geothermal conditions, most of the identified traps may turn out to be sub-permafrost accumulations of gas hydrates. In total, at least five potential accumulations of gas hydrates were discovered, confined to structural depressions ‒ Uedineniya Trough and its side included Egiazarov Step and North Mikhailovskaya Depression.