GEOLOGICAL CONDITIONS OF FORMATION AND DISTRIBUTION OF GAS-GEOCHEMICAL FIELDS IN BOTTOM SEDIMENTS OF THE LAPTEV-SIBERIAN SEA ZONE OF THE EAST ARCTIC SHELF

Gas composition of bottom sediments of the Laptev-Siberian Sea geostructural zone (LSZ) was determined to include: helium and hydrogen in concentrations of 0.0028–0.2092 and 0.0012–0.8727 cm³/kg, hydrogen sulfide – 0.0006–0.0072, carbon monoxide – 0.0016–0.0577, carbon dioxide – 0.1482–21.1602, methane – 0.0017–8.3047 and its homologues (in total) – 0.00001–0.0355 cm³/kg. It was found that the values of average concentrations of gases in bottom sediments increase with the depth of their occurrence and exceed the anomaly criteria identified for the East Arctic shelf by 2.6–27.7 times, including: CO₂, CO and H₂S – by 2.6, 3.0 and 3.4 times, ΣC₂–C₅ and He – by 9.3 and 10.8 times, CH₄ – by 27.7 times; this actually indicates a high degree of gas saturation of bottom sediments of the region. The formation of CO₂, CO, H₂S, H₂, CH₄ and ΣC₂–C₅ anomalies in sediments of the upper and the base of the middle sampling horizon is determined by relatively high contents of sapropelic and humic organic matter and gas influx from the lower horizon. Distribution of anomalies with maximum concentrations of natural gases and He in sediments of the lower horizon, with minimum contents of C_org, is associated with zones of faults, large dislocations, tectono-magmatic and seismic activity, which are the main routes of diffusion-migration of gas transfer to bottom sediments. Formation of concentrations of gases and their anomalies in sediments of the LSZ follows the rules of additivity, i.e. successive accumulation of migratory natural gases of different genesis with prevalence of gas phase and isotopic indicators of more gas-saturated parental source, epigenetic and syngenetic to sediments. It was found that maximum values of average gas saturation with CH₄, ΣC₂–C₅, He and H₂ in LSZ are typical for bottom sediments of the flanks of troughs, CO and CO₂ – for the arch parts of uplifts. In sediments of monoclinal folds (structural terraces), minimal values of average gas saturation are observed, with the exception of the South Anyui Fault Zone. In general, gas saturation of LSZ sediments is determined by the complex impact of geological factors, with the major ones being: gas content of rock complexes and gas saturation of underlying sediments, fault and fold tectonics, a high degree of tectono-magmatic and seismic activity, a geostructural position, coal content, oil and gas content, as well as the depth of occurrence, organic saturation and material composition of OM in bottom sediments. The influence of the latter predetermined the formation and distribution of five geochemical fields of CH₄, three of CO, ΣC₂–C₅, He and one of CO₂, and one of H₂ in LSZ.