Hydrocarbons from a volcanic area. Oil seeps in the Uzon caldera, Kamchatka

The isotope and hydrocarbon compositions of oil seeps in the caldera of Uzon Volcano were investigated. An experiment was conducted to simulate the thermal hydrolysis of biota in the hot springs of the caldera, which is considered as a source of carbon for the oil seeps. The same methods were applied to investigate petroleum condensate and thermal gases (CH4 and CO2) from a well in the Bogachevka field adjacent to the Uzon caldera. The measured carbon isotope composition of oils from the Uzon caldera (δ13C of −31.24 and −31.47‰) is typical of oils derived from biological materials of mostly bacterial origin. The low content of tricyclic terpanes, which is a qualitative oil maturity parameter, indicates that the oil maturity of the Uzon seeps is low. In the gas samples, the δ13C of CO2 ranges from −6.5 to −0.18‰ averaging approximately −2.8‰. The δ13C of methane is from −30.13 to −19.03‰ averaging approximately −24.0‰. A difference was detected between the isotopic fractionation characteristics of naphtides from the Uzon caldera and products of the thermal hydrolysis of biota. In turn, the isotopic fractionation characteristic of thermally hydrolyzed biota appeared to be similar to the proportion of the isotopic compositions of the respective fractions of Bogachevka oil. The identical isotopic diagrams of the Bogachevka oil and the hydrothermally transformed biota confirm the biogenic nature of the Bogachevka oil. Hydrocarbons from the Uzon caldera (Kamchatka) are mainly products of the hydrothermal transformation of the biomass of microorganisms and plant remains in the zone of thermal water circulation.