Geotemperature transients and the phenomena of heat flow define the fluid hydrocarbon regime in petroliferous sedimentary basins. The redistribution of heat and the thermo-physical properties of the rocks are mainly determined by the hydrogeology. As the temperature thresholds of smectite dehydration and kerogen diagenesis are passed, endothermic chemical changes convert solid rock mineral matter to fluids, reducing the net volume of mineral solids in each unit volume of rock and thereby increasing its porosity. As this occurs, the pore-fluid pressure rises markedly in response to the loss of load-bearing strength in the altered rock. Simultaneously, the aqueous solubility of fluid hydrocarbons is enhanced and the hydraulic permeability of the altered rock is greatly increased. Pore water carrying dissolved hydrocarbons moves through the altered rock and into adjacent aquifers, driven by steep hydraulic gradients. Subsequently, mass movement of water from the geopressure zone to the hydropressure zone migrates the dissolved hydrocarbons to traps, near which a sharp pressure drop causes exsolution.

The threshold temperature of smectite dehydration generally occurs a short distance below the top of the geopressure zone. The 100°C (212°F) isothermal surface closely approximates the top of the geopressure zone, except where water loss from the geopressure zone is in progress. At depths where temperature exceeds 150°C (302°F), petroleum occurrences are rare indeed. The abundance of natural gas, however, in both vapor phase and in aqueous solution, increases with pressure and temperature, and thus with depth, probably as a result of progressive natural cracking of petroleum residues in the rocks with deepening burial.

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