A thermobaric mechanism for the formation of low velocity zones in the crystalline crust of the northwestern Black Sea shelf: a new type of traps for abiogenic methane

Abstract

Seismic studies of the NW Black Sea shelf have delineated low-velocity zones (LVZs) in the crystalline crust at depths of 6—16 km. For the first time, this paper presents a novel thermodynamic mechanism for their formation. The active temperature regime and deep geodynamic processes of the region provide evidence in favour of this mechanism. The LVZs are mainly associated with the thermal decompaction of rocks, which is not compensated by deep pressures. The rocks of such zones are characterized by an increase in fracturing and porosity and decrease in density, elastic parameters and thermal conductivity. As the rocks are permeable and hygroscopic they are able to more freely transform, absorb and localize mantle hydrocarbon-bearing fluids which, in turn, further destroy rock matter. Within the LVZs the existence of hydrocarbons is confirmed by the occurrence about 3200 activegas seeps and 8 gas and gas condensate fields. For the first time, the spatial coexistence was revealed between LVZs and a degassing tube to which hydrocarbon fields are related. LVZs zones should be considered as a new search criteria for thermobaric traps. Favou­rable conditions exist for the accumulation of abiogenic methane at a depth of 6—16 km that can substantially expand the hydrocarbon potential of the shelf in usage modern drilling technology.