The regional thermobaric trap for mantle hydrocarbons in the crystalline crust of the Ukrainian NW Black Sea sector

Abstract

Summary The deep seismic sounding studies on the northwestern Black Sea shelf have delineated 2 low-velocity zones (LVZs) in the crystalline crust at depths of 6–16 km. For the first time, a novel thermobaric mechanism is developed for their formation in this area. The LVZs are mainly associated with the thermal decompaction of rocks that is not compensated by geostatic pressure. 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 pass, absorb and localize deep hydrocarbon-bearing fluids which, in turn, further destroy the integrity of the crystalline crust. The occurrence of hydrocarbons within the LVZs is corroborated by the occurrence of about 3000 active gas seeps and 8 gas and gas condensate fields in this area. The LVZs are first revealed to be spatially and genetically related to a mantle degass ing tube with which hydrocarbon fields are associated. They can be recommended as perspective target for the prospect and exploration of commercial hydrocarbon accumulations. This substantially allows us to expand the hydrocarbon budget of the NW shelf in using modern sophisticated drilling technology.