Abstract: Floor for Gas
Colin Barker, Marwin K. Kemp
The deepest well is more than 31,000 ft (9,450 m) but the deepest production is less than 25,000 ft (3,600 m). This gap of 6,000 ft (1,850 m) between deepest hole and deepest production is the largest in the history of the petroleum industry and prompts the question of the ultimate depth limit for producible hydrocarbons. The generation of petroleum from the organic matter in sediments causes a redistribution of hydrogen with oil/gas and kerogen as the hydrogen-rich and poor products respectively. Thermodynamic considerations show that the final products are methane and carbon and they are stable to 500°C. However, the conclusion that methane will survive at all depths in a sedimentary basin is not valid because the natural system contains water as well as carbon com ounds. As temperature increases the water oxidizes methane to carbon dioxide and this establishes the practical limit on the depth of methane occurrence. A computer model has been used to establish equilibrium compositions in the relevant multicomponent systems. A series of iterations systematically changes the concentrations of components until the composition of minimum free energy is obtained. The model shows that for a normal geothermal gradient, reaction of methane with water becomes appreciable within current drilling depth. In overpressured systems reaction is inhibited and hydrocarbons survive to slightly greater depths. The influence of sulfur, pyrite, and hydrogen sulfide at specified temperatures and pressures can be studied also. Thermodynamic calculations only give the equil brium concentrations and provide no indication of the rate at which equilibrium is approached. However, the high temperatures encountered at depths greater than 20,000 ft (6,100 m) and the long time intervals available suggest that equilibrium will be approached closely.
AAPG Search and Discovery Article #90968©1977 AAPG-SEPM Annual Convention and Exhibition, Washington, DC