Deep hydrocarbon cycle

Abstract

Research subject. Experimental modelling of the transformation of complex hydrocarbon systems under extreme thermobaric conditions was carried out. The results obtained were compared with geological observations in the Urals, Kamchatka and other regions.Material and methods. The materials for the research were a model hydrocarbon system similar in composition to natural gas condensate and a system consisting of a mixture of saturated hydrocarbons and various iron-containing minerals enriched in 57Fe. Two types of high-pressure equipment were used: a diamond anvils cell and a Toroid-type high-pressure chamber. The experiments were carried out at pressures up to 8.8 GPa in the temperature range 593–1600 K.Results. According to the obtained results, hydrocarbon systems submerged in a subduction slab can maintain their stability down to a depth of 50 km. Upon further immersion, during contact of the hydrocarbon fluid with the surrounding iron-bearing minerals, iron hydrides and carbides are formed. When iron carbides react with water under the thermobaric conditions of the asthenosphere, a water-hydrocarbon fluid is formed. Geological observations, such as methane finds in olivines from ultramafic rocks unaffected by serpentinization, the presence of polycyclic aromatic and heavy saturated hydrocarbons in ophiolite allochthons and ultramafic rocks squeezed out from the paleo-subduction zone of the Urals, are in good agreement with the experimental data.Conclusion. The obtained experimental results and presented geological observations made it possible to propose a concept of deep hydrocarbon cycle. Upon the contact of hydrocarbon systems immersed in a subduction slab with iron-bearing minerals, iron hydrides and carbides are formed. Iron carbides carried in the asthenosphere by convective flows can react with hydrogen contained in the hydroxyl group of some minerals or with water present in the asthenosphere and form a water-hydrocarbon fluid. The mantle fluid can migrate along deep faults into the Earth’s crust and form multilayer oil and gas deposits in rocks of any lithological composition, genesis and age. In addition to iron carbide coming from the subduction slab, the asthenosphere contains other carbon donors. These donors can serve as a source of deep hydrocarbons, also participating in the deep hydrocarbon cycle, being an additional recharge of the total upward flow of a water-hydrocarbon fluid. The described deep hydrocarbon cycle appears to be part of a more general deep carbon cycle.