Effect of structural features of sedimentary cover on gas composition of permafrost table in northern West Siberia

Abstract

Here, we analyzed geochemical indices of gases from core samples of permafrost table and evaluated data acquired from electric survey at the field of the Nadym-Pursk petroleum-bearing region. The Nadym-Pursk region is located in the northern West Siberia that is presently the major object of raw hydrocarbon (HC) exploration. The geologic exploration practice has demonstrated that the seismic survey efficiency is notably influenced by heterogeneous rocks of permafrost table, especially within permafrost distribution zones. The exploration efficiency for oil and gas can be enhanced using data from surface geochemical surveys that rely on vertical HC migration from deposits. The adequate interpretation of both seismic and geochemical survey data requires insights on the structure of permafrost table and on processes occurring therein. This study aimed to explore if geologic structural features of the territory affect the composition of gases dispersed in the permafrost table and identify the role of non-geological factors. The gas compositional features of the permafrost table were associated with HC migration channels, frost mounds, and probable gas hydrates within the permafrost distribution zone. The distribution of deep (He and H2) and catagenic (light methane homologues) gases stems from tectonic activity. This conclusion was made from the isotopic assay of samples from several holes, from the correlation between methane and homologues, from the presence of migration channels as per electric survey data, and essentially from the presence of deep gases. The enhanced content of methane is due to its active migration from both diagenetic zone (biogenic methane) and Cenomanian gas pay zones, which along with the respective geotemperature regime of subpermafrost strata is a favorable factor for gas hydrate generation. The natural connections between data from frost mounds point to their distinct genesis: the compositional genesis of gases is influenced by deep degassing and gas accumulation from diagenetic zones.