Lithological Composition and Hydrocarbon Anomalies of Bottom Sediments in the Western Part of the East Siberian Sea

Based on the lithological and gas-geochemical studies and a comprehensive interpretation of the available materials, the main factors of the formation and distribution of grain size distribution, water-physical properties, organic saturation, as well as concentrations and geochemical parameters of hydrocarbon (HC) gases in seafloor sediments of geostructures in the western part of the East Siberian Sea are summarized and analyzed. It has been established that the sediments in the northern and central parts of the studied region are represented by aleurite–pelites; in the southern part, by aleurite–pelite–psammites with the following parameters: C_org—0.6–2.0%, natural moisture content (hereafter, wetness)—18–43%, density—1.5–2.0 g/cm³, open porosity—17–33%, concentrations of methane and its homologs—0.001–5.934 and 0.00003–0.0312 cm³/kg, respectively, molecular mass of the HC fraction—16.05–22.6 g/mol, coefficients of “wetness”, “dryness”, and transformation—0.2–51, 1–1999, and 0.2–50.8%, respectively, and δ¹³С‒СН₄ varying from –82.7 to –38.4‰. Based on the values of gas-geochemical parameters, eleven types of gas sources were identified in bottom sediments. The results showed that the formation of HC anomalies in sediments is governed mainly by their reservoir properties, C_org content, sampling depths, integrated influence of geological factors, such as gas saturation of the underlying sediments, type of gas sources, fault and fold tectonics, position of geostructures, thickness of Quaternary sediments, content of coal and gas, and age of the folded basement. Anomalies of HC gases are associated to a lesser extent with the lithological composition and density parameters of sediments.