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 carb on 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 direc tion of <<cold>> continental geobarotherms. Below this zone, HH are in ther modynamic equilibrium, and above it they are thermodynamically unstable and convert into methane (with some admixtures of its next homologues), hydrog en, 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 characterist ics of a metastability level. It is calculated as the difference between th e internal energies of hydrocarbon system in metastable and thermodynamical ly equilibrium states per 1000 cm(3) of the metastable matter. Most of meta stable substances, including HH, have Delta U-vol no greater than 150-200 k kal under the T-P conditions of the Earth's surface. When Delta U-vol excee ds 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 1 000-2640 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 upp er 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 r ate of conversion of metastable HH into methane with admixtures of other ga ses and into solid carbon-bearing restite. The ascending flow of mantle HH, when passing through the energy barrier, d ecomposes into thermodynamically equilibrium components: methane (with admi xtures of its next homologues), hydrogen, and solid carbon - diamond and/or graphite. When transferring along deep faults, the mantle flow Feeds gas r eservoirs 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 s eismic 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 localize d above the upper boundary of the energy barrier.