Genesis of Methane Entrapped in Ground Ice, Yamal Peninsula, WS Russia
In the North of Yamal Peninsula, near Bovanenko gas field. The sediments contain ground ice bodies (large tabular ice bodies with volumetric ice content higher than 95%), icy sediments (sediments with a volumetric ice content from 50 to 90%) and syngenetic ice wedges in upper part of geological section (20 m). Various ice types were collected to measure the relative abundance of CH4 , N2 , CO2 , O2 , and H2S, as well as δC13(CH4), δC13(CO2) entrapped in the ice. The concentration of CH4 entrapped in the ice was distinguished between ice wedges, massive ground ice, and icy sediments. CH4 in ice wedges have preserved a signature of slightly modified atmospheric values, whereas methane in intrasedimental ice shown the dissolution of the gases in water before freezing. In Holocene ice wedges methane content is 0.16%, which is similar to the concentration of atmospheric methane in preindustrial Holocene period. Methane content in Late Pleistocene polygonal ice wedges is 0.0044%, which reflects the methane composition of atmosphere in Last Glacial Maximum period. The highest methane content up to 1.37% is found in icy sediments formed under conditions of syngenetic freezing of marine-lacustrine sediments. The mass spectrometer analysis showed δC13(CH4) is -70.5 ‰ indicating gas biochemical origin. In thick massive ground ice deposits formed under conditions of prolong epigenetic freezing methane is almost absent (less than 0.03%) because of oxidation processes. Results from 40 boreholes in Bovanenkovo gas field showed methane emission with similar isotope content at 60-80 m depth in Quaternary sediments of Bovanenkovo field. The gas outlets corresponded with sandy-clay frozen sediments, lenses of cryopegs, and sometimes to ice bodies. Maximum gas flow rate was measured at 14000 m3/day. Marine saline sediments up to a depth of 40-80 m contain ground ice (icy sediments 1-3 m and massive ground ice bodies more than 50 m thick). Organic carbon content in marine sands is 0.1-0.6%, in clays 0.8-1.2%. In coastal and alluvial-marine sediments the organic carbon content is 0.1-0.7% and higher. Hydrate stability zone is found significantly lower than gas outlets. It is suspected that methane contained in intrasedimental ice at 80-130 m depth is transferred to free form around 0° C. Voids filled with gas occupy cavities in ice and clay and are not a product of deep methane hydrate degassing.
AAPG Search and Discovery Article #90177©3P Arctic, Polar Petroleum Potential Conference & Exhibition, Stavanger, Norway, October 15-18, 2013