A thermodynamic criterion of metastable state of hydrocarbons in the Earth's crust and upper mantle

Thermodynamic modeling of the hydrocarbon system C-H with excess solid carbon permitted establishment of a narrow zone of heavy-hydrocarbons-methane (HH-CH4) conversion in the upper mantle. In the T-P plot it crosses the line of the diamond-graphite phase transition and the <<hot>> geobarotherm for oceanic crust, deviating from the latter toward high pressures in the direction of <<cold>> continental geobarotherms. Below this zone, HH are in thermodynamic equilibrium, and above it they are thermodynamically unstable and convert into methane (with some admixtures of its next homologues), hydrogen, and solid carbon in the form of diamond and/or graphite. From the Earth's surface to the conversion zone, KH can exist only in metastable state. A volume energy capacity Delta U-vol is used as a quantitative characteristics of a metastability level. It is calculated as the difference between the internal energies of hydrocarbon system in metastable and thermodynamically equilibrium states per 1000 cm(3) of the metastable matter. Most of metastable substances, including HH, have Delta U-vol no greater than 150-200 kkal under the T-P conditions of the Earth's surface. When Delta U-vol exceeds 300-500 kkal, the metastable state becomes unstable. In the depth range of 7-120 km, Delta U-vol of HH is greater than 300-500 kkal and can reach 1000-2640 kkal at a depth of 60-120 km. These values are close to the energy capacity of explosives. The descending flow of HII cannot overcome the upper boundary of energy barrier localized at a depth of 7-10 km, because the rate of sedimentary-rock sinking under these conditions is lower than the rate of conversion of metastable HH into methane with admixtures of other gases and into solid carbon-bearing restite. The ascending flow of mantle HH, when passing through the energy barrier, decomposes into thermodynamically equilibrium components: methane (with admixtures of its next homologues), hydrogen, and solid carbon - diamond and/or graphite. When transferring along deep faults, the mantle flow Feeds gas reservoirs in the sedimentary rocks which overlie the crystalline basement. If the ascending mantle HH flow forms intermediate chambers in the form of temporary metastable accumulations, their subsequent detonation can cause seismic activity in zones of deep faults. The mantle HH flow may also rapidly pass through the energy barrier, by the scold,, route, into the sedimentary cover and fractured basement rocks. In this case, metastable HH are conserved if their accumulations are localized above the upper boundary of the energy barrier.